Reactions of 0 2 (I&) and O2 (lCg+)BY L. W. BADER AND E. A. OGRYZLODept. of Chemistry, University of British Columbia, Vancouver 8, CanadaReceived 14th January, 1964A method of obtaining measurable concentrations of 02(1Ag) and Oz(lXC,+) for kinetic studieshas been developed, and a number of reactions of these excited molecules are described. Evidenceis presented for simultaneous electronic transitions in a weakly bound complex between two 02(1Ag)molecules resulting in emission bands at 6340 and 7030A. The phenomenon is discussed in termsof the " association theory " of gases.The (0,l) 1 -58 ,u band of the (1 A, - 3Z;) system is one of the more intense featuresof the atmospheric " day glow " (the (0,O) 1.27 p band is re-absorbed by the loweratmosphere).l A number of excitation mechanisms have been suggested by VallanceJones ; 2 however, all of them depend on assumptions about the deactivation cross-sections for O@A,) which have not yet been determined.A prominent featureof the " night air glow " is the (0,l) 8645 A band of the (1Z$-3E;) system (the(0,O) 7619 band is also re-absorbed). Here again deactivation cross-sectionshave not been determined and Young and Sharpless 3 have indicated that no satis-factory excitation mechanism has yet been proposed.Both of these species are known to be present in electrically discharged oxygentogether with oxygen atoms, where the ratio of O@Z;) : 0 : 02(1Ag) : 02(1Zi) isusually about 1 : 0.1 : 0.05 : 0.0015.4 When these products are pumped out of thedischarge into a fast flow system the atom concentration is essentially unalteredsince both the gas phase and wall recombination are sl0w.5 The 02(1Zl) concen-tration is still large; however, since it is continually being formed by the recom-bination of atoms, its reactions cannot be studied conveniently in this system.Thereis, in the literature, only indirect evidence for the presence of 02(1Ag) in the dis-charge products. Foner and Hudson6 showed the presence of a species with anappearance potential about 1 eV below that of ground-state oxygen. They sug-gested that it might be 02(1Ag), in which case about 10 % of the oxygen could be inthis state. Elias, Bgryzlo and Schiff 5 presented calorimetric evidence that wasconsistent with about 10 % 02(1Ag) in the discharge products.They also describedthe following three methods that could be used to remove atoms preferentially fromthe gas stream. (i) NO2 can be added to the stream. Atoms are rapidly removedand some calorimetrically-detectable excited molecules remain. However, thepresence of NO and NO2 then complicates any kinetic studies. (ii) A coil of silverwire placed in the gas stream rapidly recombines the atoms and more slowly de-activates the excited molecules. (iii) The distillation of mercury through the dis-charge creates a mercuric oxide deposit after the discharge which removes atomsbut not excited molecules. Using these sources of excited molecules we have begunan investigation of some of their reactions.EXPERIMENTALA typical Pyrex fast-flow system5 was equipped with Edwards needle valves and a581./min vacuum pump. The discharge was maintained in air-cooled 13 mrn Pyrex tubingby a Raytheon 2450 rncfsec, 100 W generator.Two light-traps separated it from the4L. W. BADER AND E. A . OGRYZLO 47reaction-tube. The 20mm (int. diam.) reaction-tube was jacketed and insulated so thatthe temperature could be varied between 173 and 373°K. Two different spectrographscould be placed against an optical window at the end of the reaction tube. A low dis-persion (5 4.6) Hilger-Watts glass prism monochromator was equipped with a 27 c/secmechanical chopper, an RCA-7265 (or 7 102) liquid-nitrogen-cooled photomultiplier anda conventional tuned a.c. amplifier. With this instrument very weak emissions could bedetected between 3000 and 13,OOOA.A Jarell-Ash (5 6.3) grating spectrograph blazedat 7500 A with a