Absorption Spectrum of Sulphur Dioxide in the Vacuum Ultra-violet BY I. DUBOIS* AND B. ROSEN Universit6 de Lihge, Institut d’Astrophysique, Cointe-Sclessin (Belgique) Received 18th February, 1963 A preliminary study of the absorption spectra of SOi6 and SO;* has confirmed that at least two transitions are involved in the 2300-1800 8, region and some new information has been gained concerning one of them. Further studies are needed to settle the possible geometrical asymmetry of the molecule in the upper electronic levels. INTRODUCTION Interest in the SO;! spectrum is mainly due to the possibility of a geometrical asymmetry of the molecule in some at least of the excited states. Such an asym- metry, due to a potential function with a double minimum in the antisymmetrical normal co-ordinate, has been tentatively assumed by Coon and Ortiz 1 and by Mulliken 2 to explain some peculiar intensity distributions observed in the spectrum. In fact, we 3 have been able to arrange the main bands in the far ultra-violet part of the spectrum in a vibrational scheme involving all three frequencies of the excited states in a way which would be contrary to the selection rules for a symmet- rical molecule.This arrangement was based on spectra obtained with moderate dispersion and cannot be considered conclusive. The difficulties of the analysis are even greater since it is not even known with certainty if the numerous strong bands in the 2300-17OOA region belong to a number of independent systems or form a single transition. The bands between 2300-1800 A have been arranged by Duchesne and Rosen 4 and Rosen 5 in four independent systems al, a2, a3 and a4 in general agreement with the theoretical expectation of Walsh.6 However, Riggs and Coon 7 admitted that all observed bands might belong to a single transition, in agreement with earlier assignments by Price and Simpson.8 In order to settle this question, we have re-investigated the bands of SOi6 and S 0 i 8 using spectra obtained in Dr.Douglas’s laboratory at the National Research Council in Ottawa in the 6th order of a 10-m concave grating, the dispersion being about 0.25I$/mm. We are much obliged to Dr. Douglas who kindly put these spectra at our disposal.? The spectra have been measured in Likge and this paper contains the preliminary results of the vibrational analysis.VIBRATIONAL ANALYSIS All bands measured in the absorption spectrum of SOi8 can be arranged in one system which is similar to the a2 system of SOi6 as proposed by Duchesne and * Aspirant of the Belgian National Foundation of Scientific Research. t We are also obliged to Dr. L. C. Leitch of the National Research Council in Ottawa for pre- paring a pure sample of SO\* for this experiment, as well as to Mr. F. Alberti for taking the plates. 124I . DUBOIS A N D B . ROSEN 125 Rosen. The heads and the corresponding vibrational quantum numbers are given in table 1, together with values calculated from the expression v = 42276 + 7455 + 379~; - 6&2 (1) The differences Vobs.-vcalc. are considerable and the constants in (1) are not reliable; but taking into consideration the complicated structure of the bands and the difficulties of a rotational analysis (even the K-structure being only partly resolved), the agreement is considered satisfactory.TABLE BAND HEADS IN THE SPECTRUM OF SO;* v ; v; v; obs. calc. v; v; v; obs. calc. 5 3 0 5 4 0 6 2 0 6 3 0 6 4 0 6 5 0 7 3 0 7 4 0 7 5 0 8 3 0 8 4 0 8 5 0 8 6 0 9 4 0 8 7 0 9 5 0 47 074 47 410 47 458 47 822 48 184 48 510 48 5675 48 9105 49 235 49 303 49 651 49 973 50 294 50 406 50 579 50 720 47 084 47 421 47 480 47 829 48 166 48 491 48 574 48 911 49 236 49 319 49 656 49 981 50 294 50 401 50 595 50 726 8 8 0 9 6 0 10 4 0 9 7 0 10 5 0 9 8 0 10 6 0 11 4 0 9 9 0 10 7 0 11 5 0 10 8 0 11 6 0 12 4 0 11 7 0 11 8 0 50 885.5 51 046.5 51 151 51 348 51 465.5 51 645 51 778-5 51 883.5 51 926.5 52 086 52 192 52 375 52 535 52 634.5 52 840 53 121 50 884 51 039 51 146 51 340 51 471 51 629 51 784 51 891 51 906 52 085 52 216 52 370 52 529 52 646 52 830 53 115 The proposed arrangement of the SO;* bands and their correlation with the corresponding SQk6 bands requires an adjustment of the earlier analysis of the system a2.The extrapolation of the observed isotopic shift indicates that the 0, 0 band lies for S 0 i 6 at about 42225 cm-1. All measured bands can be approxim- ately represented by the expression v = 42224 + 7554 + 422.5~; - 3.5~;2 (2) No deduction can be made from the change of the anharmonicity which might be due to the limited precision of this representation. DISCUSSION The spectra of SQi8 at our disposal do not include the region of the strongest a1 bands.The extrapolation using eqn. (2) leads, however, to conclusions con- cerning this system. In fact, the extrapolation includes all bands attributed by Duchesne and Rosen to a2 as well as all bands observed earlier by Chow 9 in the corresponding region but none of the bands which entered in the a1 system." It seems, therefore, that at least two systems are present in the 2100-19OOA region. We hope to extend the vibrational analysis to the region beyond 2100 and below 1900 A, in order to obtain further information about the other transitions * These latter bands have been used by Coon, De Wames and Loyd 10 to determine the geometrical configuration of the upper state. It seems, therefore, that the conclusion reached by these authors concerns the system MI, whose origin happened to lie near the origin of cc2.126 SPECTRUM OF SULPHUR DIOXIDE in SO2, and also to achieve a partial rotational analysis of the 1x2 bands. The only data obtained up to now concern the K structure. From the few bands in which this structure is clearly recognizable we obtain ( A ' - 2 ) = 0*90+0-02 cm-1. 1 Coon and Ortiz, J. Mol. Spectr., 1957, 1, 81. 2 Mulliken, Can. J. Chem., 1958, 36, 10. 3 Dubois and Rosen, Conference on Spectroscopy (Heidelberg, 1961). 4 Duchesne and Rosen, J. Chem. Physics, 1947, 15,63 I. 5 Rosen, J. Physique Rad., 1948, 9, 155. 6 Walsh, J. Chem. SOC., 1953, 2283. 7 Riggs and Coon, Symp. Mol. Structure and Spectroscopie (Columbus, June, 1958 ; AFOSR- 8 Price and Simpson, Proc. Roy. SOC. A, 1938, 165,272. 9 Chow, Physic. Rev., 1933, 44,638. TR-58-206, 1959). 10 Coon, De Wames and Loyd, J. Mol. Spectr., 1962, 8, 285.