摘要:

Formation of Ozone from Oxygen by the Action of IonizingRadiationsBY G. R. A. JOHNSON AND J. M. WARMANSchool of Chenistry, University of Newcastle-upon-TyneReceived 22nd January, 1964The formation of 0 3 from 0 2 and from rare gasf02 and N2+02 mixtures has been investigated.The results are interpreted in terms of the primary, radiation-produced, excited and ionic species.Some possible applications of the conclusions reached to an understanding of the chemistry of theupper atmosphere are considered.The photochemical processes which result in the formation of ozone from oxygenhave been discussed in detail.1 Less attention has been paid to the ways in whichozone can be produced as a result of ionization processes. Valuable informationabout the nature and reactions of ionic species in oxygen has been obtained frommass-spectrometric investigations.2-4 However, the manner in which reactions ofthis type can lead to ozone remains a matter of speculation.Although several investigations on the radiation-induced formation of ozonehave been made, no systematic study aimed at establishing a reaction mechanismhas been carried 0ut.sEXPERIMENTALPure oxygen was prepared by heating previously degassed KMnO4 and was distilledseveral times.Comparison with 0 2 prepared by electrolysis of triply-distilled water showedno difference between the results from the two sources. The other gases used were assupplied by British Oxygen Gases (spectroscopically pure). The irradiation techniquewas as described previously, a C060 y-ray source being used.6The irradiation vessels were washed with permanganic acid followed by H202 solutionand then several times with triply-distilled water, dried, pumped to below 10-4 mm Hg,filled with 0 2 to 760 mm and pre-irradiated for at least 24 h.Gas mixtures were prepared in the previously-evacuated irradiation vessels by intro-ducing the constituents in order of increasing partial pressure, the pressure being measuredwith a mercury manometer. Cooled traps were interposed to exclude mercury uapourfrom the irradiation vessel.For irradiations at 90 and 200°K the vessels were immersedin liquid 0 2 and solid C02+ ethanol respectively.For 0 3 determination a known proportion of the irradiated gas was trapped in a flaskcontaining 20 ml of de-aerated, frozen K1 solution (PH 7, phosphate buffer).The frozensolution was melted and shaken to ensure complete reaction of the 0 3 before measuringthe iodine spectrophotometrically as 13 at 350 mp.Dosimetry was based on measurements with the nitrous oxide gas-phase dosimeter,an initial G(N20-+N2) = 12.5 being assunied.7 The dose rate in 0 2 was 2x 1014 eVmm-1 in the 400ml vessel used. This was calculated from the dose in N20 assumingthe energy absorbed per unit volume to be directly proportional to the electron density ofthe gas.The total energy absorbed by a mixture of oxygen and a second gas may be assumedto be the sum of the energies absorbed in the 0 2 (&J and in the second gas (Ex). Itmay also be assumed that, for a given time of irradiation, &, is identical to the energy which888 FORMATION OF OZONEwould be absorbed in pure 0 2 irradiated at a pressure equal to the partial pressure p 0 2of oxygen in the mixture.Then Ex = Eo,ZxSxpX/Zo,So~O~, where Z,, and 2, arethe atomic numbers, So, and Sx are the stopping powers per electron of the two gases,andpX is the partial pressure of X. Experimental values of the stopping powers relativeto air, determined by Baily and Brown,8 have been used to calculate the ratio Sx/So, forthe different gas mixtures used. Since the stopping powers relative to air were determinedusing S-35 P-particles, the electron energy spectrum differed from that in our experiments.However, the stopping power per electron is nearly independent of electron energy overthe range involved.RESULTSIn agreement with previous worker^ the yield (per unit dose) of 0 3 from 0 2irradiated at 760 mm and 293°K was found to decrease with increasing dose.Thefall-off was marked, even at yields corresponding to an 0 3 concentration of only10-4 mole %, making it impossible to measure the initial yield accurately. Atlower irradiation temperatures (90 or 200°K) the yield was found to be directlyproportional to dose at 90°K up to an 0 3 concentration of at least 2 x 10-2 mole %(fig. 1). The initial yield was G(O3) = 12-8+0-6 (G = molecules/lOO eV).16.0-n0 . - !2.0tX Il o " !4.0' / OX'@I _-I0 4'" - - - - - - ~ 1 - . - - 1 ~ _ L - 2.0 4.0 6 - 0 8.0 10.0 I2 0radiation dose (eV x 1017)200°K (0) and 293°K (a).FIG. 1.-Irradiation of pure 0 2 at 760 mm Hg. Dependence of 0 3 yield on dose at 90°K (0),The fall-off in yield with dose at 293°K was found to be due partly to a radiationinduced and partly to a thermal decomposition of 0 3 . When the latter was minim-ized, by reducing the temperature (to 90°K) immediately after irradiation the initialyield at an irradiation temperature of 293°K approached that observed at the lowertemperatures (fig. 1).At 90 and 200"K, the initial rate of ozone formation was directly proportionalto the pressure of oxygen between 100 and 760 mm (fig. 2).Fig. 3-5 show the formation of 0 3 from rare gas+02 mixtures as a function ofthe dose absorbed in the rare gas X. The 0 3 yield shown is the contribution fromthe energy absorbed in X, i.e., the difference between the total 0 3 yield and thecontribution from the energy absorbed in the 0 2 fraction.The latter was calculateG . R . A . JOHNSON AND J . M. WARMAN 89assuming that G02(03) is independent of the partial pressure of X and equal to theG(03) = 12.8 found in pure 0 2 . GO'(O3) and GX(O3) denote yields of ozone per100 eV absorbed in 0 2 or X respectively." 8 2-oFIG.I I I200 400 600 80000 I/!PO2 (mm Hg)2.-Irradiation of pure 0 2 . Dependence of the rate of 0 3 formation onat 90°K (a), and 200°K (0).Ioxygen pressureER (eV x 10-18)FIG. 3.-Dependence of the Ar- and He-sensitized yield of 0 3 on dose absorbed in the rare gasat 90°K.pHe = 350 mm, p02 = 11.5 mm (0), = 2-0 mm (a).pAr = 350 mm, p02 = 100 mm (0), = 10 mm (a), = 2 mm (M), = 0-7 mm (A), = 0.3 mm (b.Fig.3 shows results obtained with Ar+O2 (pAr= 350 mm) irradiated at 90'K.Forp02 = 10-100 mm, the 0 3 yield was linear with dose and independent of p0290 FORMATION OF OZONEgiving an initial Gh(O3) = 4.5. At p02 < 10 mm the yield-dose plot was not linearbut at p02 = 2 mm the lowest dose used (6 x 1017 eV) gave Gh(O3) = 4.5. The con-stancy of the initial Gh(03) over the range p02 = 2-104 mm justifies the assumptionthat Go2(03) is constant in the Ar+02 mixtures used.2.0-P 2XG0E45 ,.,i z0" C EI I I I I40 8.0 12.0 16.0 20.0ER (eV x 10-17)FIG. 4.-Dependence of the Xe-, Kr- and Ar-sensitized yield of 0 3 on dose absorbed in the raregas at 200°K.pXe = 200 mm, p 0 2 = 10 mm (O), = 30 mm (0)p K r = 200 mm, p 0 2 = 10 mmI(O), = 30 mm (m)pAr = 660 mm, pO2 = 100 mm (A).ER (eV x 10-17)FIG.5.-Dependence of the Ne-sensitized yield of 0 3 on dose absorbed in Ne at 90°K.pNe = 200mm; p 0 2 = lOOmm(0); 1Omm (a), 3 mm (0).Results from He+O2 gas mixtures (90"K, pHe = 350 mm), are also shownin fig. 3. The initial GHe(O3) = 7-0, for pO2 = 2.0 and 11.5 mm.Fig. 4 gives results for Kr+02 and Xe+02 (200"K, pKr and pXe = 200 mm).P(03) = 6.6, independent of p 0 2 between 10 and 30 mm. GXe(O3) = 9.5, inde-pendent of p02 between 10 and 30 mm. The higher irradiation temperature iG . R . A . JOHNSON A N D J . M. WARMAN 91these experiments was used because of the low vapour pressures of Kr and Xe at90°K.For comparison some experiments with Ar+02 mixtures were carried outat 200°K (fig. 4); G&(O3) = 4.5 was identical to that obtained at 90°K.In Ne+02 gas mixtures (pNe = 200 mm, 90"K), GNe(O3) decreased with in-creasing dose at low doses (fig. 5). The initial GNe(O3) = 9 is therefore subjectto some uncertainty. The yield of 0 3 was independent of pO2 over the rangestudied (2-4-100 mm) up to doses of 8 x 1017 eV.In Nzf02 mixtures for pN2 = 190 and 400 mm, yQ2 = 25 mm, the initialGxz(O3) = 12. No study has been made, as yet, of the formation of the reactionproducts other than 0 3 .DISCUSSIONThe cheinically significant consequence of the absorption of ionizing radiations byoxygen is the formation of ions and of excited molecules, which may be represented :0,-+0,++eO,-+O+ + O + e0230;O,+e-+O,,where 0; can be in either the ground state OF excited and 0: represents an electron-ically excited molecule.At the 0 2 pressures considered here, reaction (4) will be thepredominant electron capture process, although dissociative capture may also occur.Since the average energy required to form an ion-pair in oxygen is W = 30.8 eV,the number of ions formed, Gi = 3-2. An approximate value for the yield of ex-cited molecules, Ge, = 4.1, may also be calculated.10In the mass-spectrometer, both Of and 0; ions have been found, the formerresulting to some extent from the dissociation of excited ions :The relative abundances of O+ and 0; in the mass-spectrum are 0-17 and 0.83respectively.11 On this basis Go+ = 0.5 and Go; = 2-7.(Here, and in the follow-ing discussion, GP is the yield of the primary species, p. For gas mixtures, (3:is the yield of p due to energy absorption in X). However, the proportion of O+may be less in the radiolysis since, at the relatively high pressures used, ion-moleculereactions may remove excited ions which, in the mass-spectrometer, dissociateaccording to reaction (9.12Some of the excited O*,+ ions are presumably sufficiently long-lived to undergo ion-molecule reactions. The reaction,has been shown to occur with a high rate constant (1-2 x 10-11 cm3 sec-1) for O$+ions of appearance potential greater than 17.3 eV.39 4 If it is assumed that excitedions are produced in radiolysis with the same efficiencies as in the mass spectro-meter, approximately 70 % of the 0; ions can react according to reaction (6).2Of ions formed in reactions (2) and (5) should undergo charge transfer,0;" -+o+ +o (5)o;+ +02-+0; +o, (6)o+ + opo+ o;, (7)0; + 0 2 - 0 3 + o;,the reported rate constant for this reaction being 6 x 10-10 cm3 sec-1.13also be assumed that the reaction,will occur readily.It may(892 FORMATION OF OZONEAt the pressures and dose-rates used in this work, reactions (6) and (7) will alwayscompete successfully with ion-neutralization. 0 ions, which do not undergoreactions (5) or (6) must eventually be neutralized by reaction with 0;.The primarily formed 0 5 may be deactivated or undergo dissociation :0;-+20, (9)0:+02-+0+0,.(10)or may react to give 0 3 :0 atoms produced in reactions (2), (5), (6), (7) and (9), and possibly in the ion-neutralization process, can lead to 0 3 by the reaction,which will predominate over the alternative reactions of the 0-atoms at the pressuresconsidered here.14In rare gas+02 mixtures, 0 3 can be produced as a result of energy absorptionby the rare gas, an effect that can be interpreted in terms of the reactions of the radi-ation-produced primary species from the rare gas.Knowledge of the primary pro-cesses is more extensive for the rare gases than for other gases.10 It appeared,therefore, that an investigation of the rare gas-sensitized formation of 0 3 mightgive some information useful in interpreting the reactions in pure oxygen.The yields of ions and excited atoms produced in the rare gases by irradiationmay be calculated.10 The values used in this discussion are listed in table 1.In0 +20,+0, + 0 2 , (11)gasHeNeArKrXeN20 2W (eV) a42.336.626.424- 121.934.930.82.42.73.84-24.62.93.2TABLE 1Gex =1.01.11.51.71.92.64.1Gx(03)calc.6.87.6> 3.0> 3.410.713.0-Gx(03)obs.7.09.04.56.69.512.012.8a values from ref. (10). calculation as described in text.addition to ionization and excitation of the rare gas, it must be taken into accountthat approximately 18 % of the initially absorbed energy remains as kinetic energyof the electrons, referred to as subexcitation electrons, which have been moderatedto energies below the lowest excitation potential of the rare gas.10, 15The rate constant for the charge transfer reaction,Ar+ + 02+Ar + 0,’ ,has been reported to be 8-6 x 10-10 cm3 sec-1.16 It may be assumed, therefore,that in Ar + 0 2 all of the Ar+ ions will undergo reaction (1 2) since the only competingprocessis Ar+ neutralization.The ionization potential, I& = 15.8 eV, so that reaction(12) can lead only to ground state 0;. Dissociative neutralization of 0; wouldlead to a yield of G(O3) = 2GA,+= 7.6. In addition to this, some 0 3 formationfrom the reactions of excited Ar atoms with 0 2 must be considered. The Gh(03)>7.6 predicted on this basis is considerably higher than the observed value of Gk(O3) =4.5. This leads us to suggest that neutralization of 0; occurs as a non-dissociativeprocess in this system and does not, therefore lead to 0 3 G.R . A . JOHNSON AND J . M. WARMAN 93In Ar + 0 2 mixtures, it is probable that 0 3 would result mainly from the reactionof excited Ar :Ar*+O,-+Ar+O;, (13)followed by reactions (9) and (1 I), or (10) and (1 1). From studies of the radiation-induced luminescence in Ar+Hg and Ar+N2 mixtures, it was concluded that Arexcited to the metastable triplet levels (3Po and 3P2) reacted with Hg or with N2 17. Asimilar process may occur in Ar+02.Any possibility of a contribution to the 0 3 yield from reactions of multiply-charged Ar, and of sub-excitation electrons, has so far been ignored. It has beencalculated that the mean energy of the sub-excitation electrons in Ar is about 4.8 eV 10and it is unlikely that dissociation of Oz(D0-0 = 5.1 eV) by these will be of im-portance.Up to about 20 % of the positive ions may be formed as multiply-chargedions 18, 19 and, if these react with 0 2 either by dissociative charge transfer or bycharge transfer to give an excited O i , a yield of 0 3 as much as G = 1.5 could beproduced from this source. Charge-transfer from multiply-charged Ar to 0 2 mustcompete with charge-transfer to Ar atoms and it is difficult to estimate the relativeprobabilities of the two processes. However, the difference between the predictedyield from Ar (=2G&* = 3.0) and the observed Gh(O3) = 4.5 may be due to multiply-charged ions.An attempt was made to establish the number of different active Ar speciesresponsible for Gk(O3) by studying the kinetics of the competition between reactionswith 0 2 , leading to 0 3 formation, and deactivation processes. Unfortunately, thisdid not prove to be feasible since, at pO2 sufficiently low to observe such a com-petition, the yield against dose plots were not linear, making it impossible to measurethe initial 0 3 yield (fig.3). The dependence of the fall-off in Gb(O3) with doseon pO2 is consistent with competition between deactivation by 0 3 of the speciesresponsible for 0 3 production and reaction of the species with 0 2 .In Kr+O2 mixtures the reaction,Kr++O,-+Kr+O,f, (14)can only give ground state Oi(I= = 13.9 eV). The observed GK-’(03) = 6.5 wasless than the value which would be expected (G = 2G,+ = 8.4) if dissociativeneutralization of 0; occurred.This result, therefore, is in agreement with theconclusion drawn from the Ar+02 experiments, that the neutralization does notlead to dissociation. In Kr+02 it may be assumed that 0 3 results mainly fromreactions of excited Kr atoms possibly together with some contribution from multiply-charged Kr.Xe + 0 2 mixtures were of interest since Ixe = 12.1 3 eV is just less than lo, = 12-2 eVand therefore charge-transfer from ground state Xef to 0 2 is much less probablethan with Ar and Kr. On the other hand it has been shown that approximately25 % of Xe+(2P,) (A.P. = 13.4) are formed,Zo which may undergo charge transfer:Xe + ( ,P%) + o,( 3~g)+ Xe( IS,) + o,+ (,n,) (15)To calculate the ozone yields from Xe+02 mixtures it is assumed that (i) the 0;formed in reaction (1 5) is removed by non-dissociative neutralization, (ii) Xe+(2Pg)ions are neutralized to give an excited atom, which can lead to 0 3 via reactions (9)and (1 l), or (10) and (1 I), and (iii) excited Xe atoms, formed directly by irradiation,also lead to 0 3 via the above reactions.This gives a predicted yield of 03, G =(2 x 0*75Gxe+ +2Gxe*) = 10.7, close to the observed GXe(O3) = 9-594 FORMATION OF OZONEIn He + 0 2 mixtures, dissociative charge transfer,He' + O,-+He+ 0' + 0, (1 6 )is feasible since IHe = 24-6 eV. Reaction (16), followed by reaction (7) and non-dissociative neutralization of the 0; so formed, would give an 0 3 yield = 2 G ~ e + = 4.8.In addition, excited He atoms may lead to 0 3 via a reaction analogous to reaction(13).More probably, ionization of 0 2 will occur, either as a dissociative process,or to give an excited O;,Either of these reactions, followed by the reactions of excited O*,+ and of O+ as-sumed above, would give a yield of 0 3 = 2 k e * = 2.0. The total predictedGHe(O3) = 2Gke+2G~,* = 6-8 is close to the observed GHe(O3) = 7.0.It may be expected that Ne+O2 mixtures would be similar to He+O2 mixtures,since again dissociative charge transfer is possible and excited Ne atoms can alsolead to 0 3 via reactions similar to those discussed for He. Making assumptionsanalogous to those discussed for He+02, gives a calculated GNe(03) = 7.5, i.e.,less than the observed GNe(O3) = 9.Any contribution to the 0 3 yield from sub-excitation electrons has been neglectedso far in discussion of He+O2 and Ne+O2.The mean energy of these electrons,calculated to be 7.6 and 6.8 eV in He and Ne respectively,lo is sufficiently high tosuggest that they might play a part in the decomposition of 0 2 . It is difficult toestimate the extent of this with any certainty but an upper limit of the yield of 0 3from this source would be G = 2.0.15The formation of 0 3 from pure 0 2 may now be considered in the light of theconclusions arrived at from the results obtained in the study of the rare gas sensitizedreaction. As discussed above, values for the yields of the primary species in 0 2are Go; = 0.8, Go;' = 1-9, GO+ = 0.5, Go; = 4.1, where 0; and O%+ representground state ions and ions of appearance potential > 17.3 eV respectively.Assumingthat the reactions (6), (7), (8), (9) or (10) and (11) occur, and that neutralization of0; does not give 0 3 , it may be calculated that GO*(03) = 13. This is consistent withthe observed GOz(03) = 12.8 +0-6. While there remains some ambiguity in the inter-pretation of the results, it is believed that the mechanism arrived at for the formationof 0 3 , both in pure 0 2 and in rare gas + 0 2 mixtures should be reasonably correct.The N2-sensitized formation of 0 3 is more difficult to interpret than the raregas-sensitized process since fewer data are available about the primary processesin N2 and, furthermore, the possibility that products other than 0 3 are formed cannotbe excluded.If excited N:! were the only species playing a part in 0 3 formation,a maximum yield G = ~ G N ; = 5-2 would be expected. The observed G"(O3) = 12.0is much higher, which suggests that N; may also be involved. To be consistentwith the conclusions arrived at above, it must be assumed that ground state NZ(15.6 eV) cannot react to give 0 3 . However, electron impact studies have shownthat most of the N l ions are formed in the B22:+, or higher states,21 which could leadto 0 3 by charge transfer to give an excited O i , followed by reactions (6), (8) and(10). It appears from this study that almost 40 % of the 0 3 formed radiolyticallyfrom pure 0 2 , is produced as a result of the reactions of primary ionic species.Also,O3 is produced apparently as a result of the reactions of excited N; ions with 0 2 .The experimental conditions used in this work were far removed from condi-tions in the upper atmosphere. Nevertheless it may be of interest to considerwhether the conclusions have any bearing on the reactions in the upper atmosphere.He* + 0,-+He + O+ + 0 + e,He* + 0 p H e + 0;' + e.(17)(18G. R. A . JOHNSON AND J . M. WARMAN 95Ionic reactions are not commonly considered as a source of 0 3 in the atmo-sphere. However, both ground state and excited 0; and N; ions will be producedby photo-ionization, X- and cosmic-ray absorption. Because of uncertainties inthe concentrations of ions, electrons and oxygen, the circumstances under whichreaction (6) would compete successfully with O*,+ neutralization cannot be calculatedwith any certainty.However, some contribution to ozone formation due to reaction(6) might be expected at altitudes less than - 100 km. Excited Og+ ions have beenobserved which are sufficiently stable to persist for at least 10-6 sec in the absenceof collisions.23 An ion of life-time 10-6 sec would persist long enough to be involvedin a collision with 0 2 at altitudes less than about 100 km and longer ion life-timesare conceivable.In our interpretation of the radiolysis results, it was necessary to assume thatneutralization of ground state 0; does not lead to dissociation and 0 3 formation.At the pressures used in the radiolysis, ion neutralization must involve the reactionof 0; with 0;.Since reaction (19) is about 1 1 eV exothermic and apparently doesnot lead to dissociation, it would be expected to lead to highly excited 0 2 molecules :Excitation to the 3Z.+U state is energetically possible and would be expected to befollowed by Herzberg band emission. As the pressure of 0 2 decreases one wouldexpect direct dissociative recombination of 0; with electrons (or 0-) (reaction (20))to take place before attachment (reaction (4)) could occur :0: + O i ( + M)-+202( +M). (19)O,++e-,O+O. (20)It is of interest to speculate whether the Herzberg bands and the 5557 A O-atomemission, which appear to originate, in the airglow, below and above - 100 kmrespectively, might be produced as a result of reactions (19) and (20).We thank Prof. J. J. Weiss for €us encouragement and for helpful discussions.1 Bates, The Earth as a Planet, ed. Kuiper (Univ. of Chicago Press, 1954), chap. 12, p. 576.2 Erost and McDowell, J. Amer. Chem. SOC., 1958, 80, 6183.3 Cermak and Herman, J. Chim. Physique, 1960, 57, 557.4 DBng and Cottin, J. Chim. Physique, 1960, 57, 557.5 Lind, Radiation Chemistry of Gases (Reinhold Publishing Corporation, New York, 1961),6 Clay, Johnson and Warman, Disc. Faraday Suc., 1963, 36, 46.7 Johnson, J. Inorg. Nucl. Chem., 1962,24,461.8 Baily and Brown, Rad. Res., 1959, 11, 745.9 Hurst and Bortner, Physic. Rev., 1959, 114, 116.10 Platzman, Int. J. Appl. Rad. Isotopes, 1961, 10, 116.11 Laidler and Gill, Trans. Faraday Soc., 1958,54,633.12 Stevenson, Rad. Res., 1959, 10, 610.13 Dickinson and Sayers, Proc. Physic. Soc., 1960, 76, 137.14 Kaufman, Progress in Reaction Kinetics (Pergamon Press, London, 1961), p. 20.15 Platzman, Rad. Res., 1955, 2, 1.16 Maushart, Ann. Physik, 1958, 44,264.17 Brown and Miller, Trans. Faraday SOC., 1957, 53, 748.18 Melton, J. Chem. Physics, 1962, 37, 562.19 Field and Franklin, Electron Impact Phenomena (Academic Prss, New York, 1957), p. 183.20 Fox, Hickman and Kjeldaas, Physic. Rev., 1953, 89, 555.21 Frost and McDowell, Proc. Roy. Soc. A , 1955,232,227.22 Robinson, Reports Progr. Physics, 1959,22, 263.23 Henglein, 2. Naturforsch., 1962, 17a, 37.24 Bates and Dalgarno, Atomic and Molecular Processes (Academic Press, New York and London),p. 83.1962), chap. 7

