Knowledge Bank

We have determined highly predissociative levels of the $A2A1$ state of $CH3S$ in a supersonic jet with degenerate fourwave mixing (DFWM) technique. The highest level observed lies $4265cm-1$ above the zero-point-energy level, much greater than corresponding values of $2979cm-1$ observed by fluorescence depletion spectroscopy and $1490cm-1$ by laser-induced fluorescence. Unlike in fluorescence spectra, relative intensities of lines in DFWM spectra closely reflect their Franck-Condon factors; vibronic assignments are thus more straightforward. A new progression involving excitation of the $CH3$ stretching mode is identified, and several lines in the range $1290-1410cm-1$ above the origin may be assigned to the $CH3$ deformation ($\nu 5$) and the first overtone of $CH3$ rocking ($2\nu 6$) modes. Observed vibrational wave numbers are consistent with theoretical predictions.