dispersion of 20 A/mm in first order was used when greater resolution wasrequired.An isothermal calorimetric detector 5 could be moved along the reaction tube. Its cobaltsurface removes all detectable excited molecules from the gas stream. By operating thecoil at a constant temperature the total " energy content " of the gas stream could bedetermined.For most work, ordinary tank oxygen was used without further purification. Whennecessary, it was dried by passing it over P205 and through a liquid-nitrogen trap. Nitrogen-free oxygen was prepared by thermally decomposing KMnO4. Other gases were obtainedfrom the Matheson Company.Ozone was prepared with a commercial generator andstored on silica gel at dry-ice temperature.Preliminary studies indicated that under conditions where atoms were completelyremoved from the gas stream by a silver wire, the excited molecule concentration was alsogreatly reduced. This was not found to be the case when mercury was used to destroythe atoms, and hence the later method was used in all studies reported in this paper.RESULTSCurve (a), fig. 1, shows a typical example of the energy liberated to a cobaltdetector by the products of electrically discharged oxygen from which all the atomshave been removed. The presence of the slightest trace of atoms is easily detectable20 3 0 4 0distance from discharge (cm)FIG. 1 .-Curve (a), heat measured by cobalt detector in excited molecule stream ; P = 3 mm Hg ;T = 300°K.Curve (b), heat measured when a partial pressure of 0.03 mm Hg of water is addedafter the discharge.by the yellow-green NO2 emission to which the eye is very sensitive. When allthe atoms have been removed, a weak red emission becomes visible. Fig. 2 showsthe emission detectable with the Hilger-Watts spectrograph and a cooled 7102photomultiplier. The most prominent peak is at 7619A. This is undoubtedlythe (0,O) band of the (lX;-3Z;) intercombination. The band at 8645A is thenthe (0,l) transition in the same system. Photographs of these bands with the highe48 REACTIONS OF 02(1Ag) AND 02(2S;)dispersion grating spectrograph show the characteristic rotational fine structure ofthe (lZl-3S;) transition, confirming the assignment.We have been unable todetect either the (1,l) band at 7708 A or the (1,O) band at 6882 A, though thetransition probabilities are comparable to those for the (0,O) and (0,l) transitionsrespectively. This is taken as evidence for the absence of appreciable concentrationsof vibrationally excited oxygen.000II///06340 7030 7619 8645 12700A <A>FIG. 2.-Emission spectrum of electrically discharged oxygen with all atoms removed obtainedwith Hilger-Watts glass-prism spectrometer, slit 500 microns, and liquid nitrogen cooled RCA-7102 photomultiplier. Spectral sensitivity of RCA-7102 given by dotted curve.The peak at 12,700A is undoubtedly the (0,O) band of the (1Ag-3Z;) inter-combination, confirming previous mass spectrometric 6 and calorimetric 5 evidence.Since the sensitivity of the photomultiplier at 12,700 A is less than 10-2 of its maximumvalue at 8000A, the emission is fairly intense.The peaks at 6340 and 7030A cannot be attributed to any known transitionin 0 2 .They have been observed in a number of chemiluminescent reactions insolution ; 79 8 however, Khan and Kasha's 9 recent assignment of these peaks tosolvent shifted (0,O) and (0,l) (1Zl-3Zi) transitions must be in error. Fig. 3shows the 6340 A band photographed with a grating spectrograph (20 &mm dis-persion). Neon calibration lines are superimposed on it. Only a structurelessdiffuse band could be obtained. To eliminate the possibility that it is an " impurityemission ", nitrogen-free oxygen was prepared and thoroughly dried with P205.The intensities of the peaks were essentially unaltered.Furthermore the additionof nitrogen and water before the discharge, and NO?, NO, H20, N20, Ar, HeL. W. BADER AND E. A. OGRYZLO 