ISSN:0366-9033    

DOI:10.1039/DF9643700087

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

Fluorescence of Nitric OxidePart 3.-Determination of the Rate Constants for Predissociation,Collisional Quenching, and Spontaneous Radiation of NO C2n[(v = 0)BY A. B. CALLEAR AND I. W. M. SMITHDept. of Physical Chemistry, University of Cambridge, EnglandReceived 15th January, 1964The intensity of the resonance fluorescence from NO C2l7(v = 0) was compared to the enhancement of the y-band intensity caused by deactivation of C2n(v = 0) to AW+. It was concludedthat the C2n(u = 0) state dissociates at a rate which is 30 times faster than the rate of spontaneousradiation to X2n. This observation demonstrated that the predissociating state has d>O, and itwas suggested that the simplest interpretation is that the a4l7 state is the connecting link betweenC2n(v = 0) and normal atoms.The D2Z+ state interacts less strongly with the a4n state becauseof the change in A.The following scheme was proposed to account for the fate of NO Czn(v = 0) :NO X217+hv NO A 2 B + h.k, = 9.7 x 10-10 cm3 molecule-1 sec-1, kF = 22x lo7 sec-1, kl = 6.6 x 108 sec-1 and k2 = 5.2 x10-16 cm3 molecule-1 sec-1.The potential curves of nitric oxide are shown in fig. 1. The X2IT, A2Z+,B2H, C2II, and D2Z+ states are well defined from the ultra-violet spectrum.1-7 Ahomogeneous perturbation results from the crossing of the C2II and B2ll potentialcurves.5~ 6 The positions of the a4IT and the flat 2Z+ curves have been calculatedfrom the data of Vanderslice, Maisch and Mason.* The excitation energy andmolecular parameters of the a4II state were estimated by comparison with 0; a4II.The position of the NO a4II potential curve can probably be relied upon to within0.2 eV or 0-05 A.The Ogawa bands 9 define the relative positions of the b4Z-and a4II states. The potential curve of the flat 2Z+ state is rather uncertain.If nitric oxide is irradiated with light from a xenon arc, the A2Z+, C2n, at- ‘D2Z+ states are excited, giving rise to the 7-, 6- and &-bands respectively. Some ofthe absorption bands of NO are shown in fig. 2. It is demonstrated in this paperthat NO C2II(v = 0) is weakly predissociated in all its rotational states. By cal-culating the Franck-Condon factors, the spontaneous emission rate of the C2II(u = 0)-X2IT transition has been determined. Hence, by means of a quantitative deter-mination of the weakening of the fluorescence due to dissociation, the rate constantfor predissociation of C2II(u = 0) has been found.It is suggested that the dissoci-ation energy of nitric oxide is close to 6.493 eV. Thus the rate of preassociation of N(4S)and O(3P) into the C2II(u = 0) state may be determined by calculation of the equi-librium constant. The photolysis of nitric oxide and the recombination of N and0 atoms are important chemical reactions in the earth’s atmosphere.There is already good evidence that NO C2II(u = 1) can dissociate; the &bandswith v’>O have never been observed in emission. However, the rotational lines9A. B. CALLEAR AND I. W. M. SMITH 97of the b-bands have Doppler width at - 183"C,5~ 6 and any predissociation maytherefore be described as weak ; the first-order rate constant must be less than about1 x 1010 sec-1.Flory and Johnston irradiated nitric oxide with light from a varietyof sources and concluded that decomposition resulted largely from absorption oflight by the (1,O) b-band,lo though a small fraction of the decomposition may haveresulted from absorption at longer wavelengths.11I *o 2 . 0internuclear distance R (A)FIG. 1.-The potential diagram of nitric oxide.In a study of the combination of N(4S) and O(3P), Young and Sharpless 12observed the (0' = 0) &bands in emission and concluded that the C2n(u = 0) statewas populated via the a4II state by a preassociation mechanism. The a4n state waspostulated as the precursor to combination in preference to either the b42- stateor the flat 2X+ state, because the observed infra-red system C2n-A2Ct exhibitedP, Q and R branches at low pressure. By considering the selection rules it followsthat the consecutive processeswould have no Q branch, and the stepswould give neither P nor R branch. The detection of the P, Q and R structure of theC2lI-A2ZS- bands therefore demonstrates that the precursor to the C2rI(v = 0)state has A>O, provided that the preassociation mechanism is correct.Thus,according to fig. 1, the a4n state is the only possible precursor to the C2n(u = 0)state, though states at present unknown could conceivably be involved. Young andSharpless suggested that A2X+(u = 0) can be populated by radiation or by collisionprocesses with N2.Their interpretation of the population of the b4Z- and theB2II states does not appear to be straightforward; our work is only indirectly2Z++C2II(u = O)+A2Z++hvb4X:-4C2rI(v = O)+A2X++hv98 FLUORESCENCE OF NITRIC OXIDErelated to these problems, and to the cause of the cut-off of the /I-bands in emissionwith v’ < 6.Young and Sharpless’ experiments provide evidence that the NO C2IT(u = 0)state is predissociated. However, there could be alternatives to the preassociationmechanism, such as bimolecular energy transfer. Our observations on the fluor-escence of the &bands, make possible a comparison of the rate of direct excitationwith the rate of 6-band emission from the C2IT(v = 0) state. The NO C2II(u = 0)molecules are deactivated by collisions with N2 and we have shown that the rateof photolysis depends in a similar manner on the N2 pressure as does the intensityof the 6-band fluorescence. These observations, taken with those of Young andSharpless, offer strong proof that NO C2IT(u = 0) has a much larger probabilityof predissociation than of spontaneous radiation.A quantitative study of theeffect of Nz on the nitric oxide fluorescence, makes possible a determination of thefirst-order rate constant for predissociation of NO C2II(v = 0).EXPERIMENTALThe apparatus and general experimental method have been described.13 The &systemwas photographed on Ilford Q3 plates using exposures of 3+-h duration. The subsequentdevelopment and plate photometry have been described.14 The calibration curve used toconvert the densities of the bands on the plates to relative intensities is shown in fig.3.///Y5 0 I00 150intensity (arbitrary units)FIG. 3.-Density-intensity curve for Ilford Q3 plates at about 2250A with the NO ybands aslight source ; time of exposure, 33 h.0, (0,O) band; a, (1,l) band; 0, (3,4) band; 8, minimum between (0,O) and (1,l) bands.The intensity of the y-bands was varied with a number of quartz plates acting as a step wedge.The variation of intensity was measured on a Unicam ultra-violet spectrophotometer.The complete curve was obtained by fitting together the separate curves obtained fordifferent regions of plate density.For measurements of the quenching of the &bands, the total pressure was kept con-stant by addition of an excess of either argon or helium.Pressure broadening the absorp-tion lines increased the intensity of the fluorescence, and also kept the rate of excitationconstant. Plate photometry of the bands was rendered difficult by the partial overlap ofthe &bands with bands from the y-system due to the use of wide slit openings. However,the (0,3) &band was suffciently isolated and strong enough for quantitative measurement,and the difficulty of overlap diminishes at high nitric oxide pressures when the (1,O) y-banFIG. 2.-Absorption spectrum of nitric oxide. NO pressure 30.0 mm ; patA. B . CALLEAR AND I . W. M. SMITH 99is considerably self-reversed. In later experiments, by improving the optical arrangementand using a new lamp, the (0,2) &band was also recorded with sufficient plate density foraccurate photometry.CALCULATION OF SOME FRANCK-CONDON FACTORS AND THE RATE OF RADI-ATION OF NO c2n(U = O) TO x2nThe mean radiative lifetime of a state can be calculated if the electronic f-value andthe Franck-Condon factors of all transitions are known.13 The f-value for the NO (0,O)&band has been measured by Bethke 15 but the Franck-Condon factors have not been re-ported for either the C2II-A2Z+ or the C2II- X2II transitions.However, Bates 16 hascompiled data from which Franck-Condon factors can be calculated for transitions betweenlow-lying vibrational levels.TABLE 1 .-FRANCK-CONDON FACTORSTc2n - A2XfV” 0 1 2V’0 -995 -001 -0001 -001 -986 a002c2n- X2IIV” 0 1 2 3 : 4V’0 ‘1 63(.153) *256(*255) .250(.238) *194(.166) : *O51(‘097)t Note added in proof.Ory (J Chem. Physics, 1964, 40, 562) has recently made an accurate calculation of the Franck-Condon factors for the 6 bands, and these are shown in brackets in table 1.The values of We, re and x,, were taken from Herzberg, 17 and Lagerqvist and Miescher. 6The Franck-Condon factors for the NO C2II(v = 0 and 1)-A2Cf are given in table 1.The equilibrium internuclear distances in the two states differ by only 0.011 A and theAv = 0 transitions are therefore much the strongest, with the Av = f l transitions weak,and the rest close to zero. The Franck-Condon factors for the (v’ = 0) 6-bands are alsoincluded in table 1.The strongest are clearly those with v”<3, for which Bates’ tablesgive accurate Franck-Condon factors. Therefore, it is possible to calculate the span-taneous emission rate for the C2n(v = 0)- X2lT transition using the f-value for the (0,O)8-band. The overlap with the weak (7,O) P-band has been allowed for by comparisonwith the f-value for the resolved /?-bands also given by Bethke. The $value used in thecalculation was fo,o(h) = 2.4 x 10-3. The rate of spontaneous emsision from C2II(u = 0) -X2lT was calculated to be 2 . 2 ~ 107 sec-1.The C2II state also radiates to the A%+ state and Young and Sharpless suggest thatthe rate is comparable to the rate at which the &bands are emitted. If this were so, thetransition matrix element for the CTI(u = O)-A2Z+(v = 0) transition would have to beabout 400 times that for CW(v = O)-XZll in order to compensate for the (frequency)3and degeneracy terms in the expression for the transition probability.This is possiblesince the f-value would be below unity ; for a given $value, the rate of radiation dependson (frequency)2. Our rate constants are determined relative to the &emission and are notdependent on the unknown rate of the C”(v = 0)-A2Z+ transition.RESULTSQUENCHING OF THE NO@’ = 0) BANDSIf nitric oxide is irradiated with light from a xenon arc, an ammonia filter canexclude excitation to states higher than A2Z+(v = O).139 14 The stationary stat100 FLUORESCENCE OF NITRIC OXIDEconcentration of molecules in this level, when excited by light filtered throughammonia, can be compared to that when the fluorescence is excited by unfilteredlight, in order to determine the extent to which this state is populated via higherenergy levels.Thus it was shown that population of NO A2C+(u = 0) from higherenergy levels by collision processes with argon or helium is extremely slow. Inthe presence of nitrogen, however, a considerable enhancement of the intensity ofthe u‘ = 0 y-bands is observed when the fluorescence is excited by unfiltered light,and this is attributed to the occurrence of two energy transfer processes :(i) vibrational energy exchange between NO A%+(u = 1) and N2 XlC+(u = 0),(ii) quenching of NO C2rI(v = 0).The rate of these processes has been measured and, at low nitrogen pressures, quench-ing of NO C217(v = 0), and not vibrational relaxation is clearly the dominantprocess.Since neither argon nor helium quench the 6-bands, the intensity of the systemin fluorescence can be increased, and kept constant, by adding a large excess ofargon or helium to the nitric oxide in order to pressure-broaden the lines.Quench-ing by nitrogen can then be observed by measuring the intensity of the (u’ = 0)&bands excited in mixtures of nitric oxide, nitrogen and inert gas. The nitric oxideand total pressures are kept constant, but the partial pressure of nitrogen is changed.Quenching of the (0,2) and (0,3) 6-bands by low partial pressures of nitrogen isclearly shown in fig. 4. Five series of experiments have been carried out at differentnitric oxide and total pressures.These results have been treated in the conven-tional manner by plotting Io/l against the partial pressure of nitrogen, where I0is the intensity of the (0,3) or (0,2) 8-band without N2 added, and I is the intensityin the presence of added N2, at the same nitric oxide and total pressure. The re-sultant Stern-Volmer diagram is shown in fig. 5. Within experimental error, fornitrogen quenching half-pressure at a total pressure of 700mm was constant, thenitric oxide pressures between 2 mm and 12 mm. From fig. 5, it appears that thenitrogen quenching half-pressure is slightly dependent on the argon pressure presentin the system. The argon quenching half-pressure for the &bands is certainly greaterthan 380 mm.The substitution of helium for argon as inert gas in these mixturesdid not affect the measured nitrogen quenching half-pressure. These results demon-strate that helium is as ineffective as argon at quenching the &bands. By comparingthe intensity of the 6-bands excited in the same excess of argon and helium, it wasshown that any difference in the rate of excitation as a result of different pressurebroadening by helium, on the one hand, and argon and nitrogen, on the other, wasvery slight.In order to investigate the result of quenching NO C2n(u = 0) by N2 in moredetail, the intensities of the various y-bands were measured with, and without, theaddition of a small partial pressure of N2. Any increase due to quenching of NO C2IIis most apparent at low nitric oxide pressures when self-quenching of the y-bandsis slight.Therefore, the fluorescence was excited in 0.6 mm of NO with (a) 700 mmargon, and (b) 45 mm N2+655 mm argon. To bring the plate densities of they-bands into the most convenient and sensitive range, exposure times were reducedto l s h . Fig. 6 shows clearly the enhancement of the (u’ = 0) y-bands and thesmaller increase in the (u’ = 1) progression on adding nitrogen. With a smallnitrogen partial pressure, this effect cannot be solely due to vibrational relaxationof NOA2C+. To treat these results quantitatively, the ratio of intensities inmixtures (a) and (b), (Ib/Ia), was predicted for each y-progression, assuming thatpopulation of the excited levels occurred only by direct light absorption and bA .B. CALLEAR AND I . W. M. SMITH 101vibrational exchange with N2 at the rates previously measured. This was done bysetting up the necessary stationary state equations for population and depopulationof the various levels and substituting in these equations the rate coefficients previously3 -2 -l o l lII I I20 4 0 6 0 00pressure of N2 (mm Hg)FIG. 5.-The ratio of the intensity of the (v’ = 0) &bands without (lo), and with (I), added nitrogen,as a function of its partial pressure, for different nitric oxide and total pressures.8 2.0 mm NO0 12.0 mm NO9 5.0 mm NO (0,3) band, (He+NO+N2) 700 mm0 5.0 mm NO(0,3) band, (Ar+NO+Nz) 700 mm0 5.0 mm NO (0,3) band (h+NO+N2) 300 mm(0,2) band}reported,l3, 14 and the relative intensities from exposures (a) and (b).For example,the concentration of NO A2Cf(u = 3) with nitrogen present, is given byr3(n31N2 = 1 + k,[NO] + kQ[M] + k,[N,]’and without nitrogen bySo for the (v‘ = 3) progression= 0.81. 1 +O*GjO*91+ 700/14001 + 0.6/0.91+ 700/14OO + 8.3 x 45/760- 102 FLUORESCENCE OF NITRIC OXIDE13 is the rate of formation of NO A~Z+(U = 3), and n3 is the concentration.rate coefficients are for the processesThekfk ,kQNO C(U = ())-+NO A(u = O)+hvNO A(u = O)+NO X+NO X+NO XNO A(zj = O)+Ar-+NO X+Ark3NO A(u = 3)+N2+N0 A(v = 2)+N2The observed and predicted ratios differ significantly for the U' = 1 and v' = 0and have all been divided by the spontaneous emission rate from the A2Z+ state.bands, and this is due to quenching of NO C2ll by nitrogenkc, NO C+NZ+NO A(u = l)+N2NO C+N2-+N0 A(v = O)+N2.It is essential to determine the relative rates of these processes in order to determinethe rate of excitation to NO C2II.It is first necessary to reconsider the stationarystate equations and equate the differences in table 2 between the observed andpredicted ratios, to the terms previously neglected. This givesobs. &/Iapredicted I,[&,TABLE 2difference -0.10 - 0.03 0.32In case (a) (nO)Ar/(dAr = 1.5% of total NOA2C+ pro-duced by quenching ofand- 22 1 - NO C2II by 45 mm of N20.7878Similar measurements have been made on the (u' = 0) and (u' = 1) y-bands ex-cited in mixtures of 6.0 mm of NO with (a) 700 mm of argon, and (6) 700 mm ofnitrogen.These results are compared to the intensity ratios predicted for thesemixtures assuming that populatiun of these levels occurs only by direct excitationand by vibrational relaxation of NO The results can then be treated in themanner which has been described above. It is thus shown that the ratio of therates at which molecules are produced in A~Z+(U = 0) and in A2X+(u = l), as A . B. CALLBAR AND 1. W. M. SMITH 103result of quenching NO CHI by 700 mm of nitrogen, is equal to the ratio of therates in the presence of only 45 mm of nitrogen.TABLE 3difference 0.47 0.75In case (a) (no>A?/(nl>A, = 2.524% of total NOA2V produced byquenching of NO C2lI by 700 mmof N276EVIDENCE FOR PREDISSOCIATION OF NO c2n(v = 0)The enhancement of the (u' = 1) and (u' = 0) y-bands as a result of quenchingof NO C2II by N2, is considerably larger than might be expected by comparing theintensity of the unquenched &bands with the intensity of the y-bands in fig.4. At700 mm, k, [N2][NO c]/(nl)N2 = 2.1, i.e., over twice as many molecules are quenchedfrom NO C2II to A2C+(u = 0) per unit time as radiate in the same time fromA2X,+(u = 1). However, fig. 4 clearly shows the (u' = 0) &bands to be very muchweaker in the absence of nitrogen than the (u' = 1) y-bands. This cannot beattributed to low Franck-Condon factors for the observed &bands; for the (0,3)6-band the Franck-Condon factor is 0.166.A second apparent anomaly in regard to the &band intensities is that at low nitricoxide pressures and in the absence of foreign gases, the &-bands are considerablystronger than the &bands.This is clearly illustrated in fig. 7. Under these con-ditions the reverse is to be expected; the two (0,O) bands have roughly comparable$values, the &band, in fact, having slightly the larger value. Moreover, the 6-band is at larger wavelength and is, therefore, in a region of greater quantum input.Finally, because of the large A-doubling in the C2II state reported by Herzberget aZ.,59 6 the (0,O) &band contains more separate rotational lines than the (0,O)&-band. Therefore, despite the comparable $values, the (0,O) &-transition resultsin a greater fraction of the incident light being absorbed near the front wall; at adepth of 2cm inside the reaction vessel, the exciting radiation suffers a greaterreduction in intensity than that exciting the (0,O) &band.Allowing for the Franck-Condon factors, calculations based on measurements from the plate shown in fig. 7show that the overall rate of radiation of the (u' = 0) &-bands is about 3 times thatof the (u' = 0) &bands. Gaydon 2 has reported that the 6 and &-bands excited in atransformer discharge show similar relative intensities.Final proof that NO C2II(u = 0) dissociates was obtained by studying the extentof photodecomposition, with and without added N2. Table 4 shows the results ofsome of these experiments in terms of the pressure of gas produced in the reactionvessel, which was condensed at- 196°C and was not removed by pumping.TABLE 4.-hESSURE OF THE CONDENSABLE PRODUCTS OF PHOTODECOMPOSITION (ITU'Tl)expt .20 rnm Not300 mm Ar 20 mrn N0+300 mm N2blank 0.041 0.452 0.503 0.500.010.190.220.21 04 FLUORESCENCE OF NITRIC OXIDEDecomposition in the presence of N2 occurs via NO C2II(u = 1) and NOD2C+(v = 0 and 1); the latter appear to undergo collisionally-induced predissoci-ation. In Ar, the NO C2II(v = 0) also predissociate. More NO C2rI(u = 0) isproduced than NO D2Z+(v = 0), because of the smaller f-values of the individuallines of the (0,O) &band and also because the (0,O) &band is at longer wavelengthsthan the (0,O) &-band. Details of this work and a full account of the study of thephotolysis will be described elsewhere.18RATE OF PREDISSOCIATION OF NO c2n(U = o), RELATIVE TO SPON-TANEOUS RADIATION TO x2nThe results show clearly that in argon or helium a large proportion of the NOmolecules which are excited to CXI(v = 0) do not revert to the X2II state and emitthe &bands, but are lost by a second process which is practically independent ofinert gas pressure.To compare the number of molecules radiating from NO C2II(v = 0) with those produced in NO A2C+(v = 0) as a result of quenching of theC2rI state, the intensity of the (8' = 0) &bands were measured relative to the y-bands in the presence of argon. This ratio was compared to the relative increasein intensity produced in the (u' = 0) bands by quenching of NO C2II in the samepressure of nitrogen.The stationary state equations for population and depopulation of the AzZ+(u = 0) level in nitrogen, and in argon arerO+kf[No](nl)Ar("o)Ar = 1 + k,[NO] + kQ[Ar]'where kl and kl are the rate coefficients forkiNO A(u = I)+N2-+N0 A(v = O)+N2,NO A(u =l)+NO X+NO A(u = O)+NO X.klIf [MI = "21 = [Ar],andThe relative concentrations no)^,, (no)&, ( n l ) ~ ~ and ( n l ) h were determined inmixtures of 6-0 mm of NO with (a) 700 mm of argon and (b) 700 mm of nitrogen,by measuring band intensities, and are shown in table 5.With the rate coefficientsreported previously,13~ 14 it is then possible to calculate k,[NO c)~,[N~l/(nl)~,.18.1-k,,[NO C] ~,[Nz]/(n 1)N2 = 1-35 - 0.07 - 0.24,kc~[No clN~[N21/(nl)N2 =8*4A .B. CALLEAR AND I . W. M. SMITH 105To compare the rate of quenching of NO C2ll to A~Z+(U = 0) with the rate of radi-ation from NO C2ll(u = o), the relative intensities of the (0,3) 6-band and the(1,l) y-band were measured in 700 mm of argon. Correcting by the respectiveTABLE ~.-RELATNE STATIONARY STATE CONCENTRATIONS(no) 100 52.5(n1) 35.1 21.06.0 mm N0+700 mm Nz 6-0 mm N0+700-mm ArFranck-Condon factors, kf[NO C ] / ( n l ) ~ , was thus determined, where kf is the spon-taneous emission rate of the 6-bands divided by the spontaneous emission rate ofthe y-bands :kf[NO C ] j ( r ~ , ) ~ , = 0.63.- rate of production of NO A2X+ju = 0) by quenching of NO C2Hrate of radiation of NO C211 to NO X211= 8-4/0.63 x 35*1/21.0 = 22.3.In order to convert this ratio into the percentage of molecules excited to C2Il(v = 0)which revert to the ground state and emit the b-bands, it is now necessary to con-sider quenching to A~X+(V = 1).It is shown below that this also occurs as a resultof quenching of NO C2ll(u = 0) and, as the rate of quenching to v = 1 is knownrelative to that to v = 0 (table 3), thentotal rate of production of NO A2C+ by quenching of NO C211rate of radiation of NO C21T(v = 0) to NO X21122.3 x 2476~ 2 2 . 3 +- = 29.3.Finally, the fraction of molecules remaining unquenched in the presence of 700 mmof N2 must be allowed for. Sorate of excitation to NO C211(v = 0)rate of radiation of NO C211(u = 0) to NO X21129.3 x 41= 29.3+ = 31.0.700The non-appearance of the (v' > 0) &bands in emission despite no apparent broaden-ing of the absorption lines has led Herzberg et aZ.59 6 to postulate a weak predis-sociation in these levels.The above evidence shows that only 3 % of the mole-cules excited to the C2II(v = 0) state radiate ; this level is also weakly predissociated.DISCUSSIONMECHANISM OF PREDISSOCIATIONYoung and Sharpless proposed that the a4ll state provides a connecting linkbetween NO C2II(u = 0) and the separated ground state atoms; their reasoning issummarized in the introduction. Our experimental results also show that theintermediate state has A>O. If the C2II state predissociated via a C state, onlyone-half of the A components would interact, since NO C2II approximates veryclosely to Hund's case (b), and the rate of radiationless to radiative depopulationof the C2II state would have a maximum value of 0-5.It is unlikely that the flat2Z+ curve crosses the C2rZ and R2Z+ curves near the minima of these curves a106 FLUORESCENCE OF NITRIC OXIDEshown by Young and Sharpless, since this would cause an observable broadeningof the rotational structure of the 6 and &-bands. Moreover, the &-bands have beenseen in emission with u’ up to 3.The observation that only some 3 % of the molecules in the C2ll(u = 0) stateradiate, carries with it the implication that C2ll(u = 0) interacts with a con-tinuum. The continuum is probably of a state which correlates with ground-stateatoms, though there are possible alternatives which are discussed below.It seemshighly unlikely that the reduction of the intensity of the 6-fluorescence to the degreeobserved could be due to a perturbation by a state that is itself only weakly pre-dissociated. Herzberg et al., following Sutcliffe and Walsh, have suggested that theperturbation of C2l-I by the B2l-I state may account for the break-off of the &bandsin emission; the B2ll state is supposed to dissociate via the 2C+ state, where a fullinteraction is possible because the former belongs to Hund’s case (a). However,rapid transitions could occur only between a restricted number of levels and sincethe predissociation of B2l-I by the 2C+ state is very weak, this explanation mustbe rejected.If the C2ll(u = 0) state is to interact with another state with A>O and whichcorrelates with ground-state atoms, the only possibilities are the a4II and the 6Hstates.The latter is continuously repulsive,8 and since the a4l-I state is reasonablywell defined, we conclude that the simplest and most reasonable explanation for theweak fluorescence of the (u’ = 0) 6-bands is to postulate a predissociation of theC2H(u = 0) state by the continuum of the a4l-I state. The rate of the predissociationis obtained by multiplying the spontaneous emission rate of the 6-bands by the ratioof the rate of predissociation to the rate of radiation.Hence, forNO C~II(U = 0) -[NO a4n] +N(4S) + O(3P)the first-order rate constant = 30 x 2.2 x 107 sec-1 = 6.6 x 108 sec-1.A detailed account of the quenching of the &-bands will be described elsewhere.It is important, however, to discuss briefly the fate of molecules in the D2Z+ state,because solution of this problem provides information concerning the characterof the state which predissociates NO C2ll(u = 0).In the past, the appearance ofthe (u’ = 1) &-bands in emission whilst the (u’ = 1) &bands have not been seen,has been particularly puzzling. Apparently the &-bands are quenched by added gasesbecause of collision-induced predissociation. Whereas NO CzII(v = 0) dissociatesat low pressures and can be deactivated by collision, NO D2C+(u = 0) radiates atlow pressures and is dissociated by collision. The &-bands are strongly quenchedby helium, argon and nitrogen, without population of radiative states lower in energythan D2Z+(u = 0).It is possible to develop a partial explanation for these ob-servations by means of the information given on fig. 1, an explanation which isconsistent with the assignment of an intermediate state with A>O. If two inter-acting states have AA = 1, a perturbation is said to be heterogeneous and thedegree of interaction increases with J, the rotational quantum number. Thus, ifa predissociation is both spin-forbidden and heterogeneous, it should have a lowerprobability in low rotational states than a predissociation that is homogeneous andspin-forbidden. Therefore the postulation of NO a4ll as the intermediate for dis-sociation of the C2II state is consistent with a comparatively weak interaction ofthe intermediate with the D2C+ state.Unfortunately the lack of informationabout this type of process makes it impossible to assess accurately the importanceof the change in A. The effect of the collision may be to increase the probabilityof the DzX+ - a4II radiationless transition. Alternatively collisions could inducA . B . CALLEAR A N D I . W. M. SMITH 107the transition D2E+-b4Z-, by breaking the AK = 0 rule; the quenching of theiodine fluorescence provides a similar example. The b4E- state would have to bestrongly predissociated by a4II in order to present a continuum to the D2X+ state.Thus, although the mechanism is not uniquely determined by the potential curvesof fig. 1, the difference in behaviour of the C2IT and D2Z+ states, at low pressure,is interpreted.Fig.8 illustrates the overlap of the continuum of the a4n state with C2II(u = 0)and CXI(u = 1). The position of the a4n state was calculated from the data ofVanderslice et al.8 and accounts without adjustment for the predissociation of the&bands. The overlap with C2ll(u = 0) is small, as it must be, compared to theoverlap with CXI(u = 1). The interaction of the a4n continuum with the BXIFIG. &-Overlap of vibrational eigenfunctions for the C217(u = 0 and l)-a4n transition. Theeigenfunction for the a417 continuum is approximate.(u = 7) state may also account for the break off of the /I-bands in emission. Theflat Z+ state may interact with the B2II state at u = 7, as suggested by Herzberget al.(though the lines are not broadened) and thus there may be two modes ofpreassociation of normal atoms into the B2II(u = 7) state. The apparent absenceof emission from B~II(v = 7) in Young and Sharpless’ work may be due to the smallemission rate of the P-bands compared with the &bands, coupled with the fact thatthe strongest (v’ = 7) P-bands either overlap with the much stronger &bands orare at wavelengths greater than 6000 A.Gaydon 19 favours another mechanism for predissociation of C2II(u = 0) andD2Z+, viz., that the C2II and D2X+ curves cross a 4A state, thus forbidding predis-sociation of the D2C+ state. The 4A state would dissociate via the a4IT state.Gaydon suggests that the change of A in the 2Z+--4II transition is insufficientrestriction to explain the difference in behaviour of the D2X+ and C2II states.A4A state cannot correlate with ground-state atoms and therefore it would have tobe strongly predissociated by the a4n state, in order to present a continuum to theC2ll(u = 0) state108 FLUORESCENCE OF NITRIC OXIDEDISSOCIATION ENERGY OF NITRIC OXIDEBy considering the percentage of NO C2II(u = 0) molecules which radiate,and the energy of the rotational levels in this state, an upper limit can be placedon the dissociation energy of nitric oxide. Certainly less than 10 % of these mole-cules radiate and this would correspond to the fraction of molecules in levels belowJ = 2+ in the higher spin-component. This level corresponds to an energy of6.495 eV.In fact, the ineffectiveness of both argon and helium in quenching NOC2II(u= 0) shows that predissociation is not limited by molecules in the lowerrotational levels possessing insufficient energy to dissociate. If this were so, addedgases would cause rotational transitions fast enough to be very effective quenchingagents. For this reason and because the &bands do not exhibit irregular intensityin emission, it is concluded that the dissociation energy lies below the energy of allthe rotational levels of CzII(u = 0). The lowest rotational level of C2II(v = 0)corresponds to an energy of 6.493 eV.The most accurate value available for D(N0) is that calculated from the spectro-scopically determined heats of dissociation of N2 and 0 2 , and the heat of formationof nitric oxide.20 This last quantity has not been directly determined for manyyears and the most accurate value is that determined indirectly from the heat of thereactionwhich leads to AHiNO(g) = 21.45, 0.07 kcal/mole,NOb) + CO(d = 3N2(g) + C02(g),D(0,) = 5.114, +Om002 eV21 and D(N,) = 9.758 fr0.0025 eV.D(N0) = *D(N,)+*D(O,)-AH; NO@)= 6.507 fr 0.006 eV.In the light of the results described in this section, it is concluded that this value isslightly high.Our own observations show that D(N0) is very close to 6.493 eV.QUENCHING OF THE NO &BANDSThe quenching half-pressures, obtained from the Stern-Volmer plots shown infig. 5, are given in table 6. It has been described how strong self-quenching ofthe y-bands causes the observed quenching half-pressure of an added gas to varyconsiderably with nitric oxide pressure.13 For the &bands, however, the nitrogenquenching half-pressure is independent of NO pressure from 2mm to 12mm.TABLE 6.-QUENCHING HALF-PRESSURES (m) FOR THE &BANDSN2 (+ - 700 mm argon)N2 (+ - 300 mm argon)NO > 15Ar >38041 mm (h8)30 mm (f5)Even allowing for the possibility that experimental scatter masks an actual variationof 16 mm in the nitrogen quenching half-pressure, the nitric oxide self-quenchinghalf-pressure must be at least 15 mm.Similarly, the insensitivity of the nitrogenquenching half-pressure to the argon pressure shows that the argon quenching half-pressure is at least 380 mm.The determination of the collisional efficiency of quenching depends on the life-time of the C2IT(u = 0) state.The mean radiative lifetime of this state is 4.6 x 10-8sec, ignoring radiation to AG+, but because of predissociation the mean lifetimeA. B. CALLEAR AND I . W. M. SMITH 109even in the absence of quenching, will be considerably shorter. The mean lifetimewith respect to radiation and predissociation can be estimated by multiplying theradiative lifetime by the fraction of molecules excited to this state which radiate.This results in a value of 1.5 x 10-9 sec. Assuming the collision diameter of NO C2IIto be equal to that of NOA2Z+, this lifetime was used to calculate the quenchingefficiencies per collision which are shown in table 7. The probability of NO C2II(v = 0) being quenched per collision with N2 is approximately unity.The shorterlifetime of NO C2II(u = 0) compared to NO A2Z+ is one cause of the differencein the extent of self-quenching for the two states.TABLE 7.4UENCHlNG PROBABILITIES PER COLLISION FOR THE &BANDSN Nr P* (-) Pnitric oxide 7 . 1 4 ~ 109 10.7 > 15 < 4.7nitrogen 7 . 3 6 ~ 109 11.1 22 3.1argon 6-55 x 109 9.8 > 380 < 0.21 atm pressure/sec.1 atm pressure, per mean lifetime at zero pressure ;argon pressure ;N, the number of collisions made by NO C2II(u = 0) with the molecules of a gas atN,, the number of collisions made by NO C2rI(u = 0) with the molecules of a gas atp+, the quenching half-pressure (mm); the N2 half-pressure has been feduced to zeroP, the probability of quenching NO C2II(u = 0) per collision = 760/iVrp+MECHANISM OF QUENCHING OF NO c2n(V = 0) BY N2The relative vibrational yields in A2C+ were shown to be independent of thepressure of nitrogen, thus demonstrating that deactivation occurs exclusively fromNO CzII(u = 0) and not from NO C2II(v = 1). Because of its shorter lifetime,we should expect the latter state to have a larger N2 quenching half-pressure thanthe former.Thus the independence of the ratio of the intensities in the two y-progressions, shows that NO C2II(u = 1) is too short-lived to be deactivated underthe conditions described.The most obvious quenching reaction isNO C211(v = O)+N, XIZZ-+NO A2Zf(v = 0 or 1)+N, XIZ,f.The experimentally determined yields in the various vibrational levels of NO AG+are slightly different from the calculated Franck-Condon factors given in table 1.It might therefore be concluded that the N-0 bond distance in the transition com-plex is little different from its value in NO A2Z+ or NO C2n; there is very littlechange in internuclear distance during the quenching process.It is possible, however, that quenching of NO C2II by N2 is not a single step,but consists of two energy transfer processes,followed byNO C211(v = O)+N2 XIZ,f+NO X211+N2 A3Cy+,N, A 3 C i + N 0 X211(v = O)+NO A2C,'(u = 0 or 1)+N2 X'C,,f.Young and Sharpless observed that addition of N2, in their experiments, gave riseto the two progressions of the y-bands and in addition the (v' = 0) P-progressionwas excited.Setser and Thrush 22 have recently described chemiluminescenceresulting from addition of NO to a mixture of atomic oxygen and cyanogen.Agai110 FLUORESCENCE OF NITRIC OXIDEthe two progressions of the y-bands were observed, together with a weak (u' = 0)p-emission. It is possible that NOAzX+ is excited by the same mechanism in allthree types of experiment, and the only possible process would be energy transferfrom Nz A-7Z;.However, we have searched in vain for the (u' = 0) P-progression in the fluor-escence experiments. Simultaneous population of A2C+(u = 0) and B2II(u = 0) bycollision of N2 A3X: with NO P I T , would provide a very unusual type of process,considering the large difference between the internuclear distances of B2ll and A2C+.Until a full interpretation of these observations has been developed, it is not possibleto draw any conclusions from the fluorescence expzriments.Quenching of C2lJ(u = 0) will be studied in the presence of other added gases, and this may providenew evidence about the mechanism. If the second mechanism is correct, it maybe possible to discover a substance which will intercept and deslctivate N2A3Clfbefore it collides with NO X2II. Our rate constants are neither dependent on theintermediate state of predissociation nor on the mechanism of quenching, providedthat one NO A2X+ is produced for each NO C2II(u = 0) quenched.CONCLUSIONSThe fate of NO C2II(u = 0) can be described by the schemekz kc(Nz)N(4S)+ O ( 3 P ) ~ [ N 0 a411];.N0 C211(v = 0)-[N, A3Z,f]-+N0 A2Z+NO X211 NO A%+kt / k F \GThe ratio of kl to k2 is the equilibrium constant which is 1.26 x 1024 molecule cm-3,if AE," is taken to be zero (this introduces a possible error of +lo %).Thus k2is determined.At 293°K:k, = 9.7 x 10-10 cm3 molecule-1 sec-1;kl = 6.6 x 108 sec-1;kF = 2.2 x 107 sec-1;kZ = 5.2 x 10-16 cm3 molecule-1 sec-1.kk is not yet known.Young and Sharpless measured the rate of fluorescence of NO C2II(u = 0) toNO PIX, as a function of the N and 0 atom concentrations. Their second-orderrate constant is 1.1 x 10-17 cn13 molecule-1 sec-1. This is equivalent to our kFkz/(kl + kF> = 1.7 x 10-17 cm3 molecule-1 sec-1. There is good qualitative and quanti-tative agreement in the interpretation of quite different experiments. However,kl%kp and consequently this overall rate is very insensitive to kl, but is largelydetermined by the equilibrium constant and kF, the rate at which the &bands areemitted.The photolysis of nitric oxide and iodine provide the simplest known examplesof photochemical decomposition of diatomic molecules resulting from absorptionof light by bands that are not broadened by predissociation. An understandingof these systems may facilitate similar photochemical studies on triatomic or evenmore complex species.The authors would like to acknowledge a brief but valuable discussion withProf. Longuet-Higgins, on the effect of changes in A on the rate of predissociation.They would like to express their gratitude to Imperial Chemical Industries for theloan of a spectrograph. I. W. M. S . is indebted to D.S.I.R. for a studentship, duringthe tenure of which part of this research was carried outA. B. CALLEAR AND I. W. M. SMITH 1111 Jenkins, Barton and Mulliken, Physic. Rev., 1927, 30, 150.2 Gaydon, Proc. Physic. SOC., 1944, 56,95, 160.3 Sutcliffe and Walsh, Proc. Physic. Soc. A, 1953, 66, 209.4 Brook and Kaplan, Physic. Rev., 1954, 96, 1540.5 Henberg, Lagerqvist and Miescher, Can. J. Physics, 1956, 39, 622.6 Lagerqvist and Miescher, Helu. Phys. Acta, 1958, 31, 221.7 Barrow and Miescher, Proc. Physic. SOC. A, 1957,70,219.8 Vanderslice, Mason and Maisch, J. Chem. Physics, 1959, 31, 738.9 Ogawa, Science of Light (Tokyo), 1954,3, 39.10 Flory and Johnston, J. Amer. Chem. SOC., 1935,57,2641.11 Flory and Johnston, J. Chem. Physics, 1946, 14, 212.12 Young and Sharpless, Disc. Faraday SOC., 1962,33,228.13 Callear and Smith, Trans. Farday SOC., 1963,59, 1720.14 Callear and Smith, Trans. Farday SOC., 1963, 59, 1735.15 Bethke, J. Chem. Physics, 1959, 31, 662.16Bates, Mon. Notes Roy. Astron. SOC., 1952, 112, 614.17 Herzberg, Molecular Spectra and Molecular Structure: I, Spectra of Diatomic Moleciiles18 Callear and Smith, to be published.19 Gaydon, private communication.20 Evans, private communication, quoting results from a Ph.D. Dissert. of Frisch.21 Brix and Henberg, J. Chem. Physics, 1954, 21, 2240 ; Can. J. Physics, 1954, 32, 110.22 Setser and Thrush, Nature, 1963,200,864.@. Van Nostrand Co., New York, 1950)

