The Spectroscopy of Formaldehyde and Thioformaldehyde

TLDR: The analysis of the rotational subband structure of several vibronic transitions in the A I A2X I A A1 band system of H2CO, in 1934, by Dieke & Kistiakowsky is recognized as the first interpretation of the electronic spectrum of a poly atomic molecule as mentioned in this paper.

Abstract: Formaldehyde has played a key role in many aspects of our current understanding of the spectroscopy of poly atomic molecules. The analysis of the rotational subband structure of several vibronic transitions in the A I A2X I A1 band system of H2CO, in 1934, by Dieke & Kistiakowsky (1) is recognized as the first interpretation of the electronic spectrum of a poly atomic molecule (2). During the intervening five decades, formaldehyde has been thoroughly studied by both experimental and theoretical tech­ niques, and continues to be a prototype system for the investigation of the spectroscopy, photochemistry, and photophysics of poly atomic molecules. In sharp contrast, thioformaldehyde is a relative newcomer to the scene. This somewhat unstable species was first unambiguously identified in the laboratory little more than a decade ago (3) by observation of its microwave spectrum. Since then, rapid progress has been made in characterizing the ground and excited electronic states of HzCS. Thioformaldehyde and formaldehyde are spectroscopically similar in many aspects. For example, in both molecules the lowest energy electronic transitions may be characterized as forbidden electron promotions to n,1C* states, giving rise to ij3 A2-X I A l and A I A2-X I Al band systems. However, in formaldehyde these transitions occur in the near ultraviolet, whereas in thioformaldehyde the analogous systems are in the visible. One of the major