RESONANCE-ENHANCED MULTIPHOTON IONIZATION SPECTROSCOPY OF METHANETHIOL

We report the first multiphoton study of excited electronic states of the methanethiol molecule. Members of four Rydberg series are identifi ed via the two- and three-photon resonance enhancements they provide i n the multiphoton ionisation spectrum of this molecule. Spectral analy sis is aided by parallel