ISSN:0366-9033    

DOI:10.1039/DF9643700096

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

Recombination of Hydrogen Atoms in the Presence ofAtmospheric GasesBY F. S . LARKIN AND B. A. THRUSHDept. of Physical Chemistry, University of CambridgeReceived 23rd December, 1963A calorimetric technique has been used to determine the rate constants of the reactions :H+H+M = H2+M,H+02+M = H02+M,in discharge flow experiments. The mechanism of the decay of hydrogen and of oxygen atoms underconditions when [HI -g [0]< [02], [MI is discussed.The dominant spectrum of the night sky is the Meinel band emission 1 of OH,which is associated with the formation of vibrationally excited OH in the reaction,;!although some OH emission from lower vibrational levels may be associated withthe formation of vibrationally excited OH in other less exothermic reactions.This emission is believed 3 to originate mainly from altitudes around 80 km.In the centre of this region, the hydrogen atom concentration is believed4 to beabout 108 molecules cm-3 as compared with an oxygen atom concentration 5 of ca.5 x 1011 particles cm-3 with 0 2 = 1014 molecules cm-3 and N2 = 5 x 1014 mole-cules cm-3.Under these conditions, reactions (1) and (2) provide a catalytic pathO+OH = HS02, (2)for the decomposition of ozone which has the same stoichiometry as the reaction,0 + 0 3 = 0 2 4 - 0 2 , (3)with which it can compete 6 since kl N 1000 k3.also cause the removal of oxygen by reactions (3) and (4) :H-l-03 = OH+02, (1)Small amounts of hydrogen atomsO+02+M = O3+M. (4)The mechanism of this process was first suggested by Kaufman' to explainthe acceleration of oxygen-atom decay in discharge flow experiments when tracesof water were allowed to pass through the discharge :H+Oz+M = HOz+MO+HO2 = OH+02O+OH = H+02.Reaction (5) is the rate-determining step in the above scheme and this com-munication reports a new determination of the rate constant of this reaction andof the reactionH+ H+ M = H2 + M.(7)The kinetics of the decay of hydrogen and oxygen atoms are discussed for condi-tions where [HI 4 [O] < [02],[M], which occur in discharge-flow experimentsand in the region of the upper atmosphere from which the Meinel bands are emitted.11F. S. LARKIN AND B . A. THRUSH 113EXPERIMENTALHydrogen atoms were generated by passing pure hydrogen or an argon or heliumcarrier containing less than 10 % hydrogen through a 17 mc/sec 100 W electrodelessdischarge. Forced air cooling was used for the discharge, which was establishedin 8 mm int.diam. quartz tubing by means of external aluminium foil electrodes.The discharge products flowed through a light trap to a Pyrex glass reaction tube,2.5 cm int. diam. and 100 cm long, at the upstream end of which was a multipleinlet jet through which reactants could be introduced with rapid mixing. The surfaceof the flow tube was coated with Drifilm which gave a surface recombination efficiencyy not greater than 10-5.Hydrogen-atom concentrations were determined with a movable calorimetricprobe which was inserted through the downstream end of the flow line. A Tygonsleeve provided a sliding vacuum seal where the glass tube carrying the calorimeterleads and head entered the apparatus.Two calorimeters were used, each consistingof 50 cm of 34 s.w.g. 13 % rhodium/platinum wire wound in a spiral. These wereconnected to separate Wheatstone bridges and mounted in tandem about 3 cmapart so that the downstream calorimeter could determine the efficiency of recom-bination on the first spiral, which was always greater than 80 %. Experiments wereconducted at pressures between 1 mm and 6 mm Hg and with flow velocities of400-1200 cmlsec. Total pressures at each end of the flow tube were measured withsilicone oil manometers. Hydrogen-atom concentrations of up to 5 % were ob-tained. Allowance was made for errors due to diffusion effects, 8 , 9 but these weresmall in all cases.Cylinder argon, " mineral " helium and " high purity " hydrogen were obtainedfrom the British Oxygen Company.Hydrogen was purified at atmospheric pressureby passing it through a Deoxo unit and a packed trap at - 196°C. Argon and oxygenwere dried at atmospheric pressure by passing through at packed trap at -80°C.RESULTSH+H+MIn the absence of added oxygen the recombination of hydrogen atoms is givenby the equation-d[HJjdt = zk,,~[H]~[M]+k8[H].Values of k7 were determined by plotting d In [H]/dt against [HI, or by plotting1/[H] against time. The former procedure was used at lower pressures where thesurface recombination term kg made an appreciable contribution to the total rate.It was also necessary to use this procedure in experiments with helium where thepresence of an oxygen impurity ( N 0-5 %) made an appreciable contribution to therecombination which was first order in [HI.At higher pressures identical valuesof k7 were obtained from the two methods. These are given in table 1 for a tem-perature of 293°K. The value for M t H2 agrees well with earlier data on therecombination reaction,lop 11 although no acceleration due to a high value 11 of k7for M = H could be detected at the highest hydrogen-atom concentrations used ( 5 %) .MTABLE 1MH2 0.68 f0.1 2.0 f0-3Ar 0-45 f0.08 1-3 f0.210-16 k7 (cm6 mole-2 sec-1) 1032 k, (cm6 molecules-2 sec-1114 RECOMBINATION OF HYDROGEN ATOMSH+02+MIn these experiments about 1 % of molecular oxygen was added to the productsof a discharge through hydrogen in an argon carrier.The hydrogen-atom con-centrations used were below 0.7 % to reduce the contribution from reaction (7).The molecular hydrogen concentration was low enough that OH radicals formedin the reaction H + HO2 gave negligible hydrogen-atom regeneration by the reaction 12OH + €32 = H20 + H. (9)3.0-2 . 0 -1.0- slope = 2.1 x 1016 c m 6 mole-2 sec-1I0 2-0 3.0107 [MI, mole cm-3 assuming kH, = 5 k ~ ,FIG. 1.0, determination by calorimetry ; 0, determination by HNO emission.and thatwhereClyne and Thrush 13 have shown that under these conditions, the reaction pro-ceeds by the mechanismH+02+M = HOz+M ( 5 )H+H02 = H2+02 (104= OH+OH (lob)= H20+O (10c)OH+OH = H20+O (1 1)O+OH = H+02 (2F.S . LARKIN AND B. A. THRUSH 115Excellent first-order decays of hydrogen atoms were observed in experimentswith added oxygen. Fig. 1 is a plot of - against pressure of argon plus1 0 2 1 dtfive times the pressure of hydrogen. This is chosen since data on the hydrogen + oxygensecond explosion limit 143 15 have shown that the ratio k5 (MrH2)lks (M EZ Ar) is 5.Our experiments which necessarily cover a very limited range of hydrogen con-centrations indicate a value of 6+2 for this quantity at room temperature.The data of Clyne and Thrush 13 who used HNO emission to follow the reactionis also shown in fig. 1. There is good agreement between the two sets of data.The calorimetric points lie on a straight line passing through the origin, showingthat surface reactions between H and 0 2 are negligible in this system.1 dlog[H]Using the value of x given above, the line drawn in fig.1 corresponds tok5 = 1.35 x 1016 cm6 mole-2 sec-1 at 293°K (M = Ar)= 3.7 x 10-32 cm6 molecule-2 sec-1 at 293°K (M 3 Ar).DISCUSSIONThe rate constants reported here have been mainly determined for argon orhydrogen as third bodies. Data on the H+02 reaction at the second explosionlimit 149 15 and on bromine and iodine atom recombination 169 17 give rate con-stants for M s N2 or 0 2 close to twice those for argon as third body. In the follow-ing discussion which applies to the upper atmosphere and to discharge flow experi-ments where the predominant third bodies are oxygen and nitrogen, values of k5and k7 twice those for M = Ar will be used.In the upper atmosphere where [H]<[0]<[02], "21 the half-life of reaction(7) will be very much longer than that of reaction (5).For the following concentra-tions in particles cm-3 corresponding to an altitude of about 80 km, [HI = 108,[O] = 5 x 1011, [02] = 1014, "21 = 5 x 1014, the half-lives would be 105 h and 5 minrespectively.The effective rate of removal of hydrogen atoms by reaction (5) is much lowerthan this since hydrogen atoms are regenerated in subsequent reactions :O+HO2 = OH+02 (6)H+ HO2 = H2 + 0 2 ( 104= OH+OH (lob)= H20+O (1 0 4OH+OH = H20+O (11)O+OH = H+02OH+H = O+Hz.The rate constants of reactions (6) and (10) are not known, but there is evidence 13particularly from Foner and Hudson's mass-spectrometric work 18 that both re-actions are very rapid.At 300"K, k2 = 4 x 10-11 cm3 molecule-1 sec-1,129 19kll = 5 x 10-12 cm3 molecule-1 sec-1,12 and a value of k12 = 4 x 10-17 cm3 molecule-1sec-1 can be calculated data on the reverse reaction 19 knowing the equilibriumconstant.12.20 It can be seen thatReactions (1 1) and (12) make a negligible contribution to the removal of OH radical116 RECOMBINATION OF HYDROGEN ATOMSand hence of hydrogen atoms. The dominance of reaction (2) determines thathydrogen atoms are removed only by reactions (1Oa) and (10c).Evidence 21 indicates that the rate constant of the reactionO+H+M = OH+M (1 3)is less than 5 x 10-32 cm6 molecule-2 sec-1. Since [O]< [02] its contribution to theoverall kinetics can be neglected.A steady-state analysis of the hydrogen and oxygen atom decays in a systemwhere [H]<[O]<[02], [MI in which the initial steps are reactions (4) and (9, inwhich ozone is removed by reactions (3) and (1) + (2), and where HO2 is removedby reactions (6) and (10) followed by (2), yields(9 -- dL-Hl - - 2ak 5 k 1 0 [HI c 0 2 1 CM3dt k6Wlandwhere- dL-Ol/dt = 2(k,COI 4- ~,[HI)CO2IL-MI (ii)a = kloa+klocklOll+ klOb+ kloc'In deriving these equations, it has been assumed that k~[0]9k~o[H].This isjustified providing [0]9[H], since k6 and klo will be shown to have similar mag-nitudes. Eqn. (i) and (ii) yield a first-order differential equation in [H]/[O]. Ifthis equation is integrated and then substituted in eqn.(ii),--= din [O] 2[02][M]{k4+E[exp (-at)-dtwhere a = 2k4[02][M].The initial rate of oxygen atom decay corresponds to an apparent value of(k4+k5[H]0/[0]0) for the rate constant of reaction (4). The term within the squarebrackets can be expanded to give a factorin the denominator of the term in k5[H]o/[O]o. The effect of hydrogen atoms inaccelerating the oxygen-atom decay will change with time unless the second termin expression (iv) remains much less than unity.The most reliable value 22 of k4 appears to be 2.7 x 1014 cm6 mole-2 sec-1for M = 0 2 at 293°K. Thus k 5 ~ 100 k4 and enough hydrogen atoms to causesignificant acceleration will be present in discharge flow experiments unless stringentprecautions are taken to dry the oxygen used.No author has reported the de-parture from first-order kinetics at longer times predicted by eqn. (iii) in experi-ments in which moisture was accidentally or intentionally present in the oxygenused. Kretschmer and Petersen23 report that addition of water to the oxygensupply increases - d In [O]/dt but that this quantity stays constant over the decay.For this reason they suggest that reaction (6) is rate-determining and deduce avalue of k6 = 5 x 109 cm3 mole-1 sec-1. There is good evidence 18 that reaction (6)is much more rapid than this but their rate-constant is consistent with reaction (5)being rate determining. Dickens, Gould, Linnett and Richmond 8 obtained thF. S. LARKIN AND B . A. THRUSH 117high value of k4 = 6 x 1014 cm6 mole-2 sec-1 using oxygen which contained somewater vapour.Their published first-order decay plots are linear for up to threehalf-lives, although it is difficult to detect curvature in such cases. We concludethat, for oxygen atom decays in which hydrogen atom catalysis predominates, ifthen0*5>2[OJ[M] [ k4+ ( 1-- a::") ks- ; 3 1 > - 1 ;.. 1.5 > aklo/k, > 0.75 iA kinetic study of the H + 0 2 reaction 13 has shown thatkloa/(klOb+ kloc) = 0.5 kO.2 and 2k10b) klo,.From these data,0.78 & 0.03 > a > 0.33 & 0.08,and hence that the ratio k1&6 lies between unity and six. The assumption in eqn.(i) and (ii) that the term klo[H] could be neglected by comparison with k6[O] for[O]>[H] is therefore justified.The ratios of the rate constants for the reactions ofhydrogen and oxygen atoms with nitrogen dioxide is N 10, whereas the correspondingratio for ozone k7/k3 N 1000 at normal temperatures.Substitution of relationship (v) in eqn. (i) shows that the presence of excessoxygen atoms reduces the rate of removal of hydrogen atoms in reaction (5) to givea rate approximately 2[H]/[0] times that of the initial reaction. For the conditionsquoted at an altitude of 80 km, this corresponds to a decrease in the rate of hydrogenatom removal by a factor of a thousand. We therefore conclude that the rate ofchemical removal of small concentrations of hydrogen atoms is very low indeed inthe upper atmosphere, and that diffusive escape 4 is the dominant removal process.The authors thank the D.S.I.R.for a special research grant and for a main-tenance award to F. s. L.1 Meinel, Astrophys. J., 1953, 111, 207.2 McKinley, Garvin and Boudart, J. Chem. Physics, 1955,23,784.3Packer, Ann. Geophys., 1961, 17, 67.5 Nicolet, J. Geophys. Res., 1959, 64, 2092.6 Clyne, Thrush and Wayne, Nature, 1963,199, 1057.7 Kaufman, Proc. Roy. SOC. A, 1958,247, 123.8 Dickens, Gould, Linnett and Richmond, Nature, 1960, 187, 686.9 Wise and Ablow, J. Chem. Physics, 1961, 35, 10.10 Amdur, J. Amer. Chem. SOC., 1938, 60,2347.11 Steiner, Trans. Faraday SOC., 1935,31, 623.12 Kaufman and Del Greco, 9th Symp. Combustion (Academic Press, 1963), p. 659.13 Clyne and Thrush, Proc. Roy. SOC. A., 1963,275, 559.14 Lewis and von Elbe, J. Chem. Physics, 1942, 10, 366.15 Hinshelwood and Willbourn, Proc. Roy. SOC. A, 1946, 185, 353.16 Rabinowitch, Trans. Faraday SOC., 1937, 33, 283.17 Russell and Simons, Proc. Roy. SOC. A , 1953, 217, 271.18 Foner and Hudson, J. Chem. Physics, 1962,36,2676,2681.19 Clyne and Thrush, Proc. Roy. SOC. A, 1963,275, 544.20 JANAF Thermodynamical Tables (Dow Chemical Co., 1960).21 Harteck and Reeves, Chemical Reactions in the Lower and Upper Atmosphere (Interscience,22 Kaufman and Kelso, this Discussion.23 Kretschmer and Petersen, J. Chem. Physics, 1960, 33, 948.4Nicolet, Bull. SOC. Chem. Be&., 1962, 71, 665.1961), p. 219

ISSN:0366-9033    

DOI:10.1039/DF9643700112

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

Optical Radar Study of the Upper AtmosphereBY ROBERT A. YOUNGStanford Research InstituteReceived 20th January, 1964Calculations indicate that the upper atmosphere can be probed by OpticalRadar to reveal the altitude (range) and density of metastable states in the atmosphere.Such a device may be of considerable significance for geophysical exploration oraurora, airglow, solar flare, and meteor trail phenomena.Basically, Optical Radar consists of (a) an emitter of extremely intense, highlyspecific, collimated, pulsed optical radiation, and (b) a receiver which measures theamplitude and delay of returning “ optical echoes ”. For Optical Radar the mostsuitable emitter is a pulsed gas laser and the most suitable receiver is a photo-multiplier.To be effective in measuring excited states, the wavelength of the radiation pulsemust be matched to the “target”.Hence, for each component of the upperatmosphere which one wishes to investigate, a laser must be developed in whichstimulated emission occurs in this component and to the pertinent state. This speci-ficity is advantageous, although it restricts the number of species which can now be de-tected. Unless extremely narrow wavelength filters are employed, Optical Radarwill be effective only at night.Only minor absorptive components of the atmosphere are possible targets,since otherwise the projected radiation is rapidly attenuated. However, these in-clude excited states of major chemical constituents which are of particular importancefor tracing and identifying energy sources in the upper atmosphere.It is now possible to construct and test an optical radar system based on therecent development of high-power (100 W or more), pulsed molecular nitrogengas lasers.* Metastable nitrogen in the A3Xf state (lifetime E 2 sec) could nowbe detected in concentrations of less than 105/cm3 in a uniform 100m region.Energy released in the atmosphere by impinging particles will produce significantconcentrations of this metastable molecule, and it is probable that measurabledensities exist in auroral features.The prospects are good for producing a pulsed molecular laser emitting theN i 1st negative bands, and the N2 2nd positive bands, which would provide an in-dependent means of obtaining the N t and N2 (B) density and their spatial distribu-tion.By seeding a pulsed molecular laser with Na, it may be possible to produce asodium laser (emitting the yellow doublet) because of the strong coupling of theresonance excited states of Na with excited vibrational levels of N2. A sodiumlaser would allow the vertical density distribution to Na to be followed throughoutthe night.For molecular resonance scattering, additional lines, other than that emitted bythe laser, probably occur in fluorescence and are a ready means of distinguishingresonance scattering from Rayleigh scattering. For atomic species this may notoccur.* Mathias and Parker, Appl. Physics Lett., 1963,3, 16.11R. A. YOUNG 119PRELIMINARY FEASIBILITY CALCULATIONSThe following elementary calculations will show that, even without improvingcurrent lasers and detectors, Optical Radar is feasible.For clarity, the N2(A3E+)state will be used as the scattering target although similar results would be obtainedwith other species such as N;(XZCi). If the concentration of metastable N*(A3Z+)molecules is [NZ], the time-integrated intensity of the radiation scattered to a detectoron the earth's surface from a beam projected vertically into the atmosphere isAf,a[N:]dhdt ergsh2 cm2'where h (cm) is the altitude of the scattering layer of thickness Ah; a (cm2) is thecross-section for resonance scattering ; I0 is the intensity of the laser pulse (photons/cm2) of length z (sec) ; and A is the laser beam area (cm2). For simplicity in thisrough calculation it is assumed that less than 10 % of the laser pulse is resonantlyscattered in passing through the entire atmosphere.This is so except below 3000 Awhere ozone absorption at the 30-km level is intense and in specific infra-red ab-sorption bands (of H20, C02, etc.). Rayleigh scattering will be negligible exceptperhaps low in atmosphere (where it is easily identified and compensated for). IfA10 is x100 W (109 ergs/sec) and z is ~ 1 0 - 6 sec, then AIoz is =lo3 ergs, i.e., ap-proximately 1014 photons. If a is = 10-15 cm2 (Le., the specific rotational-vibrationaltransition probability can be characterized by an f-number of = 10-4-a pessimisticassumption), thenI,(Ah) x ( [N;]Ah/h2)10- ,provided Ah/h is small and less than 10 % of the laser radiation is absorbed.within a resolution interval of Ah, thenIf S = ";]Ah is the total number of scattering centres along the light pathI,(Ah)x (S/h2)€O- (3)(if the assumed parameters are used) and if h x 100 km (107 cm) then Is( Ah) x S 10-15(photons/cm). For a photomultiplier with a photocathode area of = 100 cm2,of 1 % efficiency, and of gain G, the accumulated charge q per pulse is4 = s x 10-15~e (4)where e is the charge of an electron.For m repetitive and integrated pulses thetotal accumulated charge is mq. For detectors closer than 100 km to the scatteringspecies the scattered intensity increases as h-2 with a corresponding increase inmetastable detectability limits.Considering that the cathode dark current is negligible compared to cathodecurrents I b produced by unavoidable sky background, the signal-to-noise ratio R(for the detector just described) will bewhere only the photoelectron shot noise has been considered and z1 represents thedetection interval; i.e., z1 = 2Ah/c.(Shot noise accounts for e 9 0 % of the noisein pulse counting systems.) Hence, since Ibbzl> 10-15 S on the pessimistic assumptionthat zbx 109 photon/cm2 within the narrow pass band of detector filterR x 10- "[N~](~Ahrn/21,,)~120 OPTICAL RADAR STUDYwith the subsidiary condition that SIIlO14 so that the scattering region is opticallythin.For Rw 10, rn w 107, I b M lOg/cm2 and Ah = 104 cm, eqn. (6) gives Sw lOl4/cm2and [N:]w lOlo/cm3. If the detector area is increased by x 100 through utilizinga collecting mirror 20 in.diam., I b becomes w 100 times larger, but so does thescattered intensity.It can be seen that when the background radiation against which scattered lightmust be detected predominates, the optimum results are achieved when z (the emis-sion pulse length) is equal to 71 (the scattered pulse length) which determines therange resolution of the optical radar system. Furthermore, the repetition rate zris related to the target range, i.e., zr = 2(h+ Ah)/c if a continuous range againstscattered amplitude is to be derived from each pulse. For most geophysical events, thisis = 1 msec so a repetition rate of w 103/sec should be used.* A more rapid repetitionrate is possible if the range against scattering intensity curve is obtained range pointby range point through synchronous detection.When the scattering centres arelocalized in a region small compared to h, some increase in repetitive rate withoutsignificant information loss may be possible. In any case, from 103 to 104 secwill be required to detect scattering species having number densities from 109/cm3to lOlO/cm3 with a spatial resolution of w 100 m using a 100 W pulse laser.For existing lasers (having 100 W peak power and pulses of w 1 p duration)the sensitivity cannot be increased by sacrificing spatial resolution since *lo %of the emitted radiation is scattered from a region of thickness =Ah. However,as the pulse power increased, e.g., to l o 4 W/pulse, Ah can be increased in propor-tion, resulting in a decrease in [Nz] (for the same signal-to-noise ratio) which isproportional to the square root of the old to new Ah.For the chosen examplethis would decrease “,*I to 107/cm3 in a 10 km thick layer. Alternatively, increasedpulsed laser power may be used to shorten the number of integrated pulses requiredfor a given signal-to-noise ratio. Thus, with 104 W/pulse, Ah= 10 km andlO8/cm3, rn could be reduced from 107 to 105, i.e., the observing time could bedecreased to 10- 102 sec.It is evident that the greatest improvement in the sensitivity of the optical radarsystem can be achieved by increasing the power emitted by the pulsed laser. Sincemost components of the upper atmosphere probably vary with altitude as the scaleheight, which is w 5-10 km at 100 km, an increased emission pulse length combined witha higher powered laser pulse would not be particularly detrimental.This increase inpower above w 100 W/pulse probably will arise from either lengthening the excitationpulse or bunching them together in fast repetitive bursts. Unfortunately, theeffectiveness of these measures depends upon preventing the build-up of the lowermolecular state involved in the laser population inversion. It is, of course, possibleto simultaneously excite several laser tubes.The assumed background radiation of 109 photons/column may well be pessi-mistic. It cannot be reduced below the emission occurring in the single vibration-rotation line of the Nz. 1st positive band used as the optical radar probe. However,the transitions in which laser action occurs are not those which occur strongly inupper atmosphere emission. Hence it is possible that I b z 106 photons/columnwhich decreases the background radiation below the returned scattered pulse so thatThen R = 10 for rn = 107, Ah = lO4cm when [N;] = lO6/cm3 using 100 W anda 2-in. diam. collector. With iO4 W or a 20-in. collector [NZ] = 105/cm3, or both* Such repetition rates have been achieved by a commercial concern.For the parameters just mentioned [N;] becomes = 109/cm3.R = 3 x lO-*(Srn)* = 3 x 10-8([N~]Ahm)*. (8R. A . YOUNG 121together give a detectability of =104/cm3. Finally, if Ah were increased at thehigher power to 106 cm then [N;] z lO2/cm3 may be detected.The possibilities (for meteorological purposes) of non-resonance scattering ofpulsed laser radiation have already been preliminarily investigated by several groupsand the preceding calculations indicate that resonance scattering using a pulsedgas laser can be a valuable new tool in the investigation of the upper atmosphere

