118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions any particular point on the rigid surface becoming in turn negative neutral and positive these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. 118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions any particular point on the rigid surface becoming in turn negative neutral and positive these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. 118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions any particular point on the rigid surface becoming in turn negative neutral and positive these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. 118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions any particular point on the rigid surface becoming in turn negative neutral and positive these conditions arisdg in any order. The observed contact difference is the average effect of these conditions.Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. 118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions any particular point on the rigid surface becoming in turn negative neutral and positive these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. 118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions any particular point on the rigid surface becoming in turn negative neutral and positive these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. 118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions any particular point on the rigid surface becoming in turn negative neutral and positive these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. 118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions any particular point on the rigid surface becoming in turn negative neutral and positive these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. Absorption Spectrum of Bromine from 6200 to 51002$ BY J. A. HORSLEY AND R. F. BARROW Physical Chemistry Laboratory Oxford University Received 26th September 1966 The absorption spectrum of the 3110+u-1&! system of bromine between 6200 and 5100A has been photographed at high resolution using each of the separated isotopic species 79Br2 and 81Br2.A rotational analysis of ten bands of each isotopic species has been carried out. The accepted vibra- tional numbering was confirmed. The constants obtained (cm-1) for each species are :- 79Br2 W = 325.37 W = 168.88 x&$ = 1.75 Y ~ O = -00061 &' = 0.082114 Bi = 00594 U = 0.00043 y = -0oooO1 X:O; = 1.098 I$ = 0*00032 r; = 22809 r = 2-68 A. up" = 321.29 0 = 16683 ~ b i = 1.71 Y ~ O ; = -0W57 X ~ O ; = 1.064 &' = 0-080088 a," = 0000319 Bi = 00576 a; = 000038 y; = -o*m11 r; = 2.2809 ri = 2-69A. Revised constants for the most abundant isotopic species 79 81Br2 have been calculated. The last four bands immediately preceding the onset of the continuum have also been analyzed for each species. A limiting curve of dissociation has been plotted in each case leading to an average value for DO (79 81Br2) of 15,893.1 f 1.0 cm-1.The absorption spectra of the 3rIof-1Z+ systems of both C12 and 12 have been re-analyzed recently.1-3 In both cases the accepted vibrational numbering for the upper state was found to be in error and the constants for the upper state and ground state were revised. An analysis of some of the bands of the corresponding system of Br2 carried out by us in order to obtain an accurate potential curve by the Rydberg- Klein-Rees method indicated that the accepted constants for this molecule might also require revision. The absorption spectrum of the main system of bromine was first photographed at high resolution by Brown,4 who analyzed 13 bands of the most abundant species 79 81Br2 and obtained rotational and vibrational constants for this species.He also analyzed a number of bands of the