The absorption spectra of sulphur dioxide and carbon disulphide in the vacuum ultra-violet

Abstract

In the near ultra-violet the absorption spectrum of sulphur dioxide has been investigated to a greater extent than that of any other triatomic molecule. The excellent work of Clements (1935) on the temperature dependence of the bands has enabled the ν 0 of the upper state to be definitely fixed. It has resulted in a satisfactory analysis of the so-called low-frequency system (i. e. bands appearing at high pressures which are due to transitions from various initial vibrating states), and has yielded a plausible arrangement of part of the high-frequency system (bands appearing at low pressures and temperatures, and probably corresponding to transitions from vibrationless ground states to the various vibrational levels of the upper state). Asundi and Samuel (1935) have put forward an alternative analysis of these bands, but we do not favour it on the grounds that it disregards the results of temperature experiments, contains many violations of Herzberg and Teller’s selection rules (1933), and interprets several strong bands as transitions from initial vibrational states in spite of prohibitive Boltzmann factors. A photograph of the spectrum is shown in fig. 1 b , Plate 3. The bands are very strong and appear at pressures of about 1/2 mm. in a path length of 1 m. Another system several times stronger than the previous one starts in the region of 2350 A. It has been investigated by several experimenters, but the only attempt at an analysis has been made by Chow (1933 a , b ). One of the difficulties that has troubled previous experimenters is that the bands continue to shorter wave-lengths past the transmission limit of quartz, and thus go outside the range of their instruments. We have therefore photographed the bands with a vacuum grating spectrograph, and in this way have obtained the absorption spectrum of sulphur dioxide down to about 1000 A. The technique used in obtaining the absorption spectra has been described previously (Collins and Price 1934). The Lyman continuum was employed as the continuous background against which the absorption was observed.