49CO, C02, NH3 and H2 after the discharge had no significant effect on either the6340 or the 7030A peak. The 7619A peak was, however, affected by a numberof gases. Water had the most marked effect (heavy water was about half as effec-tive). A small amount of water added after the discharge completely removed the7619 and 8645A bands and left the 6340, 7030 and 12,700A bands unaltered.This observation shows that the two new bands do not involve 02(1X$).The lack of reactivity of the species responsible for this emission, the presenceof only two bands, and the large spacing between the bands allows us to eliminateother 0 2 such as C3A,, A3E; and clZ;.-rexcited states of r .r.."6217 6266 6334 6302 64021 (A)FIG.3.-The 6340A band obtained with a Jarell-Ash f 6-3 grating spectrograph, 2 0 & m dis-persion, 30 micron slit, 72 h exposure on Kodak 103 a F spectroscopic plate. Small peaks aresuperimposed neon calibration lines.A study of the relationship between the 02(1Ag) concentration and the new bandsprovided more positive evidence for their source. A direct comparison of the6340 peak and the 12,700 A peak was not possible because of the extremely variablesensitivity of the cooled photomultiplier in the 12,700 A region.However, thedetector could be used to measure the 02(1Ag) calorimetrically. Curve (b), fig. 1,illustrates the effect of water (added after the discharge) on the energy content ofthe gas stream. Spectroscopically the removal of 02(lXg) can be seen by the reduc-tion of the 7619A emission intensity to less than 1 % of its original value. The02(1Ag) concentration remains unaltered. In all of our experiments we have foundthat the 6340 A emission intensity is proportional to the square of the heat measuredby the detector. A typical set of results is given in table 1, where it is assumedthat the heat measured by the detector is due entirely to 02(1Ag).TABLE 1P = 4-4 ~llfn Hg ;T = 25°C ;0 2 flow = 160 pmoles/sec ;H20 flow = 0.5 ,u moles/sec.experiment 102(1 AJ flow (pmoles/sec) 2.5 1.7I = 6340 A emission intensity (arbitrary units) 55.0 27-5microwave power output (watts) 100 20C = 02(1Ag) concentration (moles/cm3) 3 .7 ~ 10-9 2 . 6 ~ 10-911/12 = 2-00; Ci/C2 = 142 ; (Ci/C2)2 = 2.050 REACTIONS OF 02(lAg) AND 02(1E:)In view of these observations, and the fact that twice the electronic energy of02(1Ag) is equal to 15765 cm-1 or 6343 A, we propose the following processes toaccount for the two emission bands.M2O2('A,)~O~+(O4),=, + hv(6340 A),20,(1Ag)+O:-(04),=1 +hv(7030 A). MTo determine whether the emitting species (0:) is simply a colliding pair ofmolecules or a stabilized dimer, the temperature of the reaction tube was varied.Fig. 4 shows the emission spectrum recorded with an RCA-7265 photomultiplier.16340 7 0 3 0 76 191 (A)FIG. 4.-Effect of temperature on the excited molecule emission.RCA-7265 photomultiplier,500 micron slit. Curve (a) obtained at 25°C ; curve (b) at -29°C.Because of its different spectral response it yields 7619 and 6340A peaks of com-parable height. Curve (a) was obtained at 25°C and curve (b) at -29°C. In theflow system the pressure remains constant when the temperature is changed in asmall region of the system. Assuming ideal behaviour (P = cRT), the concentra-tion of all independent species will be inversely proportional to the temperature.Hence the concentration of 02(1CB+) would be expected to increase by a factor of2981244 = 1.22 when the temperature is changed between 25 and -29°C.The ratioof the 761 9 A emission intensities at these temperatures (obtained by integrating underthe curves in fig. 4) is 1-21 which is the expected result within experimental error.The ratio of the emission intensities for the 6340A band is, however, 1.97. Since thL. W. BADER AND E. A. OGRYZLO 51emission is proportional to the square of the 02(1Ag) concentration, a factor of(1 *22)2 = 1-49 is due to the concentration change