less abundant isotopic species 79Br2 and 81Br2 and confirmed his assignment of the vibrational numbering of the upper state obtained from an earlier vibrational analysis. No further work on the rotational structure of the bromine bands has been published. Between 6,000 A and the onset of the continuum at 5108 A the spectrum shows little structure and the density of lines is great. This arises partly because natural bromine consists of a mixture of three isotopic species 79Br2 81B1-2 and 79 81Br2 with relative abund- ancies close to 1 1 2 respectively. In order to simplify the spectrum and reduce the blending caused by the high density of lines the separated isotopic species 79Br2 and 81Br2 were used instead of the natural mixture.32 The absorption spectrum of natural bromine is extremely complex. 1 I i - 1 E 0 P- 10 2 - I E 0 c I- 9 I. n 0 co 5 Y To face page 33.1 J . A. HORSLEY AND R. F. BARROW 33 EXPERIMENTAL The samples of 79Br2 and *1Br2 were prepared from isotopically pure K79Br and K8lBr. Very small amounts of bromine could not be prepared by the usual wet method oxidation of KBr with concentrated sulphuric acid as the bromine evolved was absorbed by the small quantity of acid distilled over with it. Oxidation of KBr with solid chromium trioxide was a suitable dry method of preparation. Adequate yields of bromine were obtained with no unwanted by-products. The apparatus used is shown in fig. 1. The part of the apparatus used for the preparation was sealed on to an absorption cell consisting of a Pyrex tube 1 m long and 3 cm diam.with Pyrex windows sealed on to each end. A short side-arm was joined on to each end of the cell. The apparatus used for the preparation consisted of two Pyrex tubes approximately 10 cm long and closed at one end connected by a short length of narrow tubing. The top of one of these tubes was sealed on to a side arm of the absorption cell. FIG. 1.-The absorption cell and apparatus for the preparation of isotopically pure Brz. About 20 mg of the isotopically pure KBr was placed in the open tube and about 0.1 g of dry CrO3 placed on top of it. The lower part of the other tube was cooled in liquid air. The reaction tube was stoppered and the mixture heated gently. Bromine vapour was evolved immediately and was distilled into the cooled collecting tube.When the reaction was complete the whole apparatus was evacuated. The reaction tube was sealed off and removed and %ally the whole apparatus was sealed off under vacuum. Bromine was then allowed to e orate into the absorption cell. The weight of KBr used in the preparation was sufficient bo obtain a pressure of about 2 mm Br2 in the absorption cell. The absorption spectrum was photographed over the range 62WA to the onset of the continuum at 5108 A using a 3.4 m Jarrell-Ash Ebert spectrograph which had a reciprocal dispersion of about 0-5A/mm and a resolving power of up to about 500,000 in this region. A high-pressure xenon arc was the source of continuous illumination and exposure times varied from 2 to 5 min. The plates used were high contrast Ilford R52 plates.The lines of an iron hollow cathode lamp were used to provide standard wavelengths. The lines of the spectrum were measured on a Zeiss AbbB comparator capable of reading to 0.0001 cm. The accuracy of measurement was estimated at &0.02 cm-1. Plate 1 shows part of the 12-2 band of 81Br2 and the onset of the continuum at the dis- sociation limit of 8lBr2. ANALYSIS AND CALCULATION OF CONSTANTS The transition has been established 5 as 3 1 1 g + u - 1Zl and hence the bands consist of P and R branches only. The bands show an intensity alternation the lines with odd J being stronger than those with even J. Compared with the spectrum of natural bromine considerably