The absorption spectrum of gaseous hydrogen bromide in the Schumann region I. Rotational analysis By R. F. Barrow and J. G. Stamper Physical Chemistry Laboratory, University of Oxford ( < Communicated by O. Herzberg, —F.R Received 10 April 1961) [Plates 6 and 7] The absorption spectrum of gaseous hydrogen bromide has been photographed in the region 1180 to 1500 A, using fourth and fifth orders of a 3 m grating. About forty bands have been J; observed. The resolving power sufficed for the study of most of the discrete rotational structure.
260 R. F. Barrow and J. G. Stamper The analysis of many of the bands was straightforward. Since, in hydrogei > bromide at room temperature, 99-98 % of the molecules are in the lowest vibrations level, v" = 0, all the observed bands arise from this level, and unequivocal numbering of Rnad P lines can be given. The analyses of Q branches, where they are observed " are less certain, both because they are in general less well resolved, and also becaus the only criterion for the correct numbering is that the A-type doubling is expectet h to decrease to zero as the rotational quantum number goes to zero: however, thi will be so only in the absence of perturbations.
Absorption spectrum of gaseous hydrogen bromide. I 261 \ diffuse feature stretching from 68874 to 68903 cm-1, which may perhaps be a lead, while at longer wavelengths a similar feature stretches from 68647 to 8676 cm-1. The branch itself cannot be an unperturbed Q branch, for the spacing w 2A BJ for a Q branch) is much too large. If the branch is a branch, there is no ? branch to fit with it, although the head at 68 903 cm-1 could then be the Q head, finally, if the resolved branch were the R branch, the band would be very strongly gd-degraded, and weakly bound states with small B values are not expected in this Table 2. Band  J R(J) P(J) Q(J) A 2F"(J) Trp Tq 0 67104-7 — — — — 1 120-5 67092H 48-4 67105-5 (67109) — 2 134-8 67 056-3 84-2 136-9 3 148-7 036-3 117-3 184-6 4 161-7 017-5 151-4 248-2 5 174-2 66997-3 182-8 328-8 6 186-0 978-9 215-8 424-8 7 196-6 958-4 248-7 535-2 8 206<2 937-3 663-1 9 805-5 Table 3. Band  J Q P 1 68 903 h — 2 68859-5 3 841-0 4 821-1 5 800-8 6 780-6 7 759-2 8 736-1 9 713-3 10 689-8 tart of the spectrum. Of the three possibilities, the most plausible is that the ©solved branch is a P branch and, in this case, the vanishing intensity of the R ranch may perhaps be explained by some form of uncoupling. The tentative nalysis given in table 3 leads to v0 = 68 893 cm-1, B' = 7-8 cm-1.
262 R. F. Barrow and J. G. Stamper diffuse R branch, but at the long wavelength end the last few P lines are jug distinguishable. Wave numbers of the lines are given in table 5. We obtai v0 = 70526, B' — 7-8cm-1.
Absorption spectrum of gaseous hydrogen bromide. I 263 Bands  and  Bands  and  lie close together, and under low resolving power appear as a Ingle feature. They were thus observed by Price (1938), who assigned them to 7+2w'. However, under high resolution, band  is wholly diffuse, while  hows some sharp structure. The longest wavelength diffuse lines appear to form iart of a Pbranch, and a possible numbering leads to v0 ~ 75267, B' ~ 7-0cm_1. |imilarly, from the sharp R lines in , v0 = 75345-6, B' = 8-06, = 2-29 x lO^cm"1.
264 R. F. Barrow and J. G. Stamper Band  A number of diffuse features (table 9) are assigned to a band . There are no; obvious branches, and the ground-state combination differences do not seem to occur. It is probable that part of the band is so predissociated as to be unobserved.
Absorption spectrum of gaseous hydrogen bromide. I 265 Band  ‘ A characteristic feature of this spectrum is a series of very strongly degraded bands which consist of only two branches, R and P. The longest wavelength band of his kind is  (see figure 1). The analysis (table 11) is straightforward and leads r,o the following values Vq = 76962-3, B' = 4-464, D' = 4-04 x lO^cm"1.
266 R. F. Barrow and J. G. Stamper Band  State  also gives rise to a simple two-branch band strongly degraded to the red, but the analysis, although straightforward, leads to a negative value of the centrifugal stretching constant* D' (see figure 2). The constants are v0 = 77 344*1, B' = 4*478, 4*45 x 10-3 cm-1.