ISSN:0366-9033    

DOI:10.1039/DF9643700118

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

The History of Oxygenic Concentration in theEarth’s AtmosphereSouthwest Centre for Advanced Studies, P.O. Box 8478, Dallas 5, Texas, U.S.A.Receiued 30th January, 1964Geologic evidence points to the absence of primordial atmosphere upon the Earth’s formationand suggests that the present atmosphere is of secondary origin derived from sources containedwithin the Earth. Initially, the important probable constituents, H2, N2, C02 and H20 have beenderived from volcanic effluents. Quantitatively, these effluents can account for the present volumeof the oceans and the development of an atmosphere over a period of 3-4x 109 years.Since oxygen is absent from volcanic effluents, its origin and rise have been the subject of wide-spread speculation and uncertainty. The present calculations based on complete u.-v.absorptiondata show that the photodissociation of water is self-regulated by the subsequent formation of02, thus exercising a shadowing effect on H20 on account of the non-exponential distributionof water vapour. This plus the effect of any C02 places an upper limit on the oxygenic pressurein the primitive atmosphere at <lO-3 present atmospheric level (P.A.L.). It is further apparentthat sufficient u.-v. energy is available to dissociate 0 2 in the primitive atmosphere to the extentthat 0 3 will be maintained as an important constituent near the surface. From absorption datait is shown that sufficient oxygen is released by u.-v. dissociation of water vapour to account forthe crustal oxides at primitive oxygenic level.Subsequent rise of oxygenic level can only be attributed to photosynthesis in primitive cellularorganisms.Strong fluxes of lethal u.-v. in the range 2200-3OOOA severely restrict the ecologyto bottom dwelling organisms in shallow protected pools or seas at a depth of 5-10 metres and pre-vents spread of life to the oceans. Only when the spread of such organisms in this ecology is suf-ficient to produce 0 2 at a rate exceeding its photodissociation can the building of an oxygenicatmosphere begin.As oxygen rises to the level -10-2 P.A.L., 0 3 levels will rise to the point where the spread oflife in the oceans becomes possible and coincidentally, the oxygenic level reaches the “Pasteurpoint ’’ where organisms change from a mechanism of fermentation to respiration, raising theenergy available for chemosynthesis by a factor of -30-40.The attainment of this “ first critical level ” -10-2 P.A.L.-opens widespread new evolutionaryniches, still confined to the liquid hydrosphere, and suggests search for a widespread evolutionaryexplosion in the waters of the Earth.Only one such event is evident-the beginning of the Cambrianera -600 million years ago. Then evolution took place explosively and laid the foundation for allmodern phyla. By immediate inference we identify the oxygenic level, 10-2 P.A.L., with the openingof the Cambrian.Following this a rapid increase in marine photosynthesis raised the level of oxygen to -0.1P.A.L. At this “ second critical level ” lethal radiation would be shielded largely from dry landby 0 3 levels, thus opening a new ecological niche for evolution ashore.Search of geologic evidenceshows evolutionary development of several phyla ashore between mid- and late-Silurian, withgreat forests by early Devonian. By immediate inference we identify the oxygenic level, 4 - 1 P.A.L.,with the late Silurian, 420 million years ago.From this point, life would develop very rapidly on land. With rapid increase in total oxygenic-production by photosynthesis, the oxygen would build to its present level, and perhaps overswingas a consequence of the phase difference in production peaks of 0 2 and C02. Study of this balancesuggests that oxygen may have fluctuated since the Devonian era in a damped saw-tooth oscillationaround the present quasi-permanent level.The implications of this model suggest that the Cambrian was not preceded by a long period ofevolution of advanced organisms which somehow, in spite of diligent search of favourable sedi-ments for more than a century, have not been preserved in the fossil record.Likewise, this modelaccounts for the sudden evolutionary migration of several phyla of plants and animals ashore atthe late Silurian and the prior absence of evolutionary evidence on land. It suggests that thegeologic record should be read exactly as observed, without extrapolation, and raises a new problem12L. V. BERKNER AND L. C. MARSHALL 123on the limiting rate of evolutionary development as favourable, widespread evolutionary nichesare opened.The analysis of oxygen balance during the several eras also poses the problem ofthe present stability of the oxygenic level.This paper reports on a subject which might be classified under the generalheading of “ paleoatmospheres ” or of “ fossil atmospheres ”. This subject involvescritical quantitative study of the history of planetary atmospheres. Studies of thiskind are now feasible in view of the substantial data from space probes on solarradiation at all wavelengths, together with reasonably complete data on the ab-sorption of particular spectral regions by component atmospheric gases. There isalso a growing knowledge of the succession and rates of photochemical reactionsinvolved in the absorption processes.These studies are of value in understanding the composition and organizationof planetary atmospheres which are of increasing interest in view of their growingaccessability.Interpretation of the events of the physical and biological history ofa planet may be misleading, unless at the same time some reasonable estimates ofaccompanying atmospheres can be made.The analysis of the paleoatmosphere of Planet Earth is most convenient, sincean abundance of data are available. Pertinent data relate to the composition,distribution and photochemistry of the present Earth’s atmosphere ; to the be-haviour, composition, chemistry and geology of its surface and interior ; to relatedsubjects of molecular biology, biochemistry, and molecular genetics of its livingmaterials ; as well as to the paleontological and evolutionary evidence derived frommore than a century of study.Moreover, the implications apparent from a criticalstudy of the paleoatmosphere of the Earth open new and interesting lines of inquiryand suggest reasonable solutions to many of the so-called “puzzles” which, inthe formulation of the history of the Earth, have so far remained unsolved.In this paper, attention is directed only to the history of the growth of oxygenin the atmosphere of Planet Earth. This report is a prkcis outlining only theprincipal methods and conclusions of a more extended study from which a preliminarymodel has been formulated.*Out of the work of Goldschmidt,l Harrison Brown,29 3 Spencer-Jones,4 Kuiper,5Urey,69 7 Alfven,8 Fesenkov,g Vinogradov,lo and many others, the initial premiseis adopted that upon its agglomeration, the Earth was without a primordial atmo-sphere.The relative abundance of the rare gases, illustrated in table 1, after Brown,3shows that their abundance on Earth ranges from one-millionth to one ten-billionthof their cosmic abundance.TABLE ~.-FRACTLONATION FACTORS OF RARE GASESelement atomic weight fractionation factorneon 20 - 1010argon (36) 40 - 1 0 8krypton 83 - 2x 106xenon 130 - 106Likewise, the relative abundance of the lighter elements, H and He, on the Sunand the more massive planets, and the paucity of these lighter elements on the innerplanets, shows that during their agglomeration, these inner planets lost the greaterproportion of mass attributable to the usual abundance of the gaseous elements.* The more complete study will be published in an early issue of the J. Atm.Research ( h e r .Meteorological SOC.)124 OXYGENIC CONCENTRATION IN EARTH’S ATMOSPHEREThe accumulation of Ar40, from decay of K40, corresponds to the estimated age ofthe solid Earth of about 5 billion years.Thus, all lines of evidence point to the absence of any primordial atmosphere,and to subsequent growth of an atmosphere primarily from secondary sourcesself-contained in the Earth. This view is consistent with the agglomeration ofthe Earth from planetisimals whose gravitational fields were too small to retaina primordial atmosphere.According to Urey,6P 7 Vinogradov,lo and others,s~ 8 the physical chemistry ofcompounds comprising the major portion of the Earth indicate that since its agglom-eration the Earth has never been substantially molten.The Earth consequentlyretains large quantities of gases chemically bound in various ways. These are suf-ficient to provide for the source of the secondary atmosphere.The volcanic origin of this secondary atmosphere is developed by Rubey,119 12Urey,g Holland,l3 Vinogradov,lo and others. The continents have been built atan average rate of 1-3 km3 per year from volcanic effluents, based on estimates ofSapper,l4 Verhoogen,ls Bullard,l6 and Wilson.17-19. In evaluating these estimates,on one hand following Vinogradov,lo volcanic activity should have been consider-ably greater in earlier eras, precedent to long decay of radioactive elements.Onthe other hand, following Verhoogen,ls the andesitic volcanoes, particularly in thePacific andesite ring, represent to some extent merely reworked continental materials.Wilson’s 17 estimate of a rate of average continent building of 1.3 km per yearthroughout geological time, and down to present times, seems compatible withinan order of magnitude with present estimates of total volume of continental materials.Accompanying these solid effluents are corresponding volumes of gases-primarilyprimitive water vapour, presumably released from water of crystallization, togetherwith C02, Nz, SOz, Hz, C12 in substantial quantities, accompanied by traces ofmany other gases.11920 No oxygen is released directly from volcanic effluents.The absence of a significant content of oxygen in the primitive secondary atmo-sphere is confirmed by several lines of evidence.First, there is no suitable source,as will be shown later. Secondly, the incomplete oxidation of early sedimentarymaterials (- 3 billion years of age) as demonstrated by Rankama,zl Ramdohr,22Lepp and Goldich,23 and others, and summarized by Rutten,24 suggests very earlylithospheric sedimentation in a reducing atmosphere. This evidence is in concord-ance with conclusions of extensive studies by Holland.13 Finally, the rapidly grow-ing evidence on the origin of life on Planet Earth appears to fQrbid a significantoxygen concentration until photosynthesis has been achieved.The work of Oparin25 and Bernal26 directed attention to the steps leading tothe organization of the simple biological cell, which is now recognized as a veryadvanced evolutionary entity.Oparin 25 visualized a logical series of evolutionary syntheses starting frominorganic, and simple organic materials of non-biological origin (i.e., the simplestcompounds of H, C, 0 and N) together with some traces of other elements (suchas S, P and Fe), finally ending with the organized cell replete with living functionwhich Bernal26 defines pragmatically as “ the embodiment within a certain volumeof self-maintaining chemical processes ”.Oparin 25 suggested that each step in theachievement of the whole process was the consequence of natural experimentationon a large scale guided by natural selection.The extensive literature on evolution leading to the simple cell is developed inwork by Wald,27 Rabinowitch,28 Calvin,29-31 Anfinson,32 and many others.33~ 34Synthesis of amino acids and other complex elements of cell structure is demonstrablein a reducing atmosphere, in the presence of ultra-violet light, which provides thL.V. BERKNBR AND L. C . MARSHALL 125energy for chemo-synthesis through photo-excitation (cf. Miller).33 Viable chemicalprecursors to the living cell were selected step-by-step from a thin soup of ever morecomplex organic compounds.During this phase of primitive evolution of pre-living compounds, oxygen is apowerful poison, acting to break them down as they are formed. Abelson 35 hasshown, e.g., that organic materials such as amino acids which are normally stablefor long intervals in anoxygenic atmospheres, are quickly degraded in presence ofatmospheres that are significantly oxygenic, particularly in the presence of a catalyticenergy source such as visible light.(AFTER BERNAL, CALVIN, HOERYG, OPARIN.RABINOWITCH. WALD P OTHERS)I SIWIFICANT OUANTlTlES OFOXYGEN FORBIDWN 0,cOOOl PRESENT ATMOSPHEhlC LEVELIPA L llw 1200-3003 AT 0-10 km ALTITUDEFOUMTION OFLQUm HZO)POLTMERtZATlON OF MACROMOLECULESWITH NATURIL SELECTION 1 PRDTEINS,NUCLEIC ACIDS, ENZYMES, GENES, HORMONES)YY PHOTO ASSOCIATION OFINTERMEDIATE ORGANICMATERIALS IAMINO ACIDS, ETC)AUkERWIC MCT~BOUSU OF ORDANIC UATERIALSTHROUQfl FfRMENTATON I15 CALORIES)FORMATTION OF CELL MEMBRANECbLLODAL COAZLRVATSS ASSOCIATING SYMBIOTICUACROUOLECULES WITH SURVIVAL DETERUINED81 NATURAL SELECTIONABOUT -2 7 x 109 YEARSTIME-FIG.1 .-Diagrammatic visualization of evolution of the simple living cell.Photosynthesis can proceed in anoxygenic atmospheres as shown by Hill 36 andHill and Scarisbrick,37 and in the process the whole warehouse of oxygenic com-ponents required for growth and reproduction can be synthesized as shown byCalvin and his co-workers.29 The principal steps in evolution to the complete cell,replete with photosynthetic activity, are thus summarized in an elemental way infig. 1.Such evidence of an early anoxygenic atmosphere therefore leads to inquiryconcerning the origin and concentration of oxygen in this primitive, secondaryatmosphere of the Earth.Excellent data on solar radiation in the u.-v. spectrumare now available from the space probes of many workers, including Hall, Damonand Hinteregger ; 38 Detweiler, Garrett, Purcell and Tousey.39 Available data onsolar u.-v. radiation are summarized after Nawrocki and Papa,40 in fig. 2 and 3,after Johnson’s 41 new evaluation of the solar constant.Radiation down to 1400a arises in the upper 100 km of the solar photosphere.As shown from the studies of Wilson42 on the evolutionary history of stars of themain sequence, similar to our sun, this photospheric radiation above 1400A isextremely stable-probably self-regulating and unvarying over very long periodscomparable to the age of the Earth. Radiation below 1400A arises primarily inthe chromosphere and corona, and may have been as much as three times thepresent average level (thus involving total average fluctuations of as much as 2126 OXYGENIC CONCENTRATION IN EARTH'S ATMOSPHEREto 1 in this spectral region over shorter intervals). From the point of view of totalultra-violet energy, however, by far the major bulk (99.9 %) arises from the stablephotospheric radiation above 1400 a.wavelength (A)FIG.2.Colar intensity 700-1400 A.COMPOSITE DATA FROM NAWROCKI 0 WPAI/'O-'HOO 1660 ' l8bO ' 2obO ' 22bO ' 24b0 ' 26b0 ' 28bO 'wavelength (A)FIG. 3.-Solar intensity 1400-3000 A.Absorption of this ultra-violet spectrum by various possible component gasesabove 1000 A is summarized in fig. 4 after the rather complete work of Watanabe 439 44and the contributions of many others.45-5L.V. BERKNER AND L. C. MARSHALL 127In the spectral region above l400A absorption by H2 and N2, and by the raregases He, Ne, Ar, Kr and Xe is negligible. Of the important probable atmosphericconstituents, only H20, C02, 0 2 and O3 absorb radiation strongly in this redon,with only 0 3 important as an absorbing gas above 2200 A.I I I I I I I I I1000 1200 1400 1600 I800 2000 2 200wavelength (A)FIG. 4.-Composite of u.-v. absorption in atmospheric gases.II I I I I I I I I I I1800 2000 2200 2400 2600 2000 3000wavelength (A)FIG. 5.-U.-v. absorption in 0 3 (data from Watanabe and Vigroux).In order to interpret the relative contributions of different concentrations of theseveral possible constituent gases as absorbers over the u.-v.spectrum, these dataon radiation and on absorption must be combined at each wavelength over thespectrum. To do this, the path length expressed in cm s.t.p., required at each wave-length to reduce the incident radiation to a small selected arbitrary level is ascertainedfor each constituent gas in an atmosphere.The selection of the particular reference level is not important to the subsequentinterpretation, since translation to other reference levels can be made readily. Thereference level in this paper is determined by the path length of the constituentgas required to reduce the incident energy at each wave band to 1 ergcm-zsec-1(50A)-l. This reference energy level is 10-4 of the level in a 50A band at the pea128 OXYGENIC CONCENTRATION IN EARTH’S ATMOSPHEREof solar radiation at approximately 4500 A (i.e., the transmitted energy level, afterabsorption, which is 0.01 % of the peak radiation in a 50 A band, a transmitted levelequivalent to about 50 times the brightness of the full moon).In the present dis-cussion, the path length x of any constituent gas, required to reduce the radiationto 1 ergcm-2sec-1 (50A)-l, will be expressed as the path length required for its“ extinction ”. The results of combining radiation and absorption data are shownin fig. 6 for HzO vapour.n E3lo4103I020 PATH LENGTH N.T.P.10I10’’I 0-21400 1600 I800 2000 2200 2400wavelength (A)FIG. 6.Thickness of H20 required to absorb available u.-v.to “ extinction ” [(I erg cm-2 sec-1(50 A)-1)].Here it is apparent that the wavelength, at which significant penetration throughH20 vapour can occur, is not very sensitive to concentration. It drops only aboutlOOA for a decrease of three orders of magnitude of H20 vapour in the most sig-nificant range of pressures.” In absence of oxygen, and at low C02 pressures,the important photochemical reactions can be summarized by 509 51H20 +2h~+2H + 0which provides a source of atmospheric oxygen.Urey 7 has pointed out, however, that production of oxygen by photodissociationof H20 would be limited at some self-regulating concentration by the shadowingeffect of the 0 2 so produced. Since oxygen is distributed exponentially above thebase of the stratosphere, while water vapour is precipitated out to very low con-centrations at this same level, the presence of oxygen will shadow the P I 2 0 vapour* Note also that a change of reference level from 1 erg to 0.1 erg cm-2 sec-1(50 A)-1, for example,will simply shift the curve upward by one decade in fig.6L. V. BERKNER AND L. C. MARSHALL 129in its range of photodissociation as seen from fig. 7. In this figure it is evident thatwhen the path length of 0 2 above 10 km approaches 35 cm, water vapour is com-pletely shadowed from the H20 dissociative radiation-band up to 1950A. Thispath length of 35 cm of 0 2 above 10 km corresponds to a total path length of 0 2above the surface of about 100 cm s.t.p.This path length represents an oxygen level (see fig.7) of somewhat less than0.001 present atmospheric concentration. On the premises of the above discussion,the upper limit of oxygenic pressure in the primitive atmosphere is less than 0.1of present atmospheric level (P.A.L.).AT 0 01 PA.L._._ ~_._ 0 2 PATH LENGTH ~~ N T P AT PRESENT ATMOSPHERIC LEVELAT 01 P.A L /103n 102E a 35CMFc 101I10-110-2AT 0001 P A L .-JI I I , I # ,wavelength (A)1400 1600 I800 20004 IIIIIIIIIIIIIIIIIIIIIIII 1 ‘I /! I‘ I‘ II I 1 1 ,2200 2400FIG. 7.Thickness of 0 2 required to absorb available u.-v. to “ extinction ” [(l erg cm-2 sec-1(50 &-I)].The presence of significant C02 (see fig. 8) simply adds to this shadowing ofH20 vapour, and lowers this upper limit of oxygenic concentration in the primitiveatmosphere below this value, i.e., <lo-3 P.A.L.Geologists have heretofore assumed that because of the extensive oxides foundin the pre-Cambrian (Proterozoic) lithosphere, that atmosphere must have beenhighly oxygenic.This assumption seems unnecessary, and probably invalid, whenthe pertinent reactions are reviewed :0 + O+ M+02+ M02+O+M+03+M0 3 + M,+surface oxidescoupled with the additional reactions0 + 0 3 + 2 0 20 2 + hv-+O + 0.In the primitive atmosphere the supply of 0 3 is maintained close to the surfacewhere it is removed through surface oxidation at very high reaction rates.130 OXYGENIC CONCENTRATION IN EARTH’S ATMOSPHERESince oxygen appears in the primitive atmosphere in its most effective form forrapid oxidation of lithospheric materials, one is led to inquire into the availablesupply over geologic time.In this study the potential oxygen-supply has beencalculated from the total energy available for photodissociation. It is found thatthe available energy is of the order of 100 times (or more) that which is necessary toaccount for existing pre-Cambrian lithospheric oxides. (The calculations showthat for each kilometre of completely oxidized metals in the lithosphere, oxygenequivalent to the oxidation of about one-third of the water of the oceans wouldbe required.) It is further determined that sufficient u.-v. energy is available to-- AT 30 P.A.L.105104lo3t I I1400 1600 1800 2000~ //- .. .I t I I 1 12200 2400 2600wavelength (A)FIG. 8.Thickness of COz required to absorb available u.-v. to “ extinction ” [(l erg cm-2 sec-1(50 A)-I)],dissociate available 0 2 in the primitive atmosphere to the extent that 0 3 will bemaintained as a major constituent near the surface. These calculations lead to therealization that oxidation rate of lithospheric materials in the Archeozoic and inthe Proterozoic is dependent, not so much on the absolute concentration of oxygen,as on its chenlical form and the reaction rate in that form. Consequently, the classicassumption seems unnecessary that abundance of lithospheric oxides dictate highoxygenic levels in the primitive atmosphere.Thus in the primitive atmosphere, the oxygen balance is dominated by a rate ofloss which consumes oxygen promptly upon its production.The rapid removalof oxygen from the primitive atmosphere, and its inherent self-regulation by the“ Urey ” process,7 together with other geochemical and biological evidence, leadstherefore to the conclusion that the oxygen level in the primitive atmosphere of theEarth was <lO-3 (is., c0.1 %) of present atmospheric levelsL. V . BERKNER AND L. C. MARSHALL 131The subsequent rise of oxygenic level can be attributed only to photosynthesis.The summary of Rabinowitcli28 shows that in the present atmosphere, all oxygenpasses through the photosynthetic process in -2000 years, all C02 in -350 years,and all H20 in the oceans in -2 x 106 years. These intervals are very short com-pared to geological periods.Thus photosynthesis, in oxidizing liquid water and at the same time reducingcarbon dioxide to carbohydrate, is the overpowering source of oxygen in the presentatmosphere.At any time after the primitive, the oxygen balance will be determined by :PLUS : photochemical dissociation of H2O ; photosynthesisMINUS: 0 2 and 0 3 oxidation of surface materials; decay and respiration; and0 2 in H20 solution.As Holland13 points out, the rise of oxygen occurs as a consequence of a smalldifferential between two much larger opposing effects, which relatively speaking,increase together.As the initial oxygen from photosynthesis is released, moreover,it will simply substitute for oxygen from photodissociation of H20, because of theinherent " Urey " regulation to the level 0 2 < 10-3 P.A.L.This balance is illus-trated qualitatively in fig. 9.PHOTOCHEMICAL NG PHOTOSYNTHETICRODUCTION OF 0 2+ NTRlBUTlNGOXYGEN-SED OXIDATION. RESPIRATION,, AND H 2 0 SOLUTIONFIG. 9.-Factors in early oxygen balance.Not until the net rate of production of oxygen, primarily by photosynthesis,exceeds the net rate of 0 2 dissociation, and its consequent loss as an active oxidant,can equilibrium values of oxygen exceed the levels in the primitive atmosphere andbuilding of a stable oxygenic atmosphere begin.This leads to inquiry concerning the ecology for the rise of photosynthesis.Caspersson,s2 and Davidson,Q have shown that cell absorption of u.-v. arises fromabsorption by nucleic acids primarily between 2600 and 2700& and by proteinsbetween 2700 and 2900A.Cell absorption in these bands is highly lethal to cellfunction in all forms, degrading chemical function, and stopping growth, repro-duction and survival. Only atmospheric ozone can provide protection by shadowingthe lethal radiation in these bands (see fig. 10). The distribution of ozone is shownin idealized form in fig. 11. Ozone is distributed roughly uniformly in a coluinnbetween its level of maximum production and the surface, to which it is convectedand lost. The level of maximum production is lowered as the oxygenic coacen-tration diminishes132I023 ww7OXYGENIC CONCENTRATION IN EARTH’S ATMOSPHEREwavelength (A)[(l erg cm-2 sec-1 (50 &-I)].lO.-Thickness of 0 3 required to absorb available u.-v.to “ extinction ” (1800-3000dlo-’FIG. 11.Estimated distribution of ozone for various levels of oxygenL. V. BERKNBR AND L. C. MARSHALL 133Therefore, the total path length of ozone will diminish with oxygenic concentra-tions as shown in table 2. Combining these data on path lengths of ozone, pathlengths of oxygen previously derived, and the path lengths for absorption in liquidTABLE 2.*-ESTIMATED TOTAL PATHLENGTHS OF 0 3 FOR VARYING PRESSURES OF 0 2x, P.A.L. -H, cm % of atmos. (s.t.p.) -x cm10.0 6 5 ~ 105 7 x 10-8 0.51.0 4 7 ~ 105 7 x 10-8 0.330.1 2s x 105 7 x10-8 0.20.01 1 2 ~ 105 4 x 10-8 0.050*005 1 2 ~ 105 1 . 6 ~ 10-8 0.020~001 1 2 ~ 105 4 ~ 1 0 - 9 0.005*The crude integration of total path length established in this table is all that present datacoupled with basic assumptions will justify.water using the measurements of Dawson and Hulburtp yields fig.12, whichshows the penetration of u.-v. in liquid water, in the presence of various oxygenicatmospheres.wavelength (A)FIG. 12.-Path length in liquid water in presence of 0 2 and 0 3 for various concentrations of 0 2to absorb available u.-v. to " extinction " [l erg cm-2 sec-1 (50 &-I].Here it is seen that in the primitive atmosphere, lethal radiation penetrates to adepth of 5 to 10m of water. Therefore, the requirements for primitive photo-synthesis are as follows :(a) a water depth more than about 10 m, sufficient to shadow lethal radiation but nodeeper than-needed, to permit a maximum of visible light for photo-synthesis 134 OXYGENIC CONCENTRATION IN EARTH'S ATMOSPHERE(b) a minimum of liquid convection to avoid circulation of primitive metazoatoward the lethal surface, but gentle convection to provide organic nutrientssynthesized photochemically at the surface, in the presence of u.-v., accordingto the processes described by Miller.32This rigidly restrictive ecology describes bottom dwelling organisms (greenalgae or their evolutionary precursors) in protected shallow lakes or seas.Inparticular, life in the oceans seeins unlikely.Warm pools associated with volcanic hot springs, rich in nutrient minerals andelemental compounds seem prime candidates for the origin of life and photosyn-thesis. The ancient bioherms, 2.7 billion years old, and known to have supportedphotosynthetic organisms, from the work of Hoering and Abelson,55 could wellhave been at the base of such pools, and must be very close to the seat of life.Therigid ecologic insulation between such pools suggests the possibility of multipleorigins of living organisms with natural selection among them only at a later erawhen the permissive ecologic environment became more general.Only as the continents grow with volcanic action can sufficient areas for photo-synthesis at suitable densities of activity be found to meet the criterion for a growingoxygenic atmosphere. With constzntly changing geographic areas, and corres-ponding fluctuations of climatology, the growth of the oxygenic atmosphere appearsto have awaited the proper combination of conditions to satisfy the critical criterion.Considering the lowered levels of light energy below lethal depths of water, we estim-ate that photosynthetic activity at about present densities must have covered between1 and 10 % of present continental areas before an oxygenic atniospherc could be built.As oxygen finally rises toward the level, 02-0.01 PAL.( k , 1 % present con-centration) several interesting potentialities arise :(i) the penetration of lethal u.-v. (replotted in fig. 13) diminishes to a few cmof water, opening the oceans to life;(ii) the oxygenic level reaches the " Pasteur point " where organisms change fromfermentation to respiration. Thus the energy available for chemo-synthesisjumps from - 20 to - 675 cal/mole (cf.Genevois 56 and Rabinowitch 28).(iii) At this same oxygenic concentration, many primitive organisms changeform anaerobic " photoreduction " to " photosynthesis " through rapidoxidation of an hydrogenase enzyme, thus broadening the base for evolu-tionary activity.28This leads to a search of paleontologic and geologic history for a radical andexplosive change in evolutionary forms, corresponding to the opening of entirelynew and far more widespread evolutionary opportunities as 02-+0.01 P.A.L. Thereis just one such evolutionary explosion-the Cambrian-beginning 600 millionyears ago." By immediate inference we therefore identify the oxygenic level,02--+0.01 PAL. as immediately preceding the opening of the Cambrian, followingthe earlier suggestion of one of the authors.58Prior to the Cambrian there is no evidence in the fossil record of any form oflife advanced beyond the elementary algae, fungi and bacteria, i.e., the simplestforms of thallophyta (cf.Rutten 24). Since Proterozoic sediments favourable tofossil preservation have been diligently studied for more than a century, the completeabsence of fossils representing more advanced forms prior to the Cambrian evolu-tionary explosion has been considered heretofore as a scientific " puzzle " (cf.Kummel 57).* This dating follows the most recent geologic and geo-chronologic conclusions (cf. Kummel57)135The usual assumption is that evolutionary pre-Cambrian precursors could havehad only " soft " parts that were unfavourable for fossilization (although in sub-sequent ages, fossils of this general kind are not infrequent) 57.Under the interpretation dictated by our present model, no advanced precursorsto the Cambrian evolutionary explosion should be expected until sufficient oxygenicconcentrations presaged the opening of the Cambrian.Thus, according to themodel developed in this discussion, the geologic record should be read exactly aspresented in nature.L. V. BERKNER AND L. C . MARSHALL1400 r10-3 10-2 10-1 I 10fraction of P.A.L. of oxygenFIG. 13.Penetration of u.-v. in liquid water with various combinations of oxygen and ozoneatmospheres [(intensity at extinction = 1 erg cm-2 sec-1 (50 %.)-')I.Following the opening of the Cambrian, the complexity of life is known to havemultiplied rapidly.In the next few million years more than 1200 species of differentcreatures appeared, many of very considerable size and variety of characters.During this time the foundations for all modern phyla were laid. In particular,complex and efficient forms of respiratory apparatus were evolved independentlyamong various phyla as increasing oxygen levels presented favourable opportunitiesfor selection of such evolutionary advances. These advanced respiratory systemsprovided the mechanistic basis for the concurrent development of circulatory systems,digestive tracts, central nervous systems, bisexual modes of reproduction, andsimilar characters associated with advanced biological organisms through effectivesupply of oxygen and removal of oxidized carbon.As oxygen rises with widespread marine photosynthesis to the level of 0.1 P.A.L.(10 % present concentration), fig.13 shows that lethal radiation will for the firsttime be largely shielded from dry land. This will open a new ecological niche forevolution ashore. Recent evidence indicates the possibility of microscopic palyno -logic organisms on land as early as mid-Silurian, but the geological record showsno evidence of any " advanced " form of life ashore until the late-Silurian age, 420million years ago. Then a number of different phyla of plants and animals ex-ploded on dry land. By the Early Devonian, 30 million years later, great forestshad appeared, and soon after amphibian vertebrates were found ashore136 OXYGENIC CONCENTRATION IN BARTH’S ATMOSPHEREThe late Silurian is interpreted by immediate inference as the earliest stage atwhich plants could emerge above the surface without danger of lethal “ sunburn ”.Therefore we identify the period 420 million years ago with an oxygenic level,0 2 N 0.01 P.A.L.The explosion of evolution ashore increases photosynthesis in astep function by some 20-25 %, again tilting the oxygen balance radically towardthe plus side.In examining the oxygen balance in light of all factors studied to date, therelation between production rate and final equilibrium may be represented crudely,by fig. 14. Much more refinement of this balance is necessary in future studies.Sx 10-4 E-lt : i I1441 I I I I II o9 1010 1011 1012 1d30 2 rate of production by photosynthesis(mol.cm-2 sec-1)FIG. 14.-Estimated atmospheric levels of oxygen as a function of production rates.Following the late Silurian, hgh rates of photosynthesis are induced withoutcorresponding quantities of organic materials immediately available ashore fordecay and replenishment of C02. This suggests that oxygen may have “over-swung ” the present level to a somewhat higher value as the lush life of the Carbon-iferous developed. Then, with reduction of CO2, the Earth would cool, due to lossof the “ greenhouse ” effect of CO2, leading to the ice ages of the Permian Period.As the Earth cooled, photosynthesis would sharply fall, leading to a radical lossof atmospheric oxygen.Thus, the phase difference of production of 0 2 and of 6 0 2suggests that the levels of these two atmospheric components in the post-Silurianatmosphere niust have been unstable, the instability being damped by the ever-improving adaptation of organisms to wider environmental ecologies. Pendingmore analytical study, a preliminary estimate seems justified of the order of 108years for a complete oscillation above and below the present quasi-permanent levelL. V. BERKNBR AND L. C. MARSHALL 137We must not expect complete symmetry of this function, however, since any dropof oxygen is likely to arise from a series of inter-related events involving adaptationof oxygen-producing organisms, events whose mutual interaction would force anunstable and precipitous drop of oxygenic levels. A model may therefore be devisedfrom the studies to date of oxygenic levels over geologic history as shown in fig.15.FIG.15.-Tentative qualitative model of growth of oxygen in atmosphere.The standard assumptions made in historical geology and paleontology con-cerning rates of evolutionary development are modified so profoundly, by theconclusions drawn from this model, as to require further examination of the evolu-tionary process.In the conduct of this study, it has been deemed necessary to consider evolutionin two separate roles :(0(ii)the character and geographic extent of living organisms which must be themajor contributors to the rise of the oxygenic atmosphere at any periodbeyond the primitive ;the identification of a period of explosive evolutionary change as an indicatorof the timing of critical oxygenic levels in the atmosphere, e.g., as an in-dicator of major and appropriate physical change of the environment.In connecting these two roles, the evolutionary process during any geologicalperiod has been interpreted as an intricate interaction between the oxygenic levelgenerated by living organisms, and the natural exploitation of that level for the newevolutionary niches which that oxygen level opens to evolution.As such evolu-tionary opportunities are captured, the oxygen level is then raised as a consequence,thereby permitting a new round of evolutionary development. From time to time,critical levels are reached that permit vast physical ecologic opportunities, due tocritical responses of organisms generally.There appears implicit in the literature an assumption that very long periodsof time (that is long compared to 10 million years) are required to account for theevolutionary rise of organisms from the simple and microscopic metazoa of Hoeringand Abelson 55 and of Tyler and Barghoorn 59 to the large and somewhat morehighly organized creatures suddenly appearing at the opening of the Cambrian138 OXYGENIC CONCENTRATION IN EARTH’S ATMOSPHEREIn the absence of other evidence, such an assumption might appear reasonable,though as previously observed, the absence of any Proterozoan links in the geologicrecord (in spite of favourable conditions and diligent search) has long been recog-nized as a “ puzzle ”.* Early in the current studies it became apparent that thelevels of oxygen preceding the critical levels were not suficient to permit a smoothrise in evolutionary character, but that “ quantum jumps ” in ecological activityfollowing achievement of critical oxygenic levels seemed more likely.Thus, thesudden opening of the whole oceans to population as a consequence of sufficient shadow-ing of lethal u.-v. represented such a jump. As an interesting circumstance, theoxygenic level for opening the oceans to life seems to coincide approximately withsignijicant biological processes intimately related to the same oxygenic level, thusmultiplying the probability of revolutionary opportunity for the change in the charactersof life observed at the opening of the Cambrian.Likewise, the opening of the dry-land areas to land-dwelling plants and animals at oxygenic levels sufficient to shadowthe dry land from lethal u.-v. represents a similar ecologic jump during late Silurian.The questions then arise: how fast can evolution develop in complexity in itscapture of the new environmental opportunity? Are the great evolutionary dis-continuities to be properly interpreted as clear signals of great physical ecologicchange?The fact is that large changes in evolutionary character of living organisms arefrequently observed in geologic history as rather sudden jumps without a “ long ”history of visible precursors.60 This suggests that the absence of visible precursorsarises from the very speed of change-a change occurring in a small and limitedpopulation poorly adapted to the revised ecology, and proceeding rapidly from gener-ation to generation under the encouragement of natural selection in a vast newlyopened physical-eco1ogy.t In developing this interpretation it would seem that theadvance in the evolutionary character of life does not necessarily await some*The idea of a long pre-Cambrian history (i.e., long compared to a few tens of millions ofyears) of advanced evolution of organisms to account for the diversification found at the base ofthe Cambrian, a history which has been thought not preserved due to the imperfection of the fossilrecord, is deeply imbedded in the whole literature of geology and paleontology. Consequentlydirect inferences from new evidence that lead to contrary views are not easy to accept.Never-theless, we believe these inferences should be faced squarely, since the classic views are purelyintuitive, and therefore must be taken advisedly, however Ptolemaic their authority. Critical ap-proach to the present interpretation in light of the oxygenic evidence can focus sharper attention onsearch for pre-Cambrian and lower Cambrian evidence, and may clarify and quantify the evolutionaryprocess.t While pre-storage of evolutionary characters has long been known, it has recently come sharplyto the attention of science as a major factor in evolution through the effects of the widespread useof antibiotics and industrial chemical poisons.Except for the most extreme poisons, organismsfind preadaptation to widely varied ecologic changes in some small fraction of their population,which permits their evolved propagation in the new ecology. The molecular basis for geneticpreadaptation is mentioned by Huxley 61 and typically discussed by Anfinsen.32 The wholesubject of the molecular basis for evolution has undergone such drastic expansion and revision duringthe past decade, as a consequence of the scientific emphasis on molecular biology, that many previousconceptions in evolution bear re-evaluation.The term “ preadaptation ” has unfortunately been misused, during the early discussion ofnatural selection, to refer to some form of supernatural or mythical, non-scientific predestination,whereby the organism was “ divined ” toward an ever higher, preordained “ design ”.This viewwas imposed by some who could not conceive of natural selection from natural variability as thedirecting mechanism in producing the delicately balanced features of evolutionary change, a viewthat is now wholly discredited (cf. Simpson ; 62 Romer 63). Preadaptation does, however, have auseful connotation in the strictly scientific sense, and in complete accord with current evolutionarytheory, when used to refer to storage and reproduction of mutational events, some of which becomeuseful to natural selection when the environment is modified. The problem is : how large a rangL. V. BERKNER AND L. C. MARSHALL 139mutational accident while the ecological opportunity lays fallow. Rather, in theface of continual mutation (indeed, rapid mutation in terms of geologic periods),a wide variety of evolutionary characters are continually pre-stored among theseveral members of the population, and await selection by appropriate ecologicopportunities that are provided by the opening of new evolutionary niches.Geneticvariation of the individual in the population involves many unexpressed physiolo-gical traits. A few of these have the opportunity to respond in a few members ofthe population through their favourable articulation when a new and favourableecology permits or encourages that response through natural selection. Thenevolution proceeds as rapidly as combination, selection and adaptation permit,since current rates of mutation are not a limiting factor.Evolution through naturalselection appears as a process of rapid scanning in times compared to a few millionyears for selection of ecologies offering improved adaptation, and selection of suchecologies will occur quickly after they are offered. Thus, explosive changes in com-plexity of similar evolutionary characters simultaneously among many differentphyla appears as positive evidence for the opening of new and major ecologicopportunities. Large physical-ecologic changes are then most favourable for" quantum jumps " in evolution since not only larger opportunities are opened,but the pre-existing competition may be reduced or eliminated.This reasoning does not contend that a metazoan could pre-store all of thespecialized characteristics of " a man ".Clearly, in achieving such levels of com-plexity, natural selection must proceed through many millions of steps in develop-ing suitable opportunities upon which mutation can act and from which subsequentselection must proceed. The suggestion here is merely that bccause of pre-storageof characters, the speed of evolution can be enhanced sufficiently to account formajor jumps in levels of complexity such as occurred at the opening of the Cambrianor in the Late Silurian of the order of a few units of 10 million years. Precedingsuch jumps, widespread evidence of intermediate precursors would not and couldnot exist. Long series of precedent evolutionary steps should not be expected orassumed since they would be forbidden by the precedent environment.In conclusion, a comment on the atmosphere of Mars seems in order.Becauseof the low gravitational field, coupled with its smaller diameter and mass, Mars hasprobably lost all H2, He and should be losing atomic oxygen at a rate determinedby its temperature and rates of production and of diffusion or convection. In absenceof oceans, the atmosphere of Mars would otherwise appear to be somewhat similarto the primitive atmosphere of the Earth, well prior to the Cambrian. Life onMars therefore may be subject to the same restrictive ecology found for the primi-tive atmosphere of the Earth. As seems probable for the primitive Earth, some-what different forms of life, arising from different initial precursors in separate andof variability, unexpressed in the current environment, can be prestored as a result of recessivemutations, to be released as a useful evolutionary change in a sharply modified environment?It can be readily shown from the mathematics of genetics, for example, that a " lethal recessive "gene inhabiting 10-6 of a population will be reduced only 50 % in 106 generations when the proba-bility of mating among the population is purely random.Not all, nor even a large proportion ofrecessive genes are lethal, nor are many " recessive " genes purely recessive. Consequently, anyindividual may contain a relatively large number of hidden or only partly evident characters, usefulfor selection in a modified ecology. If some number n of independent inherited traits can bearticulated in adapting to a new environmental opportunity, the probability of an individualpossessing such an articulated group will be 2-n.Thus, for example, the probability of articulationof any random group of 20 independent inherited characters with an individual would be 10-6.Since populations of micro-organisms are large, very considerable evolutionary changes can beexpected rather quickly under compulsion of a strong selective ecologic change (cf., Dobzhansky 64)1 40highly insulated pools, might be expected. The direct study of Mars and its atmo-sphere should open exciting new and powerful vistas to science.The study of paleoatmospheres involves a tremendous range of correlative in-formation, and this prCcis can only present the briefest account of much more ex-tensive analysis.The generation of a model for the Earth opens vistas to a widerange of studies relating to quantitative paleoclimatology. A wide range of cir-culatory and convective patterns is suggested as the level of ozone production riseswith increasing oxygen or concentration at different eras, and of paleoionospheresresulting from widely differing aeronomys. The purpose of this paper is to encouragemore critical studies and discussions of the subject, and to approach the muchgreater refinements that are immediately suggested.OXYGENIC CONCENTRATION IN EARTH’S ATMOSPHERE1 Goldschmidt, Norske Videnskaps-akad. Oslo, Skr., Mat.-Nat. KI., 1937, 4, 148.2 Brown, Rev. Mod.Physics, 1949, 21, 625.3Brown, The Atmospheres of the Earth and Planets, ed. Kuiper (The University of Chicago4 Spencer-Jones, Sci. Progress, 1950, 38, 417.5 Kuiper, Proc. Nat. Acad. Sci., 1951,37, 1.6 Urey, The Planets: Their Origin and Development (Yale University Press, 1952).7Urey, Symp. Int. Union Biochem. Moscow, 1957, (The Macmillan Company, New York,8 Alfvkn, On the Origin of the Solar System (Oxford Clarendon Press, 1954).9 Fesenkov, Symp. Int. Union Biochem., Moscow, 1957 (The Macmillan Company, New York),10 Vinogradov, Symp. Int. Union Biochem., Moscow, 1957 (The Macmillan Company, New11 Rubey, Bull. Geol. SOC. Arner., 1951, 62 (2), 111.12 Rubey, Geol. SOC. Amer. (special paper), 1955, 62, 631.13 Holland, Petrologic Studies: A Volume to Honour A.F. Buddington (Princeton University,14 Sapper, Vulkankunde (Stuttgart, Engelhorns, 1927).15Verhoogen, Amer. J. Sci., 1946, 244, no. 11, 745.16 Bullard, Volcanoes, in History, in Theory, in Eruption (University of Texas Press, 1962).17 Wilson, Amer. Sci., 1959, 47, 1.18 Wilson, in The Solar System, vol. I1 of The Earth as a Planet, ed. Kuiper (University of Chicago19 Wilson, Physics and Geology, with Jacobs and Russell (McGraw-Hill Book Company, New20 Tazieff and Fabre, Compt. Rend., 1960, 250, 2482.21 Rankama, Geol. SOC. Amer. (special paper), 1955, 62, 651.22 Ramdohr, Abhandl. deut. Akad. Wiss. Berlin KI. Chem., GeoIogie und Biologie, 1958, 35,23 Lepp and Goldich, Geol. SOC. Amer. Bull., 1959, 70, 1637.24 Rutten, The Geological Aspects of Origin of Life on Earth, Amsterdam, New York (Elsevier25 Oparin, Origin of Life (Dover Publications, Inc., New York), 1953, S213.26 Bernal, Proc. Physic.SOC. A , 1949, 62, 537.27 Wald, in The Phy.sics and Chemistry ofLife, ed. Simon and Schuster, New York, 1955, part 1,28 Rabinowitch, Photosynthesis and Related Processes (Interscience Publishers, Inc., New York,Press, 1952), p. 258.1959), 1, 16.1959, 1, 9.York, 1959), 1, 23.Princeton, New Jersey, 1962), p. 447.Press, 1954), p. 150.York, 1959).no. 3, p. 19.Publishing Co., 1962).chap. 1 .1951).29 Calvin and Bassham, The Photosynthesis of Carbon Compounds (Benjamin, Inc., New York,1962).30 Calvin, Symp. Int. Union Biochem. (Macmillan Company, New York), 1959, 1, 207.31 Calvin, Bull.Amer. Inst. Biol. Sci., 1962, 12, no. 5, 29.32 Anfinsen, The Molecular Basis of Evolution (John Wiley and Sons, 1961).33 Miller, Symp. Int. Union Biochem. (Macmillan Company, New York, 1959), 1, 123.34 Florkin, Symp. Int. Union Biochem. (Macmillan Company, New York), 1959, 1, 503, 578.35 Abelson, Ann. N. Y. Acad. Sci., 1957, 69, 276141 L. V . BERKNER A N D L . C. MARSHALL36 Hill, Proc. Roy. SOC. B, 1939, 127, 192.37 Hill and Scarisbrick, Proc. Roy. SOC. B, 1940, 129, 238.38 Hall, Damon and Hinteregger, 3rd Int. Space Sci. Symp. (Washington, 1962).39 Detwiler, Garrett, Purcell and Tousey, Ann. Geophysics, 1961, 17, 263.40 Nawrocki and Papa, Geophysics Corp. Amer. (Bedford, Massachusetts), A.F.C.R.L. Report,41 Johnson, J. Meteorol., 1954, 11,431.42 Wilson, Astrophys. J., 1963, 138, 832.43 Watanabe, Zelikoff and Inn, Geophysical Research Papers, no. 21 (June, 1953), A.F.C.R.C.44 Watanabe, Adv. Geophysics, 1958, 5, 153.45 Allen, Astrophysical Quantities (University of London, The Athlone Press, 2nd ed., 1963), p. 124.46 Herzberg, Molecular Spectra and Molecular Structure. I. Spectra of Diatomic Molecules(D. Van Nostrand Company, Inc., New York, 1961).47Pearse and Gaydon, The Identification of Molecular Spectra, 2nd ed. (John Wiley & Sons,Inc., New York, 1950).48 Pauling, The Nature of the Chemical Bond and the Structure of Molecules and CrystaZs (CornellUniversity Press, Ithaca, New York, 1960).491rzt. Crit. Tables (McGraw-Hill Book Company, Inc., New York, 1926-30).50 Nicolet and Mange, J. Geophys. Research, 1954, 59, no. 1, 15.51 Nicolet and Bates, J. Geophys. Res., 1950, 55, no. 3, 301.52 Caspersson, Cell Growth and Cell Function: A Cytochemical Study (Norton and Co., Inc.,53 Davidson, The Biochemistry of the Nucleic Acids, 4th ed., (John Wiley & Sons, New York,54 Dawson and Hulburt, J. Opt. SOC. Amer., 1934,24, 175.55 Hoering and Abelson, Proc. Nat. Acad. Sci. US., 1961, 47, 623.56 Genevois, Beochemisches Zeit., 1927, 186,461.57 Kummel, History of the Earth (W. H. Freeman and Co., 1961).58 Berkner, Proc. Conf. on Ionospheric Physics (July, 1950), part B. Published by GeophysicsRes. Div., Air Force Cambridge Research Centre, Cambridge, Massachusetts, pp. 13-20(Apr. 1952).Contract # AF 19(604)7405 (August, 1961).Technical Report # 52-23 (Cambridge, Mass.).New York).1960).59 Tyler and Barghoorn, Science, 1954, 119, 606.60 Romer, Vertebrate Paleontology (University of Chicago Press, 1962).61 Huxley, Evolution in Action (Harper and Brothers, New York, 1953).62 Simpson, The Meaning of Evolution (Yale University Press, 1960).63 Romer, The Vertebrate Story (University of Chicago Press, 1959).64 Dunn and Dobzhansky, Heredity, Race and Society (The New American Library, New York,1957)