more structure was visible and the analysis was straightforward. Approximately 800 lines of the spectrum of each molecule were assigned to 10 different bands.In order to confirm that the lines had been assigned correctly and to check the rotational numbering of each band a preliminary set of constants were obtained by fitting the lines of each band to the expression v = vo + 2Bm -+ ABm(m f I)+ 4Dm3 - ADm2(m + 1)’ where rn = J+ 1 for an R line = -J for a P line. 2 34 ABSORPTION SPECTRUM OF BROMINE B and D are the lower state values for these constants AB and AD are the differences between the lower and upper state values of these constants. Final values of the constants were obtained using a method devised by Aslund 6 in which the term values of the upper and lower states of all the observed lines are calculated relative to a pair of arbitrarily chosen reference levels. The rotational constants for each vibrational state were then obtained from the term values by poly- nomial fitting.This method leads to constants of greater accuracy than those ob- tained by conventional methods. The values of Bv and Dv obtained in this way are given in table 1. TABLE 1 .-ROTATIONAL CONSTANTS cm-1 XIS,+ = 0 1 2 3 3rI()+" = 9 10 11 12 13 16 17 19 8.1948 &0*0010 8.7643 f0.0007 8.1308 f04007 8.0988 f0.0007 5.4341 f0.0010 5.3687 f0.0008 5-3039 f0.0008 5.2363 f0.0007 5- 1 644 & 0.0007 4.9399 f0.0008 4.6988 f0.0008 lWD 1.8 f0-35 2.6 f0-25 1.9 50.2 2.1 f0-2 4.8 f0.4 4.4 f0.2 5.0 10.2 5.2 f0.2 5.1 f0.2 5.5 5 0 . 3 7.4 f0.2 81Br2 1 OZB 7-9904 f0.0010 7.9640 f0.0007 7.9304. f0.0007 7.8954 fO.0010 5.3020 f0.0010 5.1773 10.0010 5-1 143 f0.0008 50482 f0.0006 4.8333 50-0009 4.7620 f0.0010 4,6005 50-0008 lO8D 2.0 f0-4 4.0 f0.2 2.4 f0.2 1.0 f0.5 3.2 f0.5 3.7 50.4 4.6 f0-3 5.4 f0.2 5.6 f0.3 7.0 f 0.4 7.2 f0.3 Limits are in all cases standard deviations.The relative rotationless term values for all the vibrational levels analyzed were also obtained by this method. The rotationless term values of the four vibrational levels of the ground state gave three values of the vibrational interval AG,++ These were fitted to the linear relationship The rotationless term values of the seven upper state vibrational levels were fitted to the expression G = constant+w,(v+~)-x,w,(v+~)2 AG,+ = o,-~x,co,(v+$). The accepted vibrational numbering was assumed to be correct. Using the vibrational constants for 79Br2 the isotopic shift in the band origin was calculated for each band analyzed.The calculated values of the shift together with the observed values are given in table 2. The agreement confirms the accepted vibrational number- ing. The vibrational constants obtained for each species are given in table 3 to- gether with the calculated values of the vibrational constants for 79 81Br2. The previously accepted vibrational constants are also given in brackets after the new values. The lower state B values were fitted to the linear expression B = B,-u(v++) J . A . HORSLEY AND R. F. BARROW 35 and the upper state B values to the expression The rotational constants obtained are given in table 4. B = B,-a(u++)+y(v+*)2 band 9-3 11-3 11-2 12-2 13-2 13-1 16-2 19-1 19-0 TABLE 2 band origin (cm-1) 79-79 16,2 1 3.68 16,473.95 16,792.73 16,917-01 17,037.37 17,358.34 1 7,3 74-23 17,995.19 18,318.36 81-81 16,211-68 16,469.69 16,784.61 16,9OTW 17,027-39 17,344.41 17,362-28 17,978.35 18,297.5 1 obs.shift (cm-1) 240 4.26 8.12 9.1 1 9.98 13.93 11.95 16.84 20.85 TABLE 3.vIBRATIONAL CONSTANTS OF BROMINE 79-79 81-81 lower state calc. shift (cm-1) 2.02 4.26 8.13 9.09 9.96 13-90 11-91 16.82 20.80 79-8 1 m 325.366 f0.003 cm-1 321.29 f0-02 cni-1 323.33 cm-1 (323.2) x:COL( 1.0985 f0.0003 1.064 f0-005 1.081 (1.07) upper st ate 0 168.88 10-1 cm-1 166.83 f0.1 cm-1 167.85 cm-1 (1 69.7 1) x;w; 1.75 zto.01 1-71 10.01 1.73 (1.91) Y;wL - 0.0061 f0.0002 - 0.0057 f0.0002 + 0.0059 (- ) Previously accepted values are given in parentheses. TABLE 4.