alone. The additional decrease weattribute to the dissociation energy of 0:. If we assume that the process consists ofthe following steps :K02('Ag)+ 02('Ag)+ M+O,('Ag, 'A,)+M, (3)04('Ag, ' ~ i ~ ) - + o ~ ( ~ X ; "Xi)+ hv(6340 A), (4)kwhere M is any third body, k is the emission probability defined by I = d(hv)/dt =k[04(1Ag, lAg)], I is the 6340 A band emission intensity, K is the equilibrium con-stant for the formation of dimolecular complexes (abbreviated to dimols in the restof this paper), i.e.and the dimol concentration is governed by K (i,e., that reaction (4) is the rate-controlling step) ; then since04(1Ag,lAg) = I/k and 02(1Ag) = P/RT(where P = partial pressure of 02(lAg)),Hence, on substitution into the thermodynamic relationship,In (IlT:/I2T,2) = AU(T~-TI)/RT~T~,where AU is the change in internal energy and equals the bond dissociation energyD at some average temperature T.Substituting into this equation the values forZ and T given above, one obtains - AU = D(02(lAff) -02(1Aff)) = 600 cal.K = C04(lAg, 1Ag)llCo2(1Ag)129.'.K = RZIT2/kP2.In (K1IK2) = AU(T~-TI)/RTIT~,REACTIONS WITH OZONEThe effect of the addition of 0 3 to the excited molecule stream is shown in fig. 5.Visually the effect is dramatic since the weak red emission is masked by the strongyellow-green NO;! emission which results from the reaction : 0 + NO-,NO2 + hv.It can be seen that the 02(1Zgf) concentration is lowered by more than a factor of10, suggesting that the dissociation is due to this species, i.e.02(12,')+ 0 3 + 2 0 2 ( 3 ~ , ) + 0. ( 5 )o(~P)+ 03(1A)+02(1Ag)+ 0,(~2;). (6)On the other hand, the 6340 and 7030 A bands increase when 0 3 is added. If thismeans that the 02(1Ag) concentration increases, it may be produced in the reaction,This explanation is not entirely satisfactory in view of the effect of the addition ofwater shown in fig.6, where 99 % of the Oz(lX,+) has been removed by water.Ozone is, however, still strongly dissociated. No change in the 02(1Zi) concen-tration occurs and surprisingly the 02(1 Ag) concentration (taken from the 7030peak height) seems. to have risen. Clearly, eqn. (5) and (6) are not sufficient. Itis possible, however, that these results can be explained by some energy chain thatis only initiated by excited molecules.NO2 - GLO WWhen NO2 is added to the gas stream containing 02(1E:) and 02(1Ag), theconcentrations of the excited molecules are not greatly altered even when larg52 REACTIONS OF 02(1Ag) AND 02(1Ei)I I I I I6340 7 0 3 0 7119 8645 1270A (A)FIG. 5.-Effect of ozone on emission.Curve (a), excited molecules only; curve (b), excitedmolecules + ozone.I6 340 7030 7619 8545 12700A (4FIG. 6.-Effect of water on ozone reaction. Curve (a), excited molecules+water; curve (b),excited molecules, water and ozoneL . W. BADER AND E. A. OGRYZLO 53amounts are added. However, as the NO2 concentration is increased the normalred emission is masked by a higher energy emission that has a spectral distributionvery similar to that due to the 0 plus NO reaction shown in fig. 5. However, therecan be no atoms present in this system because of the large NO2 concentration.When H20 is added to the stream the NO2 glow decreases in intensity. It disappearswhen sufficient water is added to remove completely 02(1Z3.DISCUSSIONWe believe that the 6340 and 7030A bands described in this paper can be ex-plained only by the presence of dimolecular complexes (dimols) of 02(1Ag), witha bond dissociation energy of about 600cal.The two bands then arise fromsimultaneous electronic transitions in the loosely associated pair. The 6340 Aemission arises when only ground vibrational states of 0 2 are involved, while the7030A band arises when one of the 0 2 molecules ends up in its first excited vibra-tional level. Though such simultaneous electronic transitions do not seem to havebeen reported previously in emission, they have been proposed to account for