ISSN:0366-9033    

DOI:10.1039/DF9643700122

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

11. REACTION OF CHARGED SPECIESThermal Reactions Involving Charged ParticlesBY A. DALGARNODept. of Applied Mathematics, The Queen's University of Belfast,Belfast 7, N. IrelandReceiued 9th January, 1964The mechanisms of the production and loss of charged particles in the upper atmosphere aredescribed and attention is drawn to those processes that require further experimental study.The ionosphere is the result of a complex of reactions involving the productionand loss of charged particles. Much progress has been made in recent years inidentifying the reactions responsible and in providing reliable values of their ratesbut many uncertainties remain.PRODUCTION OF CHARGED PARTICLESAlthough corpuscular radiation is significant in the elucidation of many detailsof ionospheric behaviour, the major source of charged particles in the upper atmo-sphere is photo-ionization of the neutral particles by solar ultra-violet radiation.A calculation of the resulting ion and electron production rates involves the photo-ioniz2tion and absorption cross-sections of the main constituents.192 Cross-sections for the atomic gases hydrogen, helium and oxygen have been providedby theoretical calculations; the most precise for hydrogen are those of Burgess,3for helium those of Stewart and Webb 4 (which are in harmony with the measure-ments of Baker, Bedo and Tomboulian 5) and for oxygen those of Dalgarno, Henryand Stewart.6 Cross-sections for the molecular gases oxygen, nitrogen and nitricoxide must be provided by laboratory measurements and a compilation of themore recent investigations is given in ref.(7). Because of the severe requirementsof resolution imposed by the rapidly varying nature of the molecular cross-sections,serious discrepancies exist between the measurements of different investigators.We shall hear more about this subject later in the discussion.8The electrons which are produced by photo-ionization are initially movingrapidly. They are slowed down by collisions with the atmospheric constituentsand ultimately become thermalized, of special significance being elastic collisionswith the ambient electrons. The latter process constitutes a mechanism for selec-tively heating the ambient electron gas with the consequence that the daytimeelectron temperature may exceed the neutral particle and positive ion temperature.9In order to explore fully the possibility, it is necessary to have more detailed informa-tion on photo-ionization cross-sections than is normally provided, Depending uponthe wavelength of the ionizing radiation and upon the absorbing constituent, there mayoccur more than one possible photo-ionizing process.Thus, for wavelengths shorterthan 660A, N i may be produced in the ground state or in one or more excitedstates and the energy distribution of the photoelectrons depends upon the cross-sections of the individual process.1014A . DALGARNO 143It is possible in many cases to identify the end-products of a photo-ionizingreaction by observing the fluorescence spectrum 11 * but the analysis of fluorescenceobservations to yield photo-ionization cross-sections presents severe difficulties.It may be more profitable to attempt direct measurements of the energies of theejected electrons 13 and the work of Schoen 14 on the atmospheric gases at wave-lengths between 500 and 1000 A is encouraging.Information on the cross-sections describing the scattering of electrons by theatmospheric constituents is also fundamental to the quantitative interpretation ofthe ionospheric temperatures.Of special importance is the process of vibrationalexcitation of nitrogen,l* which has been studied recently by Schulz.ls The cross-section is large because of resonance effects.16IONOSPHERIC DATADalgarno and McElroy 17 have adopted the best available estimates of photo-ionization and absorption cross-sections and of incident solar ultra-violet intensities 18FIG.1.Too = 1OOO"K; z = oin a recent calculation of the ion production rates appropriate to a range of possiblemodel atmospheres and some of their results are reproduced in fig. 1 and 2. Severalmeasurements of the ion-density distributions in the ionosphere have been carried*The occurrence of a strong fluorescence spectrum raises the possibility of monitoring thesolar ultra-violet radiation by ground-based observations.1144 THERMAL REACTIONS INVOLVING CHARGED PARTICLESout using rocket-borne mass spectrometers.19 They show that N: is a minorconstituent at all altitudes, that NO+ is the dominant ion below about 200 km andthat Of is the dominant ion above about 200km.There is a transition regionlocated above 500 km in which He+ becomes the dominant ion and another at agreater altitude in which H+ becomes the dominant ion.20 The altitudes of thevarious transition regions vary with time.FIG. 2.Too = 1000°K; z = 90"The main purpose of my discussion is the identification of the chemical reactionswhich are responsible for transforming the production rates of fig. 1 and 2 intothe observed ion density distributions.ION-REMOVAL PROCESSESWhereas NZ is the major ion produced in the E region, it is present only as aminor ionic constituent. The process by which molecular ions are finally removedappears to be the Bates process of dissociative recombination,XY+ +e+X+Y,all other recombination processes being much slower in ionospheric conditions(except in the lower D region).21 Laboratory investigations 22 suggest that at300"K, the recombination coefficients for N i , 0; and NO+ are all approximatelA.DALGARNO 145equal but the variations with temperature of the recombination coefficients areunknown. If it is assumed that they remain approximately equal throughout theE region, it follows that N i must be transformed into another ionic species inpreference to its removal by dissociative recombination.This conclusion receives support from a calculation of the equilibrium con-centration arising from the inclusion of only photo-ionization and dissociativerecombination. With a rate coefficient of 2 x 10-7 cm3 sec-1 for the dissociativerecombination of N;,22 and an electron density of 105 cm-3, the equilibrium con-centration at noon of N i at an altitude of 160 km is several orders of magnitudelarger than has been measured.The reactions which transform N i into differentspecies must occur rapidly and so involve presumably either or both of the majorneutral constituents 0 and 02. The possible exothermic reactions areN l + O+N2 + 0'N; + O+NO+ +N (2)Nz + 02+N2 + 0;. (3)From mass spectrometric observations of afterglows, Fite et aZ.23 have derived arate coefficient of 2 x 10-10 cm3 sec-1 for (3) but there is no experimental informationon (1) and (2). Norton, Van Zandt and Denison 24 have claimed that the observedabundance of NO+ shows that (2) is more important than (1) and they suggest ithas a rate coefficient of 2 x 10-11 cm3 sec-1.Nicolet and Swider 25 have noted thatthe conclusions of Norton et al. are based upon many uncertainties and they reject(2), pointing out that it is not a simple ion-atom interchange reaction. However,it does not follow (2) is necessarily slower than the charge transfer process (1) andan experimental study to resolve the question is desirable.If we denote the reaction rates by y with the equation number as an identifyingsubscript, the equilibrium number density of N l may be writtenwhere q cm-3 sec-1 is the production rate of N;, a is the coefficient of dissociativerecombination and the n denote number densities.The value of y3 derived by Fite et al.is unexpectedly large but even if the actualrate were two or three orders of magnitude smaller it is clear that in the lower partof the E region and in the D region, NZ will be very largely transformed into 0;.At higher altitudes, N i will be transformed into Of and NO+ and perhaps only nearthe peak of the F region will dissociative recombination become the controllingprocess. The unknown variation of a with temperature prevents any reliable estim-ate of (yl+y2) from ionospheric data. It may be noted that a is determined bythe electron temperature which considerably exceeds the neutral particle temperaturein the region of 220 km during the d a y 3It was argued by Bates and Massey27 that the atomic ions such as O+ are con-verted into molecular ions by charge transfer and the argument was later modifiedby Bates 28 to conversion by ion-atom interchange processes.o+ + 0 2 - + 0; + 0 (4)O++N2-+NO' +N, (5)thereby providing a simple explanation of the later discovery that NO+ is a majorion in the E and P1 regions.The reactions are qualitatively in accord with the deductions of Ratcliffe et aZ.29from radio observations which showed that at low altitudes the decay of electro146 THERMAL REACTIONS INVOLVING CHARGED PARTICLESdensity is proportional to nz and at higher altitudes it is proportional to ne.Thusat high altitudes, the rate-limiting step is the ion-atom interchange process anddn,/di = - (y,n(O,) + Y 5 Q,))n,and at low altitudes the rate-limiting step is dissociative recombination andQuantitatively the position is less satisfactory.Thus from the persistence of theF-layer during the night Bates and Nicolet 30 conclude thatys + 0.1 674 = 1.3 x 10-13 cm3 sec-1,whereas the several, though disparate, measurements all indicate larger values.31Antonova and Ivanov-Kholodny 32 have suggested that the persistence of the noc-turnal I: layer is due to incoming soft electrons but their postulated flux is unac-ceptably high.33Throughout most of the atmosphere, the ion 0; is removed by dissociativerecombination but at low altitudes, chemical reactions may be significant. Thus0; can be transformed into NO+ throughThe rate of (6) is probably slow but it may well be rapid enough to transform allthe 0; produced in the D region into NO+. Similarly, despite the fact that it in-volves a minor constituent, it will be necessary to take into accountif it happens to be an efficient process.25The disappearance of NO+ is probably due almost entirely to dissociative re-combination. It is its production which is affected by chemical processes.Preciseknowlege of the coefficient of dissociative recombination and its variation with tem-perature is fundamental to a quantitative interpretation of ionospheric behaviour.The ions He+ and H+ are significant at high altitudes, where diffusion processesare important. A major source of H+ is the accidental resonance charge transferprocessas noted by Dungey.34 At altitudes below about 500 km, chemical equilibrium pro-bably prevails in which casedn,/dt = -an:.0; +N2-+NO+ +NO.(6)0; +N+O+NO+ (7)H + O+-+H+ + 0 (8)and at altitudes above about 700 km, diffusion is the controlling mechanism.35The transition altitudes depend upon the rate of (8). Hanson et aZ.36 have deriveda value of 4 x 10-10 cm3 sec-1 from ionospheric data and this value is not incon-sistent with the high energy measurements of Fite, Smith and Stebbings.37The disappearance of He+ has been discussed by Hanson 38 and by Bates andPatterson.39 Bates and Patterson point out that an ion-atom interchange reactionof He+ with N2 is almost certainly endothermic, but that one with 0 2 ,may proceed rapidly. In (9), HeO+ is electronically excited and (9) will be followedby a radiative transition to a repulsive levelHe++02+HeO++O, (9)HeO++He+O++AvA .DALGARNO 147The helium atoms so formed may have sufficient kinetic energy to escape from theatmosphere.39There remains the possibility that the greater density of N2 or of 0 may causethe charge transfer mechanismsHe+ + N,-+He +Nf (10)He' + 0-He + 0' (1 1)to occur more rapidly. The question is of considerable importance since the heliumproduced by (10) or (11) will have little kinetic energy and the reactions will makeno contribution to the escape rate of helium.Experimental work on the disappearance of He+ in 0 2 and Nz: has been reportedby Sayers and Smith 40 and by Fite et aZ.41 It appears that there is a fast reactionwhich removes He+ but its precise specification is uncertain.The laboratory investigation of the reactions mentioned in the discussion pre-sents formidable difficulties of interpretation. Several of the papers of this sessionare concerned with this problem.THE D REGIONThe D region is more complicated.Its formation is due to X-ray absorption,to photo-ionization of the minor constituent NO by Lya and to cosmic rays.Typical production rates have been computed by Nicolet and Aikin.42 Becauseof the increasing pressure, three-body processes become important and the formationof negative ions must be recognized. An authoritative review of the role of negativeions bas been given recently by Branscomb,43 and 1 shall mention only one aspect,which relates to the work of Farragher et aZ.44It was observed in a study of polar cap absorption that the local dawn for theincrease in absorption occurred at a time which suggested that visible light fromthe sun was not effective in causing photodetachment from the unidentified negativeion.It appeared that the ion must have a large electron affinity such that radiationshorter than 2900A was required for detachment to occur.45 The identity of thisnegative ion has been the subject of speculation. There is a possibility, noted byDonahue, that inclusion in the analysis of the effect of the high surface albedo ofthe earth, may after all permit its identification as 0;.1 Chapman, Proc. Physic. SOC., 1931, 43, 26, 483.2 Watanabe and Hinteregger, J. Geophys. Reo., 1962, 67, 999.3 Burgess, 1964, in press.4 Stewart and Webb, Proc. Physic. Soc., 1963, 82, 5325 Baker, Bedo and Tomboulian, Physic.Rev., 1961, 124, 1471.6 Dalgarno, Henry and Stewart, Planet. Space Sci., 1964, 12,235.7 Weissler and Lee, J. Opt. Soc. Amer., 1952, 42, 200. Weissler, Lee and Mohr, J. Opt. SOC.Amer., 1952, 42, 84. Aboud, Curtis, Moreure and Reuse, J. Opt. Soc. Amer., 1955, 45, 767.Sun and Weissler, J. Chem. Physics, 1955, 23, 1372. Watanabe, Adu. Physics (AcademicPress, New York), 1958, 5, 153. Weissler, Samson, Ogawa and Cook, J. Opt. Soc. Amer.,1959, 49, 338. Weissler, EncycZopaedia of Physics (Springer, Berlin), 1959, 21, 304. Ditch-burn and Opik, Atomic and Molecular Processes (Academic Press, New York, 1963). Huffman,Tanaka and Larrabee, J. Chem. Physics, 1963, 39, 902. A valuable compilation of the variousmeasurements has been prepared by Schultz, Holland and Marmo (1962, Tech.Report 62-15-N,Geophysics Corporation of America).8 Huffman, Tanaka and Larrabee, and Cook, Ching and Becker, this Discussion.9 Hanson and Johnson, Me'm. Soc. Sci. Li2ge, 1961, 4,390.10 Dalgarno, McElroy and Moffett, Planet. Space Sci., 1963, 11,463.11 Schoen, Judge and Weissler, Ionization Phenomena in Gases (North Holland, Amsterdam, 1963),Proc. 5th Con$, Paris, 27. Huffman, Tanaka and Larrabee, J. Chem. Physics, 1963,38, 1920148 THERMAL REACTIONS INVOLVING CHARGED PARTICLES12 Dalgarno and McElroy, Planet. Space Sci., 1963, 11, 727.13 Kurbatov, Vilesov and Terenin, Soviet Physics, 1962, 6, 853. Turner and Jobary, J. Chem.Physics, 1962, 37, 3007.14 Schoen, 1964, in press.15 Schulz, Physic.Rev., 1962, 125, 229.16 cf. Herzenberg and Mandl, Proc. Roy. SOC. A , 1962, 270, 48.17 Dalgarno and McElroy, 1963, unpublished.18 Hall, Damon and Hinteregger, Space Science III, 1963 (North Holland, Amsterdam).Detweiler, Garrett, PurceU and Tousey, Ann. de Geophys., 1961, 17, 263. Zirin, Hall andHinteregger, Space Science III, 1963 (North Holland, Amsterdam). Hinteregger, J. Atmos.Sci., 1962, 19, 351.19 Johnson, Ann. de Geophys., 1961, 17, 100. Taylor and Brinton, J. Geophys. Res., 1961, 66,2587. Istomin, Iskisstuennye Sputniki Zemli, 1961,11,94. Taylor, Brace, Brinton and Smith,J. Geophys. Res., 1958, 68, 5339.ZOBauer, J. Atmos. Sci., 1962, 19, 276; J. Geophys. Res., 1964, 69, 553. Bourdeau, SpaceSci. Rev., 1962, 1, 683.21 Bates and Dalgarno, Atomic and Molecular Processes (Academic Press, New York, 1963).22 Biondi, Ann.de Geophys., 1964, 20, 34.23 Fite, Rutherford, Snow and van Lint, Disc. Faraday SOC., 1962, 33, 264.24 Norton, van Zandt, and Denison, Proc. Int. Con$ Ionosphere, 1963 (Inst. Physics and Physic.25 Nicoiet and Swider,Planet. Space Sci., 1963, 11, 1459.26 Spencer, Brace and Carignan, J. Geophys. Res., 1962, 67, 157.27 Bates and Massey, Proc. Roy. SOC. A , 1947, 192, 1.28 Bates, Proc. Physic. Soc., 1955, 68, 344.29 Ratcliffe, Schmerling, Setty and Thomas, Phil. Trans. A, 1956, 248, 621.30 Bates and Nicolet, J. Atmos. Terr. Physics, 1960, 18, 65.31 Paulson, Ann. de Geophys., 1964,20, 75.32 Antonova and Ivanov-Kholodny, Space Research 11, 1961 (North Holland, Amsterdam).33 Dalgarno, Ann. de Geophys., 1964, 20, 65.34 Dungey, The Physics of the Ionosphere, 1955 (Physic. SOC., London).35 Bauer, J. Geophys. Res., 1964, 69, 553.36Hanson, Patterson and Degaonakar, J. Geophys. Res., 1963, 68, 6203.37 Fite, Smith and Stebbings, Proc. Roy. SOC. A , 1962, 268, 527. Rapp, J. Geophys. Res., 1963,38Hanson, J. Geophys. Res., 1962, 67, 183.39 Bates and Patterson, Planet. Space Sci., 1962, 9, 599.40 Sayers and Smith, Atomic CoZZision Processes (North Holland Publ. Co., Amsterdam, 1964).41 Fite, Smith, Stebbings and Rutherford, J. Geophys. Res., 1963, 68, 3225.42 Nicolet and Aikin, J. Geophys. Res., 1960, 65, 1469.43 Branscomb, Ann. de Geophys., 1964, 20, 88.44 Farragher, Page and Wheeler, this Discussion.45 Reid, J. Geophys. Res., 1961, 66,4071.SOC., London).68, 1773