-ROTATIONAL CONSTANTS OF BROMINE 79-79 81-81 79-8 1 lower state Be 0-0821 14 f0.000006 0.080088 f0-00002 0.081 101 (0.08091) re 2.2809 f0.0003 A 2.2809 f0.0003 8 2.2809 8 (2.28 A) a 0*000322 f0.000002 O.oO03 19 f0.000009 0.000321 (0.00028) D (Kratzer) 2-09 x 10-8 1.99 x 10-8 2-05 x 10-8 upper state Be 0.0594 f0-0002 0.0576 10.0002 0.0585 (0.0596) re 2-682 f0.005 A 2.690 f0.005 8 2-686 8 (2.65 A) a 0.00043 &O.ooOOl 0.00038 f0.00002 0-00041 (0.00062) Y - 0.0000 1 & 0~00OOOO4 - 0.0000 1 fO~OOOooo4 - 0.00001 (- ) D (Kratzer) 2.93 x 10-8 2.74 x 10-8 2.8 x 10-8 Previously accepted values are given in parentheses.DISSOCIATION ENERGY The region of the spectrum immediately before the onset of the continuum was examined in detail in order to obtain information about the rotational predissociation. Although the structure appeared to be very complicated it was possible to analyse the 36 ABSORPTION SPECTRUM OF BROMINE last four bands of each species (49-0 to 52-0).Determining the last line in the band proved difficult in some cases because there are so many lines in this region. How- ever it was possible to check the estimated last line by plotting the limiting curves of dissociation for the two isotopic species. The two curves obtained are almost identical which strongly suggests that the estimated last lines are correct. The two limiting curves are shown in fig. 2. The shape of the curves indicates that there is no I I I 5 0 0 1000 I S 0 0 2doo ’ JV+Q 19575 X last observed line; 0 first missing line. FIG. 2.-Limiting curves of dissociation for 79Br2 and *1Br2. maximum in the potential curve of the upper state. The two curves were extrapolated to give the term value of the convergence limit of the upper state relative to v” = 0 for each species.The two values are 79B1-2 = 19,577.2&0*5 cm-1; f31Br2 = 19,578.9 5 0 . 5 cm-1. The convergence limit corresponds to dissociation into a bromine atom in its ground state (2Ps) and a bromine atom in a 2P+ excited state. The 2P+ state lies 3,685 cm-1 above the ground state. Hence the dissociation energies D of the ground TABLE 5.-79Br2 bands immediately before the dissociation limit band vo(cm-1) B, 49-0 19,563.65 -0129 35 50-0 19,568-20 -01 14 28 51-0 19,571.77 -0097 23 52-0 19,574.47 ~0084 17 S of last line TABLE 6.-81Br2 BANDS IMMEDIATELY BEFORE THE DISSOCIATION LIMIT 49-0 19,562.28 -01 35 - 50-0 19,567-45 -0121 39 51-0 19,571.61 -0097 29 52-0 19,574.81 -009 1 20 band VO(Cm-9 B J of last line state of each species are 79B1-2 = 16,054.6 & 0.5 cm-1; f31Br2 = 16,054.3 f 0.5 cm-1.The value of DO for 79 -81Br2 is 15,893-1 cm-1 the error is not likely to exceed & 1 cm-1. For a predissociation observed in absorption the lines with upper levels above the true predissociation energy may not vanish completely but only become diffuse, J . A . HORSLEY AND R . F. BARROW 37 depending on the ratio of the transition probability to the decomposition probability. The above predissociation was observed in absorption only. So the true predissocia- tion may take place at a value of J lower than that observed as a slight broadening of the lines would not have been detected and therefore the above value may represent an upper limit for the dissociation energy of bromine. Previous measurements of the TABLE 7.