anumber of absorption bands in high pressure systems.Rank et aZ.10 reported the appearance of some new absorption bands in pureHCl and HC1 mixtures with Ar and Xe that appear at high pressures.The intensityof the bands was found to be proportional to the square of the pressure. Fromthe temperature variation of the absorption they calculated the following bonddissociation energies :D(HC1-HC1) = 2.14 kcal, D(HC1-Ar) = 1.1 kcal, D(HC1-Xe) = 1.6 kcal.Hoijtink et aZ.11 reported a study of the absorption spectrum of mixtures ofnaphthalene and 0 2 in chloroform. They observed a new band at 29,000cm-1with an intensity proportional to the concentration of naphthalene and 0 2 . Theyattribute the band to a simultaneous transition in a naphthalene-02 complex.The effect of a change in temperature was not reported so that it is not possible tosay whether dimols are responsible.More relevant to our work is the absorption spectrum of liquid and gaseousoxygen.Ellis and Kneser 12 were the first to attribute peaks observed in liquidoxygen at 6290 and 5770 to the process :where the 6290A peak arises when all the species are in their ground vibrationalstate, and the 5770A peak arises when one of the 02(1Ag) molecules ends up inits first excited vibrational level. Salow and Steiner 13 studied the pressure depend-ence of these bands in highly compressed gases, and found that they dependedon the square of the oxygen pressure and were independent of the pressure of addedgases, confirming Ellis and Kneser's assignment, More recently Dianov-Klokov 14studied the temperature dependence of the band intensities. He found that theychanged little with temperature, possibly increasing slightly between 77 and 298°K.Dianov-Klokov therefore concluded that the absorption was due to an 0 4 collisioncomplex.This result might appear inconsistent with our emission studies.However,the different temperature dependence may be due to the fact that the absorptionintensities were measured at a constant density of 1.17 g/cm3, which is the densityof the liquid. Under these conditions the average intermolecular distances arefixed at the values that would be assumed in our proposed dimol. Hence, changingthe temperature could well have no appreciable effect on the absorption intensity54 REACTIONS OF 02(1Aq) AND 02(lC;)For any given gaseous system there are two distinct sources for both simultaneousand induced transitions in molecules : (i) collision complexes, or (ii) dimols (stabil-ized dimolecular complexes).With the information available at present it is notpossible to state when the shorter intermolecular distance in the collision complexis more important to the transition probability than the longer lifetime of the dimol.However, it is certain that a number of simultaneous and induced transitions canbe explained only by dimols, and there appears to be no evidence in the literatureinconsistent with the idea that dimols are universally responsible for such transitionsat low temperatures.In a system containing 02(3X;), 02(1 A& and 02(1XB+) we might expect the follow-ing dimols to be present :(1) 04(3x;, 3q); (2) 04(?4g, lAg);(3) 0 4 ( 1 q , l q ) ; (4) 0d3q¶ ‘$1;(5) 04(3z;, l g ) ; (6) 0 4 ( 1 q , lAg).At any given temperature the dimol equilibrium concentration will depend on itsbond dissociation energy and the concentration of the appropriate molecules. Thedissociation energy of dimole (1) can be estimated from PVT data. With someassumptions about the form of the interaction potential, the interaction energy Ecan be calculated from the second virial coefficient.Assuming a Leonard Jones(6 : 12) potential one obtains 15 ~ ( 0 2 - 0 2 ) = 230 cal. Unfortunately, AU andthus the dissociation energy at some temperature T, is not easily calculated fromE. However, Stogryn and Hershfelder 15 have calculated Kp (the equilibriumconstant for dimol formation) from the second virial