ISSN:0366-9033    

DOI:10.1039/DF9643700142

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

Absorption by, and Photo-Ionization of, N2 and 0 2 inthe 830-1OOOA RangeBY G. R. COOK, B. K. CHING AND R. A. BECKERAerospace Corporation, P.O. Box 95085, Los Angeles 45, CaliforniaReceived 17th January, 1964Measured absolute photo-ionization and absorption coefficients for 0 2 and N2 are presentedfor the wavelength range 830-1OOOA. In 0 2 , the spectrum consists of band absorption superim-posed on a continuum. Many of the bands are broad, and absorption coefficients are independentof pressure. In N2, the spectrum shows many close-spaced narrow absorption bands with a weakunderlying continuum. Pressure dependence of the coefficient is found to be minimum at posi-tions of absorption minima. The question of penetration of solar radiation into the atmosphereis considered in the light of the measured results.The absorption of optical radiation by molecular oxygen and nitrogen has beenobserved by many investigators. since the initial work of Hopfield.1 The purposeof this paper is to present measured photo-ionization and absorption coefficientsof 0 2 and N2 in portions of the range between 830-1OOOA.Although absorptioncoefficients have been published by several authors, their results have often failedto reveal the full details of band spectra because of the lack of sufficient data.EXPERIMENTALThe experimental apparatus (fig. 1) and techniques have been described previously 2and will be mentioned only briefly here. A 1-m, Seya-Namioka-type, scanning vacuumultra-violet monochromator with a resolution of 0.5A in the first order was used with aHopfield continuum light source that produced a maximum flux of about 108 photons/secat the 40 p exit slit.Attached to the exit slit was a 40.6-cm long, stainless steel chamberin which a variety of measurements could be made. Photon flux was measured usingeither a platinum photocathode or a sodium-salicylate-coated photomultiplier tube. Ion-ization was measured by a parallel-plate ion chamber. Current due to photoelectricemission or ionization was amplified by electrometers. The output was then displayedon strip chart recorders as the spectrum was scanned at 25A/min. Gas pressure wasmeasured by thermocouple or ionization-type gauges, calibrated against a mercury McLeodgauge. Gas purity was monitored by a small mass spectrometer that was attached directlyto the experimental chamber.Reagent-grade 0 2 and N2 were obtained from the MathesonCompany in 1-1. glass flasks. The gas flowed into the cell through a needle valve. Theabsolute response of the platinum photocathode was determined by calibrating the ioniz-ation chamber with the rare gas atoms A, Kr, and Xe.The experimental procedure consisted of adjusting the pressure of 0 2 and N2 in theexperimental chamber by means of a leak valve. The spectrum was scanned while simul-taneous measurements were made of photon flux # and ion current ie. The scan was re-peated at eight different levels of gas pressure. From data obtained at various pressures,the absorption cross section Q and coefficient k = ~ O Q , were obtained using the equationwhere 40 and 4 are the photon flux before and after absorption by a gas of concentration14150 ABSORPTION BY GASESn and path length 1.The term no is Loschmidt's number. The ionization cross-sectionot or coefficient kj was obtained from the equation,cri = [igo/4,,e][1 - exp (- nol)]-', (2)where ig is the ion chamber current and e the electronic charge; 4 0 was found from therare-gas calibration 2 for which aj/O = 1.Data were taken in the following manner. With the absorption cell empty, the spec-trum was scanned at 25A/min to obtain z$~. Gas was then admitted to the absorptioncell, and the pressure was measured by a thermocouple gauge that had previously beencalibrated against a mercury McLeod gauge.The scan was then repeated while simultaneousmeasurements were made of ipt and is. The photoelectric currents z$ and iPt are relatedto the photon fluxes by the equations z$Je = Ypt& and i p t / e = Ypt&, where Ypt is theyield of the platinum photocathode.AIR --COMPRESSEDFIG. 1 .-Experimental apparatus.(A) 1-m Seya vacuum ultra-violet monochromator. (B) First differential pumping section : (1)mechanical pump, 13 cfm; (2) diffusion pump, 720 l./sec. (D) 1200 groove/mm diffraction gratingblazed at 800A (on turntable). (E) Main chamber pumping system: (1) diffusion pump, 1400l./sec ; (2) mechanical forepump, 13 cfm. (F) Pressure gauges : mercury McLeod, ionization,Phillip's, and thermocouple. (G) Micromanometer. (H) Partial pressure analyzer.(I) Com-bination absorption and photoionization chamber. (J) Experiment chamber pumping system :(1) cold trap ; (2) diffusion pump, 2411. ; (3) mechanical forepump, 5 cfm. (K) Gas inlet system :(1) needle valve ; (2) stopcock ; (3) gas sample.Information on recorder traces was converted to digital form on punched cards by aBenson-Lehner X-Y reader. A computer programme was used to determine Q and Q j .Accuracy of the measurements for those coefficients in excess of 10cm-1 is estimated tobe 10-15 % for k and 15-20 % for kj. Smaller coefficients were not measured accuratelyin this experiment due to the difficulties encountered in accurately measuring pressure andpath length with N2 at relatively high pressures.RESULTSAn example of data obtained using 20 p slits and a slower scanning speed oflOA/min is shown in fig.2-4. For the data presented in fig. 2, a pressure of about16p of Na was used with an 875cm path length. Under these conditions thestrong absorption bands of N2 are clearly evident. The upper trace is the back-ground continuum. At the bottom of the figure the positions and spectroscopiintensity, arbitrary unitsi"'I 3850.085 3.v) Y.m C1Y s VC 0.mwavelength, AFIG. 4 . 4 2 photo-ionization and absorption spectra, 850-990 AG. R. COOK, B . K. CHING AND R. A . BECKER 151designations of identified bands are shown. Underneath the photoelectric absorptionspectrum is a high-resolution photograph of the same region taken with a 3-m,normal-incidence spectrograph.(The authors are greatly indebted to Dr. M. Ogawafor furnishing this photograph and those in fig. 3 and 4.) An examination of thephotograph in fig. 2 shows that many of the vibrational bands have rotationalstructure, unresolved in the photoelectric trace. The effect of insufficient resolvingpower on the absorption measurements was apparent from a plot of In (ig,/ipt)against n, which showed deviations from linearity. Although rotational structurecould not be resolved, the resolution was sufficient to separate the vibrational bandseasily. Thus, with a continuum light source, the effect of measuring an absorptioncoefficient at various wavelengths over a band could be investigated. It was foundthat the absorption law, ipt = ig,exp (-nal), was more nearly obeyed when themeasurements were made in the vicinity of band heads, where the rotational levelsblend together, or at minima, where the levels are less dense.For measurementsmade in this manner, deviations from linearity in Beer’s law occurred mainly in the0-50 p pressure range, and in a slight failure of the extrapolated linear portion tointersect the origin. Measurements made in this way are considerably more re-liable than those made over band edges where there is a rapid change in intensity.Still, the absorption coefficients for N2 listed in table 1 are probably too small forthe maxima and slightly too large for the minima. Absorption nlinima were in-vestigated by increasing the pressure of N2 to 500 p ; this was sufficient to causetotal absorption at most of the maxima, as shown in fig. 3.Peaks in the recordertrace correspond to absorption minima. At this pressure several of the bands havean apparent width of more than 5 A in maxima as well as in minima. In the photo-graph some rotational structure is still evident between the more closely spacedstrong absorption bands.Indicated by arrows in fig. 2 and 3 are some of the stronger observed solar emis-sion lines. Those at 832.1 and 835.3 in fig. 2 are approximately at an absorptionminimum and maximum, respectively. In fig. 3 solar lines at 937.8, 949.7, 972.5and 977.0A are, respectively, slightly within the edge of a broad band, within theedge of a narrow band, at the maximum of a broad band, and at an absorptionminimum.TABLE 1 .-N2 ABSORPTION COEFFICIENTSA k a progression or series min and rnax834.5835.0836-5837.5840.0840.8841-5842.0843.5844.2846.2846.9848.2850.0852-5853.55249515-525.544058034960030570201481 176531900834-94 a835.160837.3 1840.55841-87843.98846.93 b849.29 a*849.7 1 a*853.19minRx4,O rnaxtRx3, 1, ssminrnaxminrnaxminrnaxminrnaxminrnaxminmaxminmw152 ABSORPTION BY GASESTABLE 1 ---(contd.)1855-0856.2856-7857.1859.0859.8860.0860.8861-3862.0862.5863.3864.2865.0866-1866.8870 087 1 -5873.2874.2875.0876.5878.3879-2880.2881.0882.0882-6885.0885.8886.4887.0888-3889.2890.5892.2893.2894-0895.5897-2898.3899.0900-5901-2902-3903-0904.0905.0907.0k5050040.55908914.910.212.212.316.119815620045016-314-259084070.512.27.415.115.171015012002204002489.620-24501324911.233011.623811.867013925026029914.615.215.616.62701856.00 b856.98860-51863.17865.05865.3 1866.76870.74871.4087587879.47880.73882.47885.66 b886-03886.69888.81890.95893.86897.19899.20 b901.36902-58903.64 b*904.73progression or seriesSRx3,Or, v' = 7r, v' = 6r, v' = 5q, v' = 4r, v' = 4of, v' = 4r, v' = 3Rx2,4hq, vt = 4r, v' = 2q, v' = 3of, vt = 3q, v' = 2r, v' = 0Rx293S'(a, v' = 1rnin and maxminrnaxminrnaxminminminmaxminrnaxminminrnaxnlinrnaxminmaxminminminmaxminmaxminrnaxminrnaxminrnaxminrnaxminrnaxminrnaxminmaxminrnaxminrnaxminmaxminmaxminmaxminmG .R . COOK, B . K . CHING AND R . A . BECKER 153I908.0910.0910.89 12.0912.69 16.09 17-7919.3920.0922.0923.0925.0927.0927-7928.7931.5932.5934.3935.0937.0938.5940.5942.5945.5946.7948.0949.22950.0950.2951.2952.0953.0954.0954.5955.5957.0958.0959.3960.0963.5965.5970.0972-5976.0978.0k20015096029745016.520.55.111.310.717.47-016.01.51.410.26.12-0518.91-5510.01-282.81.182.01.122.022-554.305-02-5529712090010098180035015050500790130052001470TABLE 1-(contd.)I progression or series907.45 f910.48 q, v' = 0912.63 of, v' = 2916.42 p, v' = 1920.46 d'920.04 Rx2,2922.75 p, u' = 0925.93 b', U' = 6928.88 o', v' = 1931.73 b', vr = 5935-15 b rn, v' = 2942.39 m, v' = 1946.12 of, v' = 1949.22 m, u' = 0955.08 I, u' = 1958-16 a Rx2,O960.21 1, u' = 0965.63 6, V' = 3972.07 6, V' = 2978.87 b b, V' = 1min and maxm aminmaxminmaxminmaxminmaxminmaxminmaxminmaxminrnaxminmaxminmaxminrnaxminrnaxminrnaxminmaxminmaxminmaxminmaxminrnaxminmaxminmaxminMaxminmaxa Ogawa and Tanaka, Can.J. Physics, 1962,40,1593.b Worley, Physic. Rev., 1943, 64, 207.c Ogawa, private communication.d Mulliken, The ThreshoZd of Space (Pergamon Press, New York, 1956), 169.1 Rx are Rydberg bands converging to the XzZg+l state of N2+.2 Rx3,O (Rxm = 3, v' = 0) are Rydberg bands with rn = 3, v' = 0, converging to the X2Z:,Cstate of N2+I54 ABSORPTION BY GASESThe observed wavelengths of absorption bands are in agreement with the resultsobtained by Worley.3 A direct comparison of k values in this region is difficultbecause the emission lines employed by other workers often fell on the side of arapidly changing absorption band and thus showed a strong pressure effect. Forinstance, at 916.5 A, k = 5.1 cm-1 was observed in the present investigation.Thisvalue may be compared with the k values of 1.2, 22, 9 and 7 cm-1 reported by Lee,4Clark,s Watanabe and Marmo,6 and Itamoto and McAllister,7 respectively. Forhigh values of k, results of this study for the broad band at 972.5 A showed k =5200 cm-1; this is in fair agreement with the results of Clark (3900 cm-I), but islower by a factor of 2 than that reported by Itamoto and McAllister. In the narrowband at 920& k = 450cm-1 was observed. This can be compared with valuesfor k (610, 280 and 200 cm-1) observed by Clark, by Itamoto and McAllister, andby Watanabe and Marmo. In view of the wide discrepancies among the publisheddata, further experiments at higher resolution would be desirable.For 0 2 it was found that the bands between 1026.3 and 860A were diffuse inboth the absorption and photo-ionization spectra, with a half-width of from 2 to 4times the instrumentzl resolving power.The broadening of these bands was prob-ably due to pre-ionization from the overlapping continua of the 02+(X2IIg) ionicstate. The appearance of ions at H Lyman was consistent with an ionizationpotential 8 of 12.075 eV (1026.7 A). Amplitude changes in band intensities reacheda minimum at about 860A, where there is a break in the absorption.Ion current was observed at all wavelengths in 0 2 from the first ionization poten-tial and was superimposed on an underlying continuum. A comparison of theabsorption and ionization spectra in a region free from emission lines showed that,in general, there are peaks in the ionization spectrum which are coincident with3.832-3834.5835.7837.9839.1842.2843-8845.5846.0848.6850.0851.5853.0854.5856.8858.5860-5862.0864.0868.5871.0875.5877.6882.5TABLE 2-02 ABSORPTION AND IONIZATION COEFFICIENTSk950400830500770350460420640270390350442305407285305300360270395250403250ki305112119963209311410815690114104139901138098871208216010224088832.33 a836.61 a838.96 a843.88 a845.94 a853.24 a856.1 b863.3 b870-0 b877.5 bI2maxminmaxminrnaxminrnaxminrnaxminrnaxminrnaxminmaxminm aminmaxminmaxminmaxmiI885-4890.0891.5892.5893-8895.5897.0898.0900.5902.5903-1908-0909.2910.69 12.0914.5915-7916.5920.5922.9924.5926.5930.6931.5932.5936.0938.9944.0947.9950.0956.1957-2959.5962.0964.5965.4971.5972.6974.3977.598 1.0983.0984-7985-7989.0993.0995.0104-01010 01025.7G .R. COOK, B . K . CHING AND R . A . BECKERk52020536530040531533525543030036015649049023029015557025038065025046043077012811701151310190137013709248027092083166027246534092072250608006038014058TABLE 2-(contd.)ki2908219314519410014011324015616691307307801209634063178408903852383007264510078573116011607885826108350057555231560120316030564934.8A885.4 b893.8 b909.17 a916.34"922.85 a924-47 a930.58 a933-27 a938.88 a947.87 a956.1 1 a957.21 a965.44 a972.57 a975.32 a983-09 a985.80 a993.04 a1003.71 aprogressionor series11M'M'MM'MM'MMH'MH'HH'HH'HH155max and rninmaxminrnaxminmaxminmaxminrnaxminrnaxminmaxmaxminrnaxminmaxminrnaxrnaxminrnaxminmaxminrnaxminminmaxmaxminrnaxminmaxminmaxmininaxniinrnaxminmaxminmaxminmaxminrn&Xa Price and Collins, Physic.Rev., 1935,48, 714.b Tanaka and Takamine, Sci. Papers Inst.Physic. Chem. Research (Tokyo), 1938, 34, 854.Rb 3 , 2 are Rydberg bands converging to the b4C; state of 0; with in = 3 and v' = 2156 ABSORPTION BY GASESabsorption maxima (minimum photoelectric current). This appears to be true forsharp bands, such as those observed in N2, as well as for the diffuse bands of 0 2 .In fig. 4, the 0 2 spectrum between 920 and 990A is shown. The upper curveis a recorder trace of the photoelectric detector measuring the total absorption.In the centre is a photograph of the absorption spectrum, while the lower recordertrace is that due to the ion current. Also shown in the upper part of the figureare the positions of absorption progressions classified by Price and Collins9 asH, H’, M and M’. Absorption measurements of the diffuse bands of 0 2 were foundto obey Beer’s law, and the measured k and kg values for 0 2 are listed in table 2.More extensive results for both 0 2 and N2 will be published elsewhere.10DISCUSSIONIt is interesting to examine the effect of the present results on the penetration ofsolar radiation into the atmosphere.For this a density distribution (table 3) wasadopted closely parallelling that of the COSPAR International Reference Atmo-sphere 11 which represents the daily mean. The mean molecular mass was chosento be 12 at 800 km and the temperature 1350°K at 800 km. The latter conforms toBlamont’s 12 determination by means of sodium release. The ratio of 0 to 0 2was chosen to conform with the determination of Schaefer and Nichols 13 between100 and 130 km.Penetrations for some solar emission lines were computed following the methodof Watanabe and Hinteregger 14 and are displayed in fig.5. The gross behaviourof the transmission results is governed by the following features : at wavelengthsgreater than 91 1 A, wide fluctuations in absorption coefficients due to band structureexist for both 0 2 and N2, with an underlying 0 2 absorption continuum whichwould become effective at low altitudes where the 0 2 concentration is appreciable;below 91 1 A, ionization of 0 would decrease transmission ; finally, below approx-imately 800 A, the fluctuations in the absorption coefficients are considerably reducedfor both 0 2 and N2.TABLE 3.-NlTROGEN AND OXYGEN COMPOSITION OF THE UPPER ATMOSPHERE (IlO.lCm3)km n(N2) 40) 4 0 2 )1101201301401501601701801902002202402602803003504005006007008001-4 x 10123-09 x 10111-1 x 10116.0 x 10103.5 x 10102.3 x 10101.45 x 10109.5 x 1096.4 x 1094.5 x 1092.5 x 1091 .4 7 ~ 1098.8 x 1085.4 x 1083.4 x 1081 . 0 8 ~ 1082.9 x 1062.5 x 1053-3 x 1072.2 x 1041-91 x 1031.7 x 10117.3 x 10103.3 x 10102.3 x 10101-6 x 10101.2 x 10109.1 x 1097-1 x 1095.6 x 1094.5 x 1093.0 x 1092 . 0 5 ~ 1091-43 x 1091 . 0 5 ~ 1097.3 x 1083.7 x 10s1.9 x 1085.6 x 1071-78 x 1075-6 x 1061.8 x 1062-7 x 10114.2 x 10101.5 x 10107-5 x 1094.2 x 1092.0 x 1091.6 x 1091.0 x 1096.4 x 1084.0 x 1082.0 x 1081.12x 1086-3 x 1073.6 x 1072.1 x 1079.0 x 1045.5 x 1035-7 x 1061 .4 8 ~ 1063.3 x 10G . R . COOK, B . K. CHING AND R . A . BECKER 157The line 977.0 A is absorbed mainly by 0 2 , though it lies near the edge of an ab-sorption region of Nz.; hence its apparent intensity in an actual satellite or probemeasurement would be sensitive to the exact location and width of the acceptancewavelength band of the detector. Molecular oxygen also is the principal absorberfor 989.8-991-5 A. On the other hand, 972.5 A lies in a region of high NZ absorp-tion for which the presently measured absorption coefficient represents but a lowerlimit. Further work at higher resolution may show a different transmission for972-5 A than is indicated in fig. 5. Similarly, Ogawa and Cairns 15 have shown thatthe relative absorption by N2 over the width of Lyman y (972.5 A) is not uniform.L , ; .,0 0.1 0.2 0:3 0:4 0.5 0:6 O i 7 2 8 0 4 0transmissionFIG. 5.-Atmospheric transmission at selected wavelengths.The observed solar emission peak,l6 832.1-835.3& is probably mainly due tothe 0 I1 lines 832.7, 833.3, and 834.5A. The relative transmission of these threelines is concerned with the circumstance that the middle line occ~~rs near a narrowabsorption maximum (fig. 2), while the two outside lines both occur at absorptionminima. The minimum for 834.5A is too narrow to be revealed by the presentmeasurement; it does, however, appear in Ogawa's photograph. In the regionfrom about 700 A to the lower end of the Hopfield continuum (- 580 A) there existslittle structure in the absorption coefficients of N2 and 0 2 ; hence little variationin the transmission of solar lines over this range is to be expected.In the calcul-ations the theoretical results of Dalgarno 17 for 0 were identified with the existenceof an absorption continuum. Lyman 6, 949.7 A, and Lyman 8, 937.8 A, were notincluded in fig. 5 because they appear to lie on the edges of'absorption regions ofeither or both N2 and 0 2 .A calculated curve of penetration depth (altitude at which solar intensity is re-duced to l/e) as a function of wavelength is presented in fig. 6. The wide fluctu-ations are owing to band structure in 0 2 and N2. Below 91 1 A, the depth o158 ABSORPTION BY GASESmaximum penetration is increased because of absorption through the ionization of 0.The values were calculated only for the peaks and valleys of the absorption traces.The large fluctuations in fig.6 are drawn with straight lines joining these points.180a>"2).wavelength, AFIG. 6.Penetration depth for reduction to l/e of initial intensity.120The present work suggests that 0 2 and Nz laboratory absorption measurementsshould be made at improved resolution and at a range of temperatures and pres-sures corresponding to upper atmospheric conditions. Likewise, there is a greatneed for laboratory measurements of absorption by 0. Finally, solar flux measure-ments are needed at sufficiently high altitudes so that corrections due to absorptionby residual atmosphere are minimized.-1 Hopfield, Physic. Rev., 1938,36,789 ; Astrophys. J., 1930,72,133 ; Physic. Rev., 1927,29, 356.2 Metzger and Cook, J. Opt. SOC. Amer., to be published.3 Worley, Physic. Rev., 1943, 64, 207; 1953, 89, 863.4 Lee, J. Opt. SOC. Amer., 1955,45, 703.5 Clark, Physic. Rev., 1952, 87, 271.6 Watanabe and Marmo, J. Chem. Physics, 1956,25,965.7 Itamoto, Jr., and McAllister, Scient9c Report no. 4, Contrib. no. 29 (Hawaii Inst. Geophysics,8 Watanabe, J. Quunt. Spectr. Radiative Transfer, 1962, 2, 369.9 Price and Collins, Physic. Rev., 1935, 48, 714.10 Cook and Metzger, J. Chem. Physics, 1964,41,321.11 COSPAR Int. Reference Atmosphere 1961, ed. Kallmann-Bijl et al. (North-Holland Publishing12 Blamont et al., Space Research 11, ed. van de Hulst (North-Holland Publishing Co., Amster-13 Schaefer and Nichols, COSPAR, 4th Int. Space Science Symp. (Warsaw, 1963).14 Watanabe and HintFregger, J. Geophys. Research, 1962, 67, 999.15 Ogawa and Cairns, Planet Space Sci., 1964, 12, 656.16 Hall, Damon and Hinteregger, Space Research 111, ed. Priester (North-Holland Publishing17 Dalgamo, GCA Tech. Report 6 0 - 5 4 (Geophysics Corp. of America, Boston, 1960).Honolulu, 1961).Co., Amsterdam, 1961).dam, 1961).Co., Amsterdam, 1963), pp. 745-759