-79Brz BANDS IMMEDIATELY BEFORE THE D~SSOCIATION LIMIT Wavenumbers of observed lines cm-1 49-0 50-0 51-0 52-0 J R(J) P(J) RQ P(J) R(J) P(J) R(J) P(J) 3 4 5 19,561-72 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 560.89 559.97 558.89 557.66 556.30 554.80 553.17 551.37 549.44 547.37 545-23 542-82 540.33 537.71 534.93 532.02 528-94 525.73 522.38 5 18-90 5 1 5-25 51 1-44 507.53 503-43 499.20 490.32 485.63 480.85 - 19561 -45 560.60 559.60 558.47 557- 1 8 555.79 554-22 55253 550.70 548.70 546.61 54-36 541.97 539.41 536.75 533.94 530.96 527.86 524.64 52 1 *24 517.71 514.04 510.21 506.23 502- 1 3 497.92 488.95 484-22 479.34 474.3 6 469.18 - 19,566.90 566.22 19,565.97 565-39 564.42 563-31 562-04 560.60 559.13 557-45 555.62 553.67 551.55 549.22 546-89 544.36 541.65 538.83 535.82 53271 529-39 526.02 522.38 5 18-73 514-78 565.10 564.08 562.92 561-64 560.20 558-63 556.89 555.03 553.03 550.88 548.57 546- 1 3 543.56 540.85 537.93 534.93 53 1.78 528.43 525.01 521.39 517.64 513.72 509.70 505.52 19,570-99 19,570-85 19,573.66 19,573-52 570.45 569-73 568-88 567.88 566.73 565.47 564.08 562.48 560.75 558.89 556.89 554.73 552.44 549.97 547.37 544.62 541.74 538.70 535.48 570.24 56950 568.61 567.61 566.42 565.10 563.65 562.04 560.28 558.38 556.30 554.15 551.80 549-32 546.7 1 543.91 541 *oO 537.93 534.68 527.76 - 573.08 572-36 570-45 569.34 568.02 566.56 564.95 563-20 561.31 559.27 557.06 554.73 - - 572.17 570.24 569-06 567.71 566.22 564.58 562.74 560.89 556.53 554.22 55 1 -62 548.97 - I position of the convergence limit of 79 81Br2 gave a value of 15,890 cm-1 for the dissociation energy,7 in good agreement with the above value.The information used to construct the limiting curves is summarized in table 5 for '9B1-2 and table 6 for 81Br2. The wavenumbers of the lines assigned to the last four bands of each species are given in tables 7 and 8. Enriched samples of 79Br and 8lBr were supplied by the Atomic Energy Research Establishment Harwell. 38 ABSORPTION SPECTRUM OF BROMINE TABLE 8.-SlBr2 BANDS IMMEDIATELY BEFORE THE DISSOCIATION LIMIT Wavenumbers of observed lines em-1 49-0 50-0 5 1 4 52-0 J R(J) P(J) N J ) P(J) R(Jl P(J) R(Jl P(J) 3 19,570-86 19,570.71 4 5 19,560-44 19,560.17 19,565.55 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 559.65 558.75 557.73 556.55 555.22 553.79 55220 550.48 548-61 546.63 54-51 542-26 539-86 537.35 534.66 53 1 *80 528.90 525.8 1 522.61 51 9-22 51 5-76 5 12.09 508.20 504-24 500.06 559.33 558.38 557.27 556.05 554.72 553.1 7 551.56 549.78 547.88 545.82 543.64 541.37 538.92 536.33 533.61 530.76 527.77 524.65 521.38 51 7.98 514.44 5 1 0.76 506.96 502.98 498.85 564-78 1 9.564.46 563.84 562.78 561.60 - 558-75 557.13 555.35 553.48 551 -45 549.27 546.96 544.51 54 1 -92 539.17 536-33 533.29 530.1 8 526-88 523-43 51 9.87 516.13 51 2.28 508.25 504.12 499-80 495-3 1 490.74 485.95 48 1 -08 476.03 470-79 465.47 563.50 562-37 561.12 - 558.21 556.55 554.72 552.76 550.74 548.52 546.1 5 543.64 541 -05 538.28 535.32 532.32 529- 1 1 525.8 1 522.35 518.73 5 14-94 51 1.07 507.05 502.84 498.52 494.03 489.42 484.59 479.67 474.61 469.42 458.52 452.88 - 570.30 569.59 568-84 567.86 566.76 565.55 564.16 562.66 559.22 557.27 555.22 552.99 550-63 548.1 1 545.49 542.72 539.76 536.66 533.45 530.1 8 526.80 523.28 515.51 - - 570.14 56940 568.58 567.51 566.42 565-14 563.70 562.1 7 560.44 558.65 556.68 554.54 552.34 549.92 547.38 544.70 541.92 538.92 535.81 532.52 529.1 1 525.64 522.03 51 8.21 514.15 19,572078 570.97 569.86 568.58 567.16 565.66 563.94 562.1 7 560.1 7 558.03 555.78 553-38 550.97 548-1 1 - 19,572.60 570-71 569-59 568.25 56676 565.25 563.50 561.60 559.65 557.49 555.22 552.76 - 1 A.E. Douglas C. K. Mdler and B. P. Stoicheff Can. J. Physics 1963 41 1174. 2 W. G. Richards and R. F. Barrow Proc. Chem. Soc. 1962,297. 3 J. I. Steinfeld R. N. Zare L. Jones M. Lesk and W. Klemperer J. Chem. Physics 1965,42,25. 4 W. G. Brown Physic. Rev. 1932,39,777. 5 R. S. Mulliken Physic. Rev. 1930,36,364. 6 N. Aslund Arkiv Fysik 1966,30 377. 7 A. G. Gaydon Dissociation Energies (Chapman & Hall London 1953) p. 66.