15min:

W. J. LAFFERTY, Optical Technology Division, NIST Gaithersburg, MD 20899, USA; J.-M. FLAUD, A. PERRIN, Laboratoire Inter Universitaire des Systemes Atmosphériques, CNRS, Universités Paris 12 et 7, 61 Av du Général de Gaulle, 94010 Créteil Cedex France; H. BECKERS, Y. S. KIM, H. WILLNER, Anorg. Chemistry, University of Wuppertal, D-42119 Wuppertal, GERMANY.

The infrared spectrum of thioformaldehyde (CH₂S) is of interest since this species is one of the myriad of molecules found in the interstellar space. From a spectroscopic point of view, it is of even more interest since the two lowest vibrational modes, the in-plane rocking mode, ₆ (B₂), and the out-of-plane wagging mode, ₄ (B₁), fall at 990.18 and 991.02 cm^-1 respectively. This separation of only 0.84 cm^-1 leads to a massive z-type Coriolis resonance where many of the rotational levels of each of the two vibrational states are mixed nearly 50% with each other. To make the situation even more interesting the C=S stretching vibration, ₃, with A1 symmetry occurs nearby at 1059 cm^-1. This vibrational level also interacts with the two low frequency modes which complicates the assignment and analysis. CH₂S was produced by low pressure thermolysis of a gas flow of C₃H₅SCH₃/Ar (560^o C) and CH₃SCl/Ar (1150^o C) in the entrance of the multipath white cell (optical path length 32 m). At a total pressure of 0.15 mbar, 40 scans were recorded for the range 750 to 1400 cm^-1 on a Bruker HR120 TFIR spectrometer at a resolution of 0.005 cm^-1 (maximum optical path difference). The initial line assignment was not straightforward. There are strong series apparent in the spectrum, but the features expected for a b-type and c-type bands were not obvious near the band center. The centers of these three vibrations have been determined from medium resolution FT spectra as well as laser Stark measurements. An initial calculation was made using this information as well as guessed values for the band intensities. This permitted the identification of several low K_a series. Finally after numerous iterations, the transitions in the spectrum were identified leading to an excellent set of ro-vibrational constants.