ISSN:0366-9033    

DOI:10.1039/DF9643700149

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

Nitrogen and Oxygen Absorption Cross-Sections in theVacuum Ultra-violetBY R. E. HUFFMAN, Y. TANAKA AND J. C. LARRABEEAir Force Cambridge Research Laboratories, Bedford, Massachusetts, U.S.A.Received 23r.d December, 1963The total absorption cross-section (or coefficient) curves of nitrogen and oxygen recently measuredin the 580-1050 A wavelength region using a Hopfield helium continuum background light sourceare discussed in relation to the absorption of solar radiation in the earth’s upper atmosphere. Similarmeasurements using the hydrogen many-line spectrum background in the 1375-1450 A region andthe 1620-164OA region of the oxygen Schumann-Runge continuum are also described. A tablelisting cross-sections of nitrogen and oxygen at all of the important solar emission lines in the 580-l050A region is given.Our measurements are compared with values given by others wherepossible.The absorption of solar ultra-violet radiation in the atmosphere produces ions,atoms, and electrons which then generally undergo further reaction with each otherand with the ambient atmospheric constituents. In order to understand the numerousreactions that occur in the atmosphere, it thus becomes essential to know the primaryphotochemical formation rates of these reactive species. For this purpose, it isnecessary to know the solar radiation flux incident on the upper atmosphere, thenumber densities of the atmospheric constituents, the total absorption cross-section,and the cross-section for the specific product of interest.The purpose of this paper is to make readily available to upper atmospheretheorists recently measured total absorption cross-sections of nitrogen and oxygen,primarily in the 580-1050 A wavelength region.These measurements were obtainedwith a continuum background light source, in contrast to most other measurementswhich were obtained with line emission background light sources and which there-fore cannot adequately measure the complicated structure usually present. Ageneral discussion of these measurements and their relation to the molecular struc-ture has been previously given.l.2 In this paper we wish to describe in more detailthe measurements at important solar emission lines and also recent measurementsin the oxygen Schumann-Runge continuum.EXPERIMENTALThe experimental method will not be given in detail here, since it has been previouslydescribed.192 A repetitive, condensed discharge (0.002 ,uF, 5 kc) through helium (38 mmHg) gives an intense Hopfield continuum 3 from 580 to 1100 A.The radiation then is dis-persed with a 2-21 8 m normal incidence monochromator equipped with differential pumpingat the 100 micron entrance slit. A 10 cm long, windowless gas absorption cell is placeddirectly behind the exit slit. The radiation is detected with a sodium-salicylate-coatedglass disc at the end of the cell a few mm from a photomultiplier (EM1 9514B). Pressureswere measured with a McLeod gauge. Reagent-grade gas from the Air Reduction Com-pany was used. Wavelengths were located to f0.18, using impurity lines of known wave-lengths, and the bandwidth, as measured from the impurity line half-widths, was slightlyless than 05A.15160 N2 AND 0 2 CROSS-SECTIONSThe absorption cross-section o or absorption coefficient k is defined by the expression,I = I , exp [ - kx] = I , exp [ - onox], (1)where 10 and I are the relative intensities without and with gas in the cell, x is the absorptionpath length reduced to s.t.p., and no is 2-69 x 1019 cm-3, Loschmidt’s number.The ab-sorption cross-section is given in units of megabarn (Mb), which is equal to 10-18cm2,and the absorption coefficient in units of cm-1. In all cases, a second 10 scan was madeafter measuring the absorption to be sure that the light source intensity had remainedconstant.A small scattered light correction of from 0-5 to 3 % was subtracted from all intensities.At wavelengths less than 661 A in N2 and 722 A in 0 2 , it was necessary to make smallcorrections for ffuorescence of the gas in the absorption cell.In the 1375-145OA region,it was necessary to correct for fluorescence from the lithium fluoride windows.The measured cell pressures were corrected for the pressure gradient along the light pathduring windowless operation by measuring the absorption of oxygen around the peak ofthe Schumann-Runge continuum (1375-1450 A) both during normal flow operation andwith a lithium fluoride windowed cell. From these results, the measured absorptioncoefficients were increased by the factor 1.15.A B S OR P T I 0 N C R 0 S S - SE C T ION S , 580- 1050 AIn this section, absorption cross-section curves averaged from at least five scans atdifferent pressures are given.In all cases, the curves agreed well with previous spectrataken in this laboratory with a 6.8 m grazing incidence spectrograph.Nitrogen absorption cross-sections are shown in fig. 1, covering 1000-800 A and in fig. 2,covering 800-600A. Many sharp molecular bands whose rotational structures are notresolved with the present bandwidth are observed between 1000 and 796& the first ion-ization threshold. For these unresolved bands, the absorption cross-section increases ast ”2‘ooot; !800 - - - - & - - - ~ o o 8 50wavelength (A)FIG. 1 .-Absorption cross-sections of nitrogen in the 1000-800 A region.the pressure decreases. This “apparent pressure effect” is probably due to extremelyhigh absorption cross-sections in the sharp rotational lines which generally produce totalabsorption at the pressures used.In this case, the given band maxima represent a lowerlimit, and much higher resolution will be needed to study these bands. There is no con-tinuum observable between these bands larger than the approximately one Mb lower limiR. E. HUFFMAN, Y. TANAKA AND J. C. LARRABEE 161for accurate cross-section measurements in these experiments. The significant rise inthe continuum beginning near 810 A is probably due to unresolved Rydberg series membersconverging to the first ionization threshold at 796A. Because of the absence of a con-tinuum and of the sharp appearance of the bands in higher resolution spectra,4 it is unlikelythat absorption by nitrogen in the 1000-796 A region will lead to formation of nitrogen atomsby photodissociation in the atmosphere.It is possible that predissociation may occurin some of these bands ; however, the necessary high resolution studies have not been done.At wavelengths between 1000 and l050A, the nitrogen absorption bands are much weaker(upper limit about 0.01 Mb 5) and could not be measured in the present case.-3000 -h 1 'E28p moo+-2a 2000 - .8m YE0 .r(N2Nz++B 2 C "II4360PEAK6 C O 650 700 750 a ocwavelength (A)FIG. 2.-Absorption cross-sections of nitrogen in the 800-600 8, region.At wavelengths less than 796A, nitrogen can be ionized to the X2I=$ ground state ofNZ, ana ng.L snows tne presence 01 me expecrea cominuum in mis reBon. inere arealso many bands, especially in the 796-725 A region, which have been found to be extensivelypre-ionized.6 Around the N:A2IT, threshold at 743 A, there is a gradual rise in the con-tinuum towards shorter wavelengths. However, at the NgB2I=Z state threshold at 661 A,wavelength limt of 580 A. In the region below 661 A, fluorescence irom N; has beenobserved.6 This radiation is probably the first negative bands of N i , B2X$-+X2Xz.Oxygen absorption cross-sections are shown in the 1060-83OA region in fig. 3 andin the 830-6OOA region in fig. 4. The observed continuum increases very slowly belowthe first ionization threshold at 1026.7 ( P I T , state of O;), probably because of largedifferences in equilibrium internuclear distances between the ground states of 0 2 and 0;.At the other ionization thresholds shown, the continuum also rises gradually.In addition,dissociation continua 2 may contribute to the observed continuum.All of the bands at wavelengths less than the ionization threshold of 1026.7 A are diffuseand have been found to be pre-ionized.7 At wavelengths less than about 722 A, fluorescencefrom 0; was observed.6 This was probably due largely to the second negative bandsof O;, A211,+X21Tg. In general, the oxygen curve is much better known than the nitrogencurve. The bands are more diffuse due to pre-ionization.In fig. 1-4, the experimental error is estimated to be -+lo % under the most favourableconditions.This error applies for absorption cross-sections between about 2 and 150 Mb,arid becomes larger outside these limits. Where necessary in later discussions, specificestimated errors will be given.absorption coefficients (cm-1)c d0 ;I000 ul0absorption cross-sections (10-1s cm2)absorption coefficient-000 0I i Y==; -'-W00I I I Iabsorption cross-sectionR . E. HUFFMAN, Y. TANAKA AND J. C. LARRABEE 163been measured, and the strongest bands have generally been estimated to be no larger than0.01 Mb.5 Predissociation has been found in some of the Lyman-Birge-Hopfield bandsin this region.8On the other hand, oxygen has an intense, complicated absorption spectrum in thisregion.5 The Schumann-Runge (SR) continuum between roughly 1300 and 1750 A is gener-ally believed to be responsible for most of the atomic oxygen found above 9Okm in theatmosphere.Absorption in this continuum yields one 3P ground state atom and one IDexcited state atom per photon absorbed. Predissociation can occur in the SR bands?In this case, two 3P ground state atoms are formed in the dissociation.We have measured the absorption cross-section of the SR continuum around its maximumusing a lithium fluoride windowed absorption cell and the hydrogen molecular spectrumas background. Measurements in the broad maximum (1375-145OA) of the continuumand at a few wavelengths near 1630 A are given in table 1. At the observed peak of theTABLE ABSORPTION COEFFICIENTS OF OXYGEN IN THE SCHUMANN-RUNGE CONTINUUM.AT THE MAXIMUM OF THE BEST CURVE THROUGH THE MEASUREMENTS GIVEN, THE /C-VALUEAVERAGES OF 9 TO 7 MEASUREMENTS.LONGER WAVELENGTH RANGE VALUES AVERAGES OFIS 4-03 Cm-1 AND THE WAVELENGTH IS 1415 f 5 A. SHORTER WAVELENGTH RANGE VALUES3 MEASUREMENTS. ESTIMATED ERROR IS LESS THAN &lo %.;".(A)1376.61379-51393.31395.51396.71398-21401-31401.81406.51409.51412.01426.81429.11430.21431.81432.11433.3k ( a - 1 )3693753933953953934004013974024064014004003984014004 A )1434-21435-61436.71437.21438.11440.11442.61445.21622.21622-61624.81627.51632.81633.21634.31635.51638.01639.4k (cm-1)39839939739839439639439690898580767574727068continuum, the cross-section was 15.0 f 1.5 Mb or 403 f40 cm-1.This value is in goodagreement with several other photoelectric measurements.ss 7.10 However, it is roughly20 % lower than several early photographic technique measurements.11 The problem ofreconciling these measurements has been discussed,79 12 and it is apparent that precisemeasurements by several independent techniques must be carried out in an attempt to fixthe best value of this important cross-section.We have not measured cross-sections with comparable accuracy at any other wave-lengths in this region. It is planned to study this entire region with continuum backgroundlight sources in the future. In particular, the region 1050-1350 A will be studied, since it hasnot been possible to interpret the cross-section curves well in this region.UPPER ATMOSPHERE ABSORPTIONThe solar radiation flux @(h,A) in photonlcm2 sec at any altitude h is given by the ex-pressio164 N2 AND 0 2 CROSS-SECTIONSwhere a&) is the flux incicent on the earth's atmosphere, o-i is the total absorption cross-section for the jth constituent, and Nj is the number of atoms in a vertical or slant cm2column along the path of the incident radiation.The production rate of a new speciesby absorption of solar radiation is given byj k ( h ) = z'-jk(A)nj(h)@O(A) exp ( - c ' j N j ) s (3)A jwhere Ojk is the cross-section for absorption by the jth constituent leading to process k andn&) is the number density of theith constituent.The cross-section ajk is the fraction ofthe total absorption cross-section which yields the product of interest. This cross-sectionmay describe ionization, dissociation, dissociative ionization, fluorescence, product excit-ation or other processes. It will be observed in (3), that both the specific process cross-section ojk and the total absorption cross-sections for all atmospheric constituents are neces-sary before accurate production rates at a given wavelength can be calculated.At certain wavelengths and altitudes there are effectively only one or two absorbingconstituents.13 In addition, from measurements of the solar flux at a number of wave-lengths as a function of altitude, the number densities, and total absorption cross-sectionsin some cases, can be found.13The solar flux below about 1300 A consists almost entirely of emission lines, althoughthe hydrogen Lyman continuum does extend throughout the region 913-8OOA.In orderto calculate production rates, it is essential to know the cross-sections at these lines. Ourmeasurements of the total absorption cross-sections are given for nitrogen and oxygenin table 2 together with the proposed classification and the general type of absorptionoccurring at the solar line. We have selected these lines from recent photoelectric in-tensity measurements.14 A number of other lines have been found by several inves-tigators,lsl16 but they are weaker.The hydrogen Lyman continuum can be readily observed in the upper atmospheresolar spectra extending from 913 to about 800 A.With some improvement in noise levels,it will probably be possible to observe the absorption spectra of nitrogen and perhaps oxygensuperimposed on the continuum. For identification of spectra and possible numberdensity measurements, the appropriate sections of fig. 1 and fig. 3 may be utilized.The estimated errors in table 2 are in several cases larger than the estimates previouslygiven. This is in order to take into account special factors which increase the error estim-ates, such as impurity lines in the region, low background light intensity at the edges ofthe continuum background, " apparent pressure effect " in unresolved bands, and cross-sections outside the range of most accurate measurement.The values are considerablybetter relative to each other than absolutely.Several intense lines in table 2 deserve special discussion. At 1025.7 A, H Ly p, we findoxygen has a moderate cross-section of 1-5 Mb. Other values have been 2*O,17 1~6~181.6,' 1.9,lQ and 2.2 Mb.20 There does not appear to be any reason to place less weighton any of these results. The average of 1.8 Mb is probably the best value at this time, andall measurements are within f 15 %, which is the smallest error claimed.The nitrogen cross-section at 972.5 A, H Ly y, is very large. We find the cross-sectionto be 300 Mb, and other measurements are 370,21 150,17 11 7 and 190.20 There is a large" apparent pressure effect " because of insufficient resolution, and the measured valuesshould be considered to be lower limits.With this consideration and also because speciallow pressure measurements were done, the most reliable value is probably the largest one,or 370 Mb. This value is within the estimated error of our measurement. Our oxygen-cross-section at H Ly y is 31 Mb, which is identical with another recent measurement.19A higher value of 50 Mb has also been obtained.17The He I line at 584.3 A is one of the most intense lines in this wavelength region, butunfortunately it is relatively weak and also partially self-absorbed in our light source.Therefore the % error shown in table 2 is larger than for most of OUT measurements. Fornitrogen, our measurement is 36 Mb. Other measurements are 18,17 26,22, 15,23 19 24and 30.20 As mentioned earlier, we corrected for fluorescence, which may have affectedsome earlier measurements. For oxygen, we find the cross-section to be 23 Mb.OtheR . E. HUFFMAN, Y . TANAKA A N D J . C. LARRABEE 165investigators find values of 16,17 20,18 and 21.19 It is apparent that the differences be-tween all these measurements is larger than desirable, but it is not possible to rule out anyof these at this time. It is probably significant that the photographic technique measure-ments,17* 18, 22923 in which the gas is placed in the spectrograph itself, are generally lowerthan the photoelectric technique measurements,19-219 24 in which the gas is confined to acell. The average of all values is 24 Mb for nitrogen and 20 Mb for oxygen.TABLE 2.-ABSORPTION CROSS-SECTIONS AT IMPORTANT SOLAR EMISSION LINES.CROSS-SECTIONS r~ IN UNITS OF MEGABARN, OR 10-18cm2. FOR MEANING OF SYMBOLS, SEE NOTE1 (A)1037.61031.91025.7991.5989.8977.0972.5949.7937-8930.7904835.3835.1834.5790.2790.1787.7780.3770.4765-1703.8702.3686.3685.8685.5685.0629.7625610599.6584.3desig.0 VI0 VIN I11N I11 c I11H LY yH LY rHLYtHLYPHLYEc I10 I110 III0 I10 IV0 IV0 IVNe VIIINe VIIIN IV0 1110 I11N I11N I11N I11N I11o vMgXMgX0 111He I%---1-9 f0.41.1 f0.40.6 f0.3300 f705*2f1*010 f44.8 f 1.96.3 f 1.015 f 526 f103.3 f0.525 f 429 f 512 f 219 f 315 f 278 12026 f 326 f 327 f 327 f 425 f 426 f 435 f 736 6 734 IL-735flO36 f12BELOW.remarks-j-bandcont .cont.P, k-bandrn-band edgeP, n-bmdp , Ryd; A , 21p, R Y ~ ; x, 40p, R Y ~ ; x, 40Ryd; A, 33Ryd; A , 33cont.cont.cont.cont.cont.P, bandcont.cont .cont .cont.cont.cont .F, cont.F, cont.F, cont.F, cont.F, cont.002< 0.4< 0.61.5 f0.31.7 f0.36.0 f2.04.7 f0.731 f 56.3 f l .05.0 f0-727 1411 f l - 513 fl.512fl-513 f1.532 f 432 f429 f 431 f422 f 324f332 f 624 1 524 f 523 f 524 f 530 f641 f 835 f 744f1236 f1223 f 8remarkscont .cont.cont.cont.bandcont.H-bandM-band edgecont .bandcont.cont.cont.bandbandbandbandcont.bandF, cont.F, cont.F, cont.F, cont.F, cont.F, u-bandF, cont.F, cont.F, cont.F, cont.M-bandF, R Y ~ ; x , 41No=.-The following symbols are used to indicate region of the absorption spectrum of the solarline : cont., is continuum ; band, band showing designation previously given ; Ryd, A 30 Rydbergband, member of series having apparent quantum number 3 and converging to the t) = 0 levelof the A state of the molecule ion; P, band shows apparent pressure effect, Fmeans fluorescencecorrection applied.CONCLUSIONTotal absorption cross-sections of molecular oxygen and nitrogen have been given forthe wavelength region 580-1050A.Cross-sections for oxygen have also been given in theSchumann-Runge continuum.These cross-sections can be used for calculation of forma-tion rates of reactive species and of particle concentrations in the atmosphere. They canalso be used to interpret solar flux measurements as a function of altitude. Comparisonwith other measurements has been made where possible166 N2 AND 0 2 CROSS-SECTIONSAgency, Reaction Rate Program, WEB 07010.This work has been supported in part by the U.S. Defense Atomic Support1 Huffman, Tanaka and Larrabee, J. Chem. Physics, 1963, 39, 910.2 Huffman, Tanaka and Larrabee, J. Chem. Physics, 1964,40,356.3 Huffman, Tanaka and Larrabee, AppI. Optics, 1963, 2, 617.4 Ogawa and Tanaka, Can. J. Physics, 1962,40, 1593.5 Watanabe, Ado. Geophy~ics, 1958, 5, 153.6 Huffman, Tanaka and Larrabee, J . Chem. PhyJics, 1963,38, 1920.7 Watanabe and Marmo, J. Chem. Physics, 1956, 25, 965.8 Douglas and Herzberg, Can. J. Physics, 1951, 29, 294.9 Wilkinson and Mullikan, Astrophys. J., 1957,125, 594. Carroll, Astrophys. J., 1959, 129, 794.10 Metzger and Cook, Report no. ATN-63(9218)-1, Aerospace Corporation, El Segundo,11 Ditchburn and Heddle, Proc. Roy. Sac. A , 1953, 220, 61. Schneider, J. Chem. Physics, 1937,12 Heddle, J. Chern. Physics, 1960, 32, 1889.13 Hinteregger, J. Atmos. Sci., 1962, 19, 351.14 HaIl, Damon and Hinteregger, Space Research 111, ed. by Priester (North-Holland Publ. Co.,15 Violett and Rense, Astrophys. J., 1959, 130, 954.16Detwiler, Garrett, Purcell and Tousey, Ann. Geophys., 1961, 17, 263.17Clark, Physic. Rev., 1952, 87, 271.18 Weissler and Lee, J. Opt. SOC. Amer., 1952, 42, 200.19 Mansunaga and Watanabe, contrib. no. 33, Hawaii Institute of Geophysics, U. of Hawaii,California, 1963.5, 106. Ladenberg and Van Voorhis, Physic. Rev., 1933,43, 315.Amsterdam, 1963), p. 745.1961.20 Cook and Metzger, Report no. ATN-63(9218)-4, Aerospace Corporation, El Segundo, Calif.,1963.21 Itamoto and McAllister, contrib. no. 29, Hawaii Institute of Geophysics, U. of Hawaii, 1961.22 Weisser, Lee and Mob, J. Opt. SOC. Amer., 1952, 42, 84.23 Astoin and Granier, Compt. rend., 1957,244, 1350.24 Marmo, quoted by Watanabe, Adu. Geophysics, 1958, 5, 190

