Spectra of SeO and SeO 2

Abstract

An analysis of the near ultra-violet bands of SO 2 , proposed recently has shown, that the energies of excitation are almost equal in SO and SO 2 and that the frequency of the symmetric valence vibration of SO 2 is about equal to the vibrational frequency of SO not only in the ground states of the two molecules, but also in their respective excited states. These results seem to be of considerable interest, particularly with regard to the theory of valency, since they indicate that the S=O bond is almost identical in both molecules. It seemed necessary, therefore, to investigate the spectra of SeO and SeO 2 in order to see whether such results are fortuitous or whether they are confirmed by the analysis of similar mole­cules. Such an investigation appeared desirable also, from the point of view that the method of analysis of the spectra of polyatomic molecules has not made so much progress as could be wished for and the bands of formaldehyde and sulphur dioxide are indeed the only ones, for which a complete vibrational analysis has been offered at the present moment. An extension to polyatomic molecules of the clearer insight into the constitution of diatomic molecules, afforded by band spectroscopy, seems to us essential in the present state of knowledge. Emission Spectrum of SeO Experimental —The spectrum of SeO has not been described in litera­ture. Various methods of producing bands of SeO, however, suggest themselves. In analogy with SO the best method probably would be one of a discharge through a vacuum tube in running vapour of SeO 2 or with high pressure SeO 2 vapour. SeO 2 being a solid with rather a high melting point (340° C), such a method could not be adopted because of the want of a suitable quartz discharge tube and a suitable furnace. A glass discharge tube, with a quartz window for observation, containing SeO 2 was heated locally by a Bunsen burner, but the spectrum obtained showed only bands due to Se 2 . An arc on 110 volts D. C., with a current of 2·5 amps, between carbon or metal electrodes filled with SeO 2 , produced again the bands of Se 2 or the lines of Se as did also the arc on 220 volts D. C. with a current of 2 to 5 amps. The substance was then introduced in a solid state and in aqueous solution into the Bunsen flame. The flame of the solid substance gave only Se 2 bands, but the spectrum of the flame of the solution showed an unmistakable progression of bands, different from those of Se 2 between 4500 and 3200 A. In the hope of exciting more vibrational levels of the emitter of these bands and thus extending the system, the oxy-coalgas flame giving a higher temperature was used. Under this condition the spectrum showed a further set of bands towards the ultra-violet until about 2400 A. The bands are all degraded towards longer waves; they are diffuse and in some cases a close double head can be distinguished. The intensity of the bands is not at all high; and increased exposure only resulted in a more pronounced overlap of the bands with the strong continuum of the flame itself. The best plates were obtained, in the blue and near ultra-violet regions, with an exposure of 45 minutes, and in the ultra-violet region, of about two hours, using a Hilger medium quartz spectrograph as the resolving instrument.