coefficient.Curve (a) in fig. 7shows a plot of loglo Kp against 1/T from their data. From the slope of the curveat 273”K, - AH = 840 cal, and therefore - AU(273”K) = D(O2-02) = 300 cal.These results, however, still depend on the assumed form for the interaction potential.A more direct estimate of AH can be obtained from “association theory”.16Assuming that the deviations from ideal gas behaviour at moderate pressures isdue to (a) the size of the molecules, and (b) the formation of dimols, trimols, etc.,we can write the equation of state (for one mole of gas) :where b = excluded volume = 4 (molecular volume) ; KD and KT are the equilbriumconstants for the formation of dimols and trimols respectively.If we consider conditions where only the dimol term is significant we obtain uponrearranging :P(V-b)= RT(1-pKD-p2KT-.. .),PV/RT = 1 - P(K, - b/RT).A plot of PV/RT against P for oxygen at several temperatures is shown in fig. 8.The slope is equal to -(Kp-b/RT). Assuming a value of b = 0.0318 the valuesof Kp obtained are plotted in fig. 7, curve (b). From the slope of the curve at273”K, -AH = 940, and - AU = D(O2-02) = 400 cal. It is not possible todecide between this value and that obtained from Stogryn and Hershfelder’s data.Dimol (2) with a dissociation energy somewhat greater than dimol (1) gives riseto the 6340 and 7030A bands. Dimols (3) and (6) could give rise to bands at 3810and 4760A respectively. We have not been able to detect any emission at thesewavelengths.This may be due to the lower concentration of O#C:), or to a smallerdissociation energy for the dimols. It is possible, however, that one of them isresponsible for the N02-glow. From the change in the 7619A emission bandintensity with temperature it appears that emission from dimol (5) does not adL. W. BADER AND E. A . OGRYZLO 55to the emission in this band. This is consistent with the fact that the molar ex-tinction coefficient for the highly compressed gas does not differ significantly fromthat of the low pressure gas.139 14 Because the 12,700A band intensity could not01/Tx lo3 (OK-1)FIG. 7.-A plot of the loglo of the equilibrium constant Kp for dimol formation against l/Tfor oxygen.Curve (a), from the data of Stogryn and Hershfelder; curve (b), from associate theory.P (atm)FIG.8.-Plot of PV/RT against P for oxygen from the data of Birdsall, Jenkens, Dipaslo, Beattieand Apt, J. Chern. Physics, 1955, 23,441.be accurately measured, it was not possible to determine whether dimol (6) con-tributed to this band. From the magnitude of the pressure-induced absorption 14one might expect it to be significant at about 1 atm pressure. However, the rota-tional fine structure observed under these conditions by Herzberg and Henberg16suggests that it is a simple molecular absorption56 REACTIONS OF 02(1&) AND 02(1xi)The research for this paper was supported by the Defence Research Board ofCanada, Grant number 9530-31 and by the Air Force Office of Scientific ResearchGrant number OSR-158-63.1 Noxon and Vallance Jones, Nature, 1962, 196, 157.2Vallance Jones and Gattinger, Planet. Space Sci., 1963, 11, 961.3 Young and Sharpless, J. Geophys. Res., 1962, 67, 3871.4 Noxon, Can. J. Physics, 1961, 39, 11 10.5 Elias, Ogryzlo and Schiff, Can. J. Chem., 1959, 37, 1680.6 Foner and Hudson, J. Chem. Physics, 1956, 25,601.7 Seliger, Anal. Biochem., 1960, 1, 60.8 Bowen and Lloyd, Proc. Chem. Soc., 1963, 305.9 Khan and Kasha, J. Chem. Physics, 1963,39,2105.10 Rank, Sitaram, Glickerman and Wiggins, J, Chem. Physics, 1963, 39, 2673.11 Dijkgraaf, Sitters and Hoijtink, J. MoZ. Spectr., 1963, 6, 643.12 Ellis and Kneser, 2. Physik, 1933, 86, 583.13 Salow and Steiner, 2. Physik, 1936, 99, 137.14 Dianov-Klokov, Opt. i. Spektr., 1959, 7, 377.15 Stogryn and Hershfelder, J. Chem. Physics, 1959, 31, 1531.16 Ogryzlo, J. Chem. Educ., to be published.17 Herzberg and Herzberg, Astrophys. J., 1947, 105, 353