ISSN:0366-9033    

DOI:10.1039/DF9643700159

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

Ion and Charge Exchange Reactions involving AtmosphericGasesBY J. SAYERS AND D. SMITHElectron Physics Dept., The University, Birmingham 15Received 15th January, 1964In order to study reactions between ions and neutral particles, a series of experiments have beencarried out which utilize time resolving mass-spectrometric monitoring of the afterglows in atmo-spheric gases produced by radio-frequency discharges. The following reactions have been investig-ated : (i) the charge exchange rates of the O+ ion with oxygen and nitrogen ; (ii) the charge exchangerates of the He+ ion with oxygen and nitrogen. Rate coefficients and cross-sections are presentedfor some of these reactions, together with suggestions concerning the mechanisms of the reactions.Since measurements of the rates of reaction between ionized and neutral atomsof atmospheric gases were required at thermal energies, the afterglow method wasused. Microwave methods could not be used in the present studies since the chargetransfer reactions investigated produce no obvious effect on the electron loss process.The method is essentially that of Dickinson and Sayers 1 and consists of ionizingthe gases under investigation with a high power radiofrequency discharge pulse andobserving the ion density decay rate in the afterglow. The method assumes that thecurrent flowing to the mass spectrometer collector is proportional to the instan-taneous undisturbed ion density in the afterglow plasma.The discharge plasma isproduced in a large glass envelope.In the afterglow, the ions and electronsrapidly reach thermal equilibrium with the gas. In this way, ions are producedhaving a Maxwellian energy distribution and their reaction with neutral species canbe studied at thermal energies. Similar work has been carried out by Fite and hisco-workers.2, 3The great advantage of the mass-spectrometric method is that positive identifica-tion of the reactants and products in a given reaction can be obtained and also themass spectrometer can be used as a monitor of any impurities present in the after-glow. These measurements are now possible due to the development of mass spectro-meters with short ion path-lengths which reduce considerably the chance of reactionstaking place between the ions and neutral species in the mass spectrometer itself.In all the measurements carried out in the various gas mixtures, neutral heliumwas the dominant species present and He+ was the dominant ion formed in theionizing pulse.In all cases, the helium was at a sufficiently high pressure so as toact as an effective buffer to the diffusion of ions and electrons to the walls of thedischarge vessel. In one series of measurements described, the rate of decay ofthe He+ ion in the presence of neutral oxygen and then nitrogen was studied. Themost important product of the reactions was in the first case the Of ion and in thesecond case the Nf ion. The rate of decay of the O+ ion in the presence of mole-cular oxygen and then molecular nitrogen is also described.EXPERIMENTALThe discharge vessel consisted of a Kodial glass cylinder of diameter 14-5 cm and overalllength about 30 cm.The electrode system consisted of two parallel flat nickel discs of16168 ION AND CHARGE EXCHANGE REACTIONS11 cm diam. inside the vessel placed about 16 cm apart and two external sleeves of copperabout 5cm wide placed about 7cm apart. The mass spectrometer was contained in aside-arm of the main discharge vessel, the ions formed in the discharge tube entering themass spectrometer through a small circular orifice of 0.18 mm diam. at the centre of aNilo-K disc of 2.5 cm diam. which was effectively sealed to the wall of the discharge vessel.Particular attention was paid to obtaining good vacuum conditions. Kodial glass wasused in preference to Pyrex glass: the latter is reputed to give off chloride ions whenbombarded in a gas discharge. Bakable metal taps were used throughout the system tothe exclusion of all grease joints and rubber gaskets.The discharge vessel was baked to350°C using an oven and the rest of the tubulation, gas train, etc., was outgassed usingheating tapes. Where possible, metal parts were outgassed by eddy current heating. Thisallowed the residual pressure in the system to be reduced to 10-7 mm Hg. Cold traps wereincluded above mercury diffusion pumps and in the gas circulating system. The massspectrometer was differentially pumped, the gas being returned to the main dischargechamber. Hence, during operation, the system was isolated from the evacuating pumps.The gases used in the experiments were spectroscopically pure gases obtained from theBritish Oxygen Co.Ltd., and the glass containers were sealed to the system via metal taps.Pressure measurements were made with a McLeod gauge.The ionizing pulses were of 10 psec duration at a frequency of 7 Mc/sec and a repetitionfrequency of 50 pulses/sec synchronized to the a.c. mains supply. The power in the pulsecould be varied continuously from the minimum required to initiate the discharge to about200 kVA. A constant discharge power of about 25 kVA was usually used in these ex-periments. Ion currents flowing to the mass spectrometer collector were usually - 10-9 Aand in order to measure these currents as a function of time they were passed throughthe grid resistor of a wide-band amplifier, the resulting time-varying amplified voltage beingdisplayed on an oscilloscope screen.To obtain the gas mixture in the required proportions, the gas required at the lowestconcentration, usually oxygen or nitrogen at a pressure of - 10-3 mm Hg, was first intro-duced and its pressure measured accurately with the McLeod gauge.Helium gas was thenquickly introduced to raise the total pressure to approximately 0.5 mm Hg pressure.Measurements of decay rates of various ion species were obtained by photographing theoscilloscope display together with a calibration time trace. From the photographs, theconcentration of the tuned ion species as a function of time in the afterglow was obtained.In some cases, it was found necessary to use a cine camera to record the oscilloscope traces,since a rapid movement of the oscilloscope trace resulted during the first few ionizing pulses.RESULTSOXYGENPreliminary measurements of the decay rate of the Of ion density in oxygenwere made by Dickinson and Sayers.1 The discharge tube was not baked, the onlyattempt at cleaning it was by running a discharge in helium for several hours.Fromthe semi-logarithmic plots of ion density as a function of time in the afterglow,a decay rate 13. can be obtained for different partial pressures of oxygen. A plotof A as a function of oxygen partial pressure should produce a straight line, theslope of which provides a value for the rate coefficient k of, in this case, the reactionThe value given by Dickinson and Sayers iso++02+o,’+o.k = 2-5+0*4 x 10-11 cm3 sec-1 at 293°K.The experiment was repeated in the present apparatus without baking and thevalue obtained for the rate coefficient wask = 2.6-&0-4 x 10-11 cm3 sec-1 at 293°KJ .SAYERS AND D. SMITH 169It was noticed, however, that under these conditions an ion of mass 12, probablycarbon, was present in the afterglow as an impurity.The experiment was repeated using the improved vacuum techniques describedin the previous section. The decay of the 0' ion was much slower than beforeand the graph of decay constant against partial pressure of oxygen showed lessscatter of the points about a straight line(see fig. 1). Also, ionized carbon was not0 4 6 8 10 I2oxygen partial pressure in mm Hg x 103FIG.1.-Decay constants for the Of ion density in oxygen afterglows at 293°K.observed in the afterglow. The reaction rate coefficient determined from theseresults isk = (1.64+0-05) x 10-11 cm3 sec-1 at 293°K.The corresponding cross-section isCT = (2*15+0.06) x 10-16 cm2.There seems little doubt that the lower value obtained is due to the more stringentvacuum techniques adopted in this work.The temperature dependence of the rate coefficient for the O+ ion reaction withoxygen has also been investigated. Prolonged baking was required in order to ob-tain measurements at the higher temperatures in order to reduce the impurity level.Measurements were also made below room temperature by surrounding the dis-charge vessel with solid carbon dioxide.Rate coefficients and correspondingcross-sections for the various temperatures are given in table 1. Over this restrictedrange of temperature, the rate coefficient appears to be approximately proportionalto the inverse square root of the temperature.TABLE 1temp. rate coefficient."K cm3 sec-1 x 1011452 1-35 -f0*05388 1-72 f0.06293 1.64 f0-05210 2.w *@locross-section,cmzx 10161.32 f0.051-95 f0.072.1 5 f0-063-70 f0-2170 ION AND CHARGE EXCHANGE REACTIONSThe charge exchange reaction occurring here is0++0~-+02++0~and it has been suggested4 that the dominant mechanism is ion-atom interchange.The observed increasing cross-section as the temperature decreases is in qualitativeagreement with the theory of this process.However, on the basis of the Gioumousisand Stevenson equation 5 the theoretical rate coefficient for the reaction is 9 x 10-9cm3 sec-1 which is an order of magnitude greater than the present results.By studying the emission of the oxygen red line (6300A) from the night-time I:layer, which he assumed was emitted from decaying excited oxygen atoms formedin the above charge exchange reaction, Nakamura 6 has calculated a value for therate coefficient ask = 1.3 x 10-11 cm3 sec-1at the temperature in the F layer ( N 1400°K). This value is in close agreement-perhaps fortuitously-with the present work. The phenomena of clean-up ofoxygen was always noticeable in the afterglow to some extent. Hence the photo-graph of the oscilloscope trace had to be taken as soon as possible after the initiationof the discharge.0XYGEN-I-NITROGEN MIXTURESThe reaction between the ionized oxygen atom and the nitrogen molecule is ofespecial interest in the upper atmosphere for it is known that the concentration ofmolecular nitrogen is considerable.The method of examining the reaction is toadd nitrogen to a mixture of oxygen and helium, such as that used in the previousexperiment. Any change in the decay rate of the Of ion must be due to the addedcomponent. An effective total rate coefficient kT may be given by the relationwhere ko is the rate coefficient for ion loss due to the oxygen molecule, kN is therate coefficient for ion loss by the nitrogen molecule, and a is the ratio of the nitrogenpartial pressure to the oxygen partial pressure.The measurements were made in two series, in one of which the nitrogen-oxygenratio was approximately 1 : 1 and in the other series the ratio was approximately2 : 1.It was not consistently possible to obtain nitrogen and oxygen pressuresthat had exact 1 : 1 or 2 : 1 ratios, but these ratios were maintained to within 10 %.The results observed for the decay rate ;1 as a function of oxygen or nitrogenpartial pressure were somewhat scattered, and to obtain reasonable accuracy inthe h a 1 value of the rate coefficient kT, many more measurements were taken thanwith pure oxygen. The values of the effective total rate coefficient kT, obtained for thetwo different mixtures at 293"K, together with the value for the rate coefficient ofthe O+ ion in pure oxygen, are shown in fig.2. From this graph the rate coefficientfor the loss of oxygen by reaction with molecular nitrogen at 293°K iscorresponding to a cross-section ofThe addition of a third gas to the discharge increases considerably the possible ion-exchange reactions. However, most reactions can be excluded in favour of thereaction,kT = ko+akNkN = (2-7 0-2) x 10- l1 cm3 sec- ',0 = (3*6&0.3)x cm2.O++NpNO++N+l.l eVJ . SAYERS A N D D. SMITH 171and it is assumed that this is the most important reaction mechanism. The resultobtained for this reaction can be compared with the value of the charge exchangecross-section measured by Hasted 7 using beam techniques. The value he obtainedwasCJ = 2.8 x 10-16 cm2at an energy corresponding to 32,000"K, and this value tended to zero as the energywas reduced.The value obtained for the cross-section in the present work, althoughcarried out at a lower energy, is larger than the above value.The results described in this and the previous section have been described inmore detail by Batey.8ratio of nitrogen to oxygen partial pressuresFIG. 2.-Variation of the effective rate coefficient for the reactions of the O+ ion in Nz+ 0 2 mixtureswith the composition of the mixture at 293°K.HELIUM -k NITROGEN MIXTURESThe partial pressures of nitrogen were in the range 10-4 to lO-3mmHg andhelium was added to make the pressure up to about 0.6 mm Hg. In fig. 3 the decayconstant ;1 for the He+ ion density is shown as a function of the nitrogen partialpressure for two different discharge powers.The curve is expected to be a straightline with a small intercept on the decay constant axis corresponding to decay ofthe He+ ion by diffusion and conversion to the molecular ion only. However, thecurves in fig. 3 show intercepts on the nitrogen partial pressure axis.In order to explain this it has been suggested that a significant amount of dis-sociation of molecular nitrogen, which is assumed to be the neutral reacting species,occurs in the ionizing pulse. It must be further assumed that the number of dis-sociations is independent of the nitrogen partial pressure so that the partial pressureof molecular nitrogen is decreased by a constant difference from the measuredvalue.The value of this constant difference would decrease with a decrease in th172 ION AND CHARGE EXCHANGE REACTIONSdischarge power, and so the forms of the curves in fig. 3 are explained. This as-sumption is not very satisfactory and the effect could be due to the phenomena ofclean-up of nitrogen in the discharge.3 0 -x0- I I I I I I I I I I 0 I 2 3 4 5 6 7 8 9 10nitrogen partial pressure in mm Hg x 104FIG. 3.-Decay constant for the He+ ion density in helium+nitrogen afterglows as a function ofthe nitrogen partial pressure for two discharge powers at 293°K.On the assumption that the neutral reacting species was N2, the rate coefficientat various temperatures-determined from straight lines drawn to the results asindicated in fig.3-and the corresponding cross-sections are given in table 2.TABLE 2temp. rate coefficient, cross-section,O K c m 3 sec-1 x 109 ~ m 2 x 1014503 1 -03 0.59408 1 -40 0.89293 1 -45 1 -09195 1 *75 1 -60Similar measurements have been made by Fite and his co-workers2 who findcross-sections of - 10-15 cm2. The present results are not sufficiently accurate todefine the form of the dependence of the rate coefficient on temperature. However,since the cross-section decreases with increasing temperature, it may be deducedthat if the mechanism of the charge exchange process is charge transfer the energydefect for the transfer is very small and the process may be accidentally resonant.Bates and Patterson9 have suggested that the complex ion HeN+ is unlikely tobe formed, and so the mechanism of the process is unlikely to be an ion-atominterchange reaction. In the present experiments the dominant nitrogen ion inthe afterglow was found to be the atomic ion N+, contrary to the observations ofFite who found the N i ion to be dominant.The mechanism of the exchangereaction in the present experiments therefore appear to be dissociative charge transfer :He+ + N p H e + N+ + N + 0.3 eV,although the energy defect is rather larger than would be expected on the basis oJ . SAYERS AND D. SMITH 173the experimentally determined variation of the cross-section with temperature.Some evidence was found of the presence in the afterglow of a very small densityof a positive ion of mass 18 a m.u., but it is possible that this was the ion HzO+released from the walls of the discharge vessel rather than the ion HeN+.HELIUM -k OXYGEN MIXTURESThe first measurements in helium+oxygen mixtures were made by a similarmethod to that used for the experiments in helium+nitrogen mixtures. Oxygenpartial pressures between 10-4 to lO-3mmHg were used, and helium was addedto make the total pressure 0.6 mm Hg.However, it was noticed that the spectro-meter signals representing the decay of the He+ ion density changed with time quiterapidly after initiation of the discharge, and that the value obtained for ;1 was de-pendent on the time between the initiation of the discharge and the exposure ofthe recording film. At a given measured pressure the values obtained for ;1 variedby up to a factor of three depending on t h s time.In order to investigate this effectfurther, the signals were photographed with a cine camera. The decay constantsdetermined from various frames of a typical cine film are shown in fig. 4 as a function'Ol-----II I I I I4 8 12 I6 2 0 24I tl3m frame number (time after initiation of discharge)FIG. 4.-Typical variation of A for the He+ ion density in heliumfoxygen afterglows with timeafter initiation of the discharge. Initial partial pressure of oxygen is 3 x 10-4 mm Hg.of the frame number. The frames were numbered starting from " 1 " at the firstafterglow to be recorded. The time after initiation of the discharge is also indicatedin fig. 4.The shape of the curve in fig.4 is largely explained by the phenomenon of clean-up. During an ionizing pulse, and in the afterglow following it, a proportion ofthe oxygen ions diffusing to the walls of the discharge vessel becomes permanentlyattached there. This process occurs during every ionizing pulse, but the numberof available sites for oxygen on the walls becomes smaller as the number of occupiedsites increases, and so the clean-up rate may be expected to decrease with time.The decay constant for the He+ ion density by charge exchange with oxygen may,therefore, be expected to decrease rapidly at first, and then more slowly as the rateof clean-up of oxygen decreases. The diffusion of oxygen from other parts of th1 74 ION AND CHARGE EXCHANGE REACTIONSapparatus into the discharge vessel would tend to reduce the rate of decrease of theoxygen partial pressure in the discharge vessel.The initial slow rate of decreaseof the decay constant is difficult to explain. It seems that before the oxygen may beadsorbed on to the walls, the walls must first be prepared by bombardment withoxygen ions. A possible reason for this is that the sites for oxygen must first bemade available by the removal of other less active atoms, such as helium, which mayinitially occupy the sites.If the curve of the decay constant against frame number is extrapolated to zeroframe number, the decay constant indicated should be that which would be ob-served if the clean-up were not important. In fig. 5 the values of the extrapolatedoxygen partial pressure in mm Hg x 104FIG.5.Variation of the extrapolated h for the He+ ion density in helium+oxygen afterglowswith the partial pressure of oxygen at 308°K.decay constant at 308°K have been plotted against the partial pressure of oxygen.As the point at zero oxygen partial pressure, which corresponds to the decay of theHe+ ion density in pure helium, is more accurately known than all the other pointsthe best fit line to the experimental points has been drawn to pass exactly throughthis point. The points lying below the line, and marked with dots, represent decayconstants that have been determined from a particular frame number of the cinefilms. They show how clean-up may affect the value obtained for the decay con-stant if the measurements are not made immediately after initiating the discharge.It would be possible to fit lines to the results modified by clean-up in the same waythat lines were fitted to the results of the helium+nitrogen experiments, i.e., withan intercept on the partial pressure axis (see fig.3). However, the intercept in theseresults is smaller than that in the helium+nitrogen results. As oxygen is a morereactive gas than nitrogen, the clean-up of oxygen was expected to be faster thanthat of nitrogen, and so a larger intercept was expected in the helium+oxygenresults on the basis of the suggested clean-up mechanism. A reliable predictionof the variation of the intercept with the gas concerned cannot be made on thebasis of the dissociation mechanism suggested in order to explain the intercept,because the cross-sections for the dissociating reactions are not known.The useof the cine camera technique in further helium + nitrogen experiments is, thereforeJ . SAYERS AND D. SMITH 175required before the cause of the intercept may be satisfactorily explained. Therate coefficient determined from the line drawn in fig. 5 on the assumption that theneutral reacting spscies is the 0 2 molecule isk = (1.05 f. 0.08) x 10-9 cm3 sec-1,and the corresponding cross-section isB = (7.7+0-6) x 10-15 cm2.The errors in these measurements are probably due largely to inaccuracies in themeasurement of pressure. Fite and his co-workers2 have recently made similarmeasurements and found a rate coefficient of 5 x 10-10 cm3 sec-1, and a corres-ponding cross-section of about 5 x 10-15 cm2.In the present experiments, as in those of Fite and his co-workers, the dominantoxygen ion in the early afterglow was the atomic ion Of.This suggests that theprocess occurring is either dissociative charge transfer,He+ + 0 p H e + O+ + 0 + 5.9 eV,where some of the energy released is taken into excited states of the products, orthe ion-atom interchange reaction suggested by Bates and Patterson,9He++02+(HeO+)’+O7followed by a radiative dissociation,(HeO+)’+He+ O++hv.Evidence was found to suggest the presence of a very small density of a positiveion at mass 20 a.m.u. in the very early afterglow. This signal appeared only whenoxygen was admitted to the discharge vessel, and so was possibly the ion HeOf.In fact, the rate coefficient for the He++02 reaction determined by the presentexperiments is in agreement with the value indicated by Bates and Patterson forthe ion-molecule reaction (on the basis of the Gioumousis and Stevenson equation).If the ion-molecule reaction is the dominant charge exchange process, the HeOfion must dissociate very rapidly after formation because the mass 20a.m.u. ionwas present only in very small quantities in the very early afterglow. Fite andhis co-workers found no trace of the HeOf ion in heliumfoxygen afterglows, andthey suggest that dissociative charge transfer is the dominant process.Design and construction of the apparatus used in the initial stages of the work,together with the results concerning the O+ ion, are due to Dr. G. R. Court andP. H. Batey.This research was supported in part by the Geophysics Research DirectorateAFCRL of the Air Research and Development Command, U.S.A.F.1 Dickinson and Sayers, Proc. Physic. SOC., 1960, 76, 137.ZFite, Smith, Stebbings and Rutherford, J. Geophysic Res., 1963, 68, 3225.3 Fite, Rutherford, Snow and Van Lint, Disc. Faraday SOC., 1962,33, 264.4 Bates, Proc. Physic. SOC. A , 1955, 68, 344.5 Gioumousis and Stevenson, J. Chem. Physics, 1958,29,294.sNakamura, J. Geomag. Geo.-elec., 1961,12, 114.7 Hasted, Proc. Roy. SOC. A , 1951,205,421.8 Batey, Ph. D. Thesis (University of Birmingham, England, 1962).9 Bates and Patterson, Planet. Space. Sci., 1962, 9, 599

ISSN:0366-9033    

DOI:10.1039/DF9643700167

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

New Technique for the Study of Ion-Atom InterchangeBY C. H. BLOOMFIELD AND J. B. HASTEDUniversity College, LondonReceived 17th January, 1964The mean cross-section for an ion-atom interchange process is deduced from an analysis of thetwo mobility peaks displayed by a mass-analyzed ion-beam injected into a drift tube. The experi-ment is described and a mean cross-section 0 . 4 ~ 10-16 cmz at a temperature of 540°K is deducedfor the process,NeAr++ Ar +Ne+ Arz ;the cross-section falls sharply with increasing temperature. Various findings concerning the mobil-ities and identification of ions 0+, 02, Ar+, Ar+2 Ne+ in Ar and N2 are reported.Ion-atom interchange processes, often termed ion-molecule reactions, can playimportant roles in the ionic kinetics of the upper atmosphere.Among the nineoxygen-nitrogen positive ion-atom interchange processes which may occur in theionosphere, listed by Bates and Nicolet,l are the following :O++N2-+NO+ +N (1)o+ + 0 p O f + 0. (2)The unexpectedly low rates of these processes are believed to be responsible for thepersistence of Of ions long after sunset.21 3Various estimates of the rates of these processes have been reported. 289 29 Usingthe Dickinson and Sayers 4 technique of mass-spectrometric monitoring of mon-atomic ion density in gas discharge afterglow, we have measured 5 rates as follows :k(02) = (1-8 k0.2) x 10-12 x 10-12 cm3 sec-1 molecule-1,k(N2) = (5.4k0.5) cm3 sec-1 molecule-1,at temperature T = 300°K. Experimental data can only be obtained over a narrowtemperature range by oven-heating the apparatus.Since mass-spectrometersource techniques 6 apply only to much higher energies, and since the pulsed electronbeam technique 7 is strictly for laboratory temperatures, there is an intermediatetemperature range 600-5000°K, hitherto inaccessible experimentally.It might be supposed that the temperature dependence of these rate coefficientscould be deduced from classical orbiting theory,g-lO were it not for the exceedinglysmall transmission coefficients. Since these cannot be explained by competingcharge transfer processes,11 or by repulsive forces 12 it is possible that the energyof the orbiting complex is not equally distributed among all degrees of freedom; 10under these circumstances the classical orbiting theory is not dominant, but givesan irrelevant upper limit.The temperature region with which we are concerned (0.04 eV < 3/2kT< 0.25 eV)can be attained in shock tubes, and also by means of a drift tube, in which an ionswarm drifts through a gas under the action of a uniform electric field.14 In sucha swarm the velocity distribution of the ions, of mass m+, in gas of mass rng is17C .H. BLOOMFIELD AND J . B. HASTED 177Maxwellian, and the root-mean-square velocity 6 is determined by the random(thermal) and drift velocities 13 6,. and v :The drift velocity (cm sec-1) is proportional to field strength X and inversely pro-portional to pressure ; reduced to unit field (V cm-1) and pressure 760 torr, it isknown as the reduced mobility X.Now the reduced mobility of an ion in a (non-parent) gas is given 15 byX = 35*9/(ap)*, (4)where a is the gas molecule polarizability in (a.u.)3, r(i the reduced mass in a.m.u.Thisrelation is adequately based in classical theory and there are sound reasons whyquantum theory affects the situation only slightly. The relation holds, and themobility remains constant, up to a “critical value” of X/’, above which it risesslowly. The critical X/p is mass-dependent, varying, in aitrogen,l4 from N 5 V cm-1torr-1 for Rb+ ions to -55 V cm-1 torr-1 for Lit ions.However the mobility of an ion in its parent gas will be smaller than that of otherions, being affected by the symmetrical resonance charge transfer process, and cal-culable 16 from its average cross-section if, measured in units of nu; :Moreover the mobility of an ion in its parent gas decreases with increasing X/p.Thus a positive identification of an ion by means of its mobility can often be made.Suppose that a drift tube contains a gas inert to inelastic collision processeswith low energy ions A+, of known species, which are injected into the tube aftermass analysis.A single mobility “ peak ” is observed. Into the tube is introduceda small proportion of reactant gas B, which in collision with A+ ions, will form oneor more ions by charge-transfer or ion-atom interchange processes. For the presentwe consider a single process, which we represent asA+ + B-+ A + B+.A second mobility “ peak ’’ will be observed, corresponding to those B+ ions formedby collisions during the drift.From the magnitude of the peak can be deducedthe rate constant for the process; from the mobility, the mass number of the pro-duct ion; and from eqn. (3) the appropriate temperature. Although some similardeductions concerning inelastic collision processes have been made from conven-tional mobility experiments with unidentified primary ions 17 and with mass-analysis ofthe detected ions,ll the injection of a mass-analyzed ion beam into a tube designedto measure ion-atom interchange or charge transfer rate coefficients has not hithertobeen proposed.* We now discuss the proposal in greater detail, report measure-ments made in argon and in nitrogen, and comment briefly on the merits and defectsof the method in comparison with others.x = 2.10 x 104/z(p~>+.( 5 )(6)THEORY OF THE METHODFor the drift velocity measurement we use a pulsed version (similar to that ofref. (19)) of the original “ four gauze ” method of Tyndall and Powell ; 20 thesubstitution of pulses for sinusoidal potentials is designed to overcome certaincriticisms.21 The diagram of the experiment (fig. 1) includes ion injection equip-ment and a chamber into which the steady lO-9A ion beam enters at orifice 0and drifts in the gas under the action of a uniform 10 V cm-1 electric field produced* The proposal was originally made by Sir Harrie Massey in discussion178 ION-ATOM INTERCHANGEby electrodes ABCDEI-~G, until it is collected at Faraday cylinder F. The swarmmust pass through four grids ABCD, which are normally held so that no positiveions can pass, as in the potential diagram of fig.2 ; since the ions are drifting atnear-thermal velocities, they are unable to " travel uphill '' in this diagram. ByFIG. 1 .-Diagram of the apparatus.FIG. 2.Definition of symbols for mobility tube distances, potential diagram,and block shapes.the application of short voltage pulses to A and D (fig. 3), " bloksc " of ions areadmitted (fig. 2). Suitable variation of the delay period T between these pulses allowsthe ion block to pass through both pairs of grids, so that time-integrated " peaks "of current f a r e measured at FC . H. BLOOMFIELD AND J. B. HASTED 179The @T) functions for primary ions A+ are triangular in shape (fig.4). Wesuppose for simplicity that each primary ion A+ is already thermalized at 0 andf t-0IFIG. 3.-Diagram of applied pulses.FIG. 4.-Diagram of peaks.drifts with velocity VA, possessing a probability /? of forming a secondary ion Bfin each 1 cm of drift :is the mean cross-section for interchange process (6), iir the random thermalFor initial A+ ion density n O ~ cm-3 the primary ion peak equations are :velocity of A+, and k the rate constant.forTA, < T < T A ~ , 1~ = (~,A~~)[T-TA,+~,-((~IV*)] eXP (-B(a+d+B)I; (8180 ION-ATOM INTERCHANGEforwithT A 2 G T g T A 3 9 ?A = j n O A / z ) [ T - T A 2 - t p + ( g / v A ) ] exp ( - P ( a + d + g ) ) , (9)T A 2 = (d+y)/VA T A l + t p - ( g / u A ) , (10)TA3-tp+(g/VA) = TA2, (1 1)and other quantities as defined in fig.2, 3.ion velocity VB is smaller than VA (case l), and when VB> VA (case 2).The secondary ion peak equations assume different forms when the secondaryIn case 1,(12)withandForwithFor(14)withIn case 2, VA<VB, the primary ion peak equations remain unchanged, but thesecondary ones become :forwitC. H. BLOOMFIELD A N D J . B . HASTED 181withandThus the exponential sections of the secondary ion peaks, A3-B3 (case l), or B2-B3(case 2) may be analyzed with the aid of eqn. (15), (22), to yield the collisonprobability p.The assumption of primary ion thermalization at 0 is of course not fulfilled.It is necessary to be certain that thermalization takes place well before A; this isachieved by variation of the distance OA until primary ion mobility peaks are ofthe correct shape and the measured mobilities are in agreement with establishedvalues.It is possible that secondary ions are formed, with a kinetic energy dis-tribution and with a distribution of probabilities, during the thermalizing drift.If this effect contributes, the peaks will be enhanced in the region of B4 (case l),or B1 (case 2).Since in the two-peak structure we do not observe such enhancement nor un-desirable distortion of the exponential B2-B3 sections, it is permissible to neglectpre-thermalization processes, provided that the correct peak shapes are observed.Possibly in such cases the collision probability falls off with some rapidity with in-creasing temperature.EXPERIMENTALThe ion source, S in fig.1, is of the oscillating electron type due to Finkelstein,22 withion extraction parallel to the magnetic field H I , which is of magnitude 1000G, produced bya ring-shaped permanent magnet. All source orifices S1-5 are + in. dim. and the filamentis of 0.03 in. x 0.001 in.x 0-7 in. tungsten ribbon. The ion beam is focused by a threeorifice lens and mass-analyzed by a 90" sector magnet H2, 1850 G, permanent hut withvariable gap width; the magnet is mounted inside the vacuum chamber, but is adjustablefrom outside, along the diagonal X.Part of the ion beam passes through orifice 0, 1 mm diam. in 1 mm ferry sheet, into thegas-filled drift tube. After an adequate thermalizing distance found to be 1 cm, the swarmpasses through nine 1-in.apertures in 0.032 in. ferry sheet, to be collected at the Faradaycage F. The apertures A and B, mutually separated by 1 mm, are fitted with grids wovenof 0-03 f l z ~ dam. tungsten wires spaced 0-5 mm apart. C and D are identical grids, andthe spacing of all apertures is 1 cm. The drift tube is electrically screened, and screene182 ION-ATOM INTERCHANGEconnection is made from F to an EKCO 1079C vibrating reed electrometer. BakeableElectrosil metal-oxide stabilized resistors for maintenance of aperture potentials are mountedinside the vacuum chamber.DISCUSSIONFor confidence to be placed in a quantitative estimate of a rate coefficient by thismethod, a two-peak analysis is mandatory. We have therefore commenced in-vestigations by searching for two-peak patterns, using as injected ions Of, O i ,Ar+, A$+, Nef, and as drift tube gases Ar, Ne, N2, pure and as mixtures.At present, only two satisfactory two-peak patterns have been observed : we con-sider first the injection of Nef ions into Ar.The pattern for X/p = 15 V cm-1torr-1 is displayed semi-logarithmically in fig. 5. The primary mobility Z = 2.6683 ’- 1 IA- A 2 3 2 t fA 3k 1 I I160 2 0 0 ms 240TFIG. 5.-Peak diagram for injection of Ne+ ions into Ar at 0475 mm pressure ;field strength 75 V cm-1.cm2 sec-1 V-1, but at low X/p this is reduced to 2-50 cm2 sec-1 V-1. Thus, the prim-ary ion is not Nef. The mobility corresponds well with the ion NeAr+ when eqn.(4) is applied.The very careful exclusion of water vapour, etc., is believed to pre-clude the possibility of clustering. The secondary mobility Z = 1.76 cm2 sec-1 V-1is in agreement with the middle one of the three argon mobilities measured byBeaty,lg for which the identification Arz has been made mass-spectrometrically.30For Arf injected into Ar we obtain Beaty’s lowest mobility 1-57 cm2 sec-1 V-1 atXj’p = 21 Vcm-1 torr-1. Because it is decreasing with rising X/p, this mobilityhas been presumed to correspond to the Ar+ ion. We measure the identicalmobility for Ar2f injected into Ar. In this case, conversion by one electron cap-ture in the thermalization region is likely in view of the cross-sections for thisprocess recently measured by beam techniques.27Analysis of the section A3-B3 of fig.5 according to eqn. (15) yields a conversionprobability /3 corresponding to a mean cross-section 5 = 0-4 x 10-16 cm2 at aC. H. BLOOMFIELD AND J . B. HASTED 183effective temperature of 540”K, determined by the application of eqn. (3). Theconversion process may be writtenNeAr’ +Ar-+Ne+Ar:. (27)However, when the X/p is raised to 20Vcm-1 torr-1 the secondary peak ismuch reduced, and at 25 V cm-1 torr-1 it has disappeared altogether. Thus themean cross-section may be presumed to have fallen by an order of magnitude fora temperature rise of order 102°K; this sharp and unusual cut-off need not be as-sumed if a competing dissociation of Arz were taken into account. However, itexplains why the NeAr+ ions are not destroyed in the thermalization region.Thereremains the question of their formation.Diatomic ions composed from two rare gas atoms are well-known in electricaldischarges, and have been supposed to arise either in collisions of metastableatoms,23 or in three-body processes similar to that which we propose here :Rate coefficients for such processes are available 24 although not the coefficientfor this particular collision. In view of the wide range of energies in the thermal-izing region we make no attempt to estimate a rate coefficient. Ne2+ ions injectedinto argon behave in an identical fashion to Nef, which in this case can be formedby one electron capture?However, the injection of O+ ions into Ar yields a low X/p single mobility peak at2.6 cm2 sec-1 V-1, with a critical X/p of 10 V cm-1 torr-1.This is similar to that ofNeArf, and might be identified as OArf. 0; injected into argon behaves in anidentical fashion, the ion being dissociated to Of in the thermalization region.The OAr+ formation precludes the study of O+ processes in argon buffer gas, sothat we are confined to the use of pure nitrogen.The identification of ions from their mobilities in nitrogen is on satisfactorytheoretical ground,25 and for N i ions into nitrogen we observe a low Xlr, mobility2.7 cm2 sec-1 V-1, with a critical Xj’p of 17 V cm-1 torr-1. This is in good agree-ment with the Bristol data 14 and with more recent measurements ; 26 the inter-pretation is believed to be NB.The injection of Of into pure nitrogen yields a single mobility 2-7 cm2 sec-1V-1 at X/p 10 V cm-1 torr-1, and critical X/p of 20 Vcm-1 torr-1.Throughout the low Xfp OfN2 experiment, a flattening of the peak structurewas observed on the low mobility side.At high Xj’p, this developed into a secondpeak, the mobility of which was -2 cm2 sec-1 V-1 at X/p 40 V cm-1 torr-1 fallingto -1-8cmzsec-1V-1 at Xlp 60Vcm-1 torr-1. An identification as N t cantherefore be made; the mobility is in reasonable agreement with the high energydata of Varney.26From analysis of the two-peak structure, a mean cross-section 5 - 9 x 10-17 cm2at a mean energy 0.5 eV is calculated. However, at present, the data and interpre-tation for this process must be regarded as preliminary.Ne++Ar+Ar-+NeAr++Ar. (28)We are grateful to Sir Harrie Massey for the facilities of his laboratory and tothe Department of Scientific and Industrial Research for financial support of thiswork, and a grant to one of us (C.H. B.).1 Bates and Nicolet, J. Atmos. Terr. Phys., 1960, 18, 65.2 Bates and Nicolet, J. Atmos. Terr. Phys., 1961, 21, 286.3 Herzberg, 1961, J. Atmos. Terr. Phys., 1961, 20, 177.4 Dickinson and Sayers, Proc. Physic. Soc. A, 1960,76,137184 ION-ATOM INTERCHANGE5 Langstroth and Hasted, Disc. Faraday SOC., 1962, 33, 698.6 Schissler and Stevenson, J. Chem. Physic., 1955, 23, 1353 ; 1956, 24, 926 ; 1958, 29, 282.7 Talroze and Frankevich, Zhur. Fiz. Khim., 1960, 34, 2709.8 Langevin, Ann. Chim. Phys., 1905,5, 245.9 Gioumousis and Stevenson, J. Chem. Physic., 1958, 29, 294.10 Firsov, Zhur. eksp. teor. Fys., 1962, 42, 1307.11 Talroze, Pure Appl. Chem., 1962,5,455.12 Boelrijk and Hamill, J. Chenz. Physic., 1961, 34, 730.13 Wannier, Bell. Syst. Tech. J., 1953, 32, 170.14 Tyndall, The Mobility of Ions in Gases (Cambridge University Press, 1938).15 Dalgarno, McDowell and Williams, Phil. Trans. A, 1958, 250, 41 1.16 Dalgarno, Phil. Trans. A, 1958, 250, 426.17 Kovar, Beaty and Varney, Physic. Rev., 1957, 107, 1490.18 McDaniel, Martin and Barnes, Reu. Sci. Instr., 1962, 33, 2.19 Beaty, Proc. 5th Int. Conf: Ioniz. Phenomena in Gases, Munich (North-Holland Publishing Co.,20 Tyndall and Powell, Proc. Roy. SOC. A , 1932, 136, 165.21 Crompton and Elford, Proc. Physic. SOC., 1959, 74, 497.22 Finkelstein, Rev. Sci. Instr., 1940, 11, 94.23 Hornbeck and Molnar, Physic. Rev., 1951, 84, 615, 621.24 Pahl, 2. Naturforsch., 1959, 14a, 239.25 Dalgarno, Ann. geophys., 1961, 17, 16.26 Varney, J. Chem. Physics, 1959, 31, 1314.27Hasted, Lee and Hussain, Proc. 3rd Int. Conf: Electronic and Atomic Collisions (London)28 Potter, J. Chem. Physics, 1955, 23, 2462.29 Talroze, Markin and Larin, Disc. Faraday SOC., 1962, 33, 257.30 McAfee, Amer. Physic. SOC. Gaseous Electronics Conference, 1963.Amsterdam, 1961).(North-Holland Publishing Co., Amsterdam, 1964)

ISSN:0366-9033    

DOI:10.1039/DF9643700176

出版商:RSC    

年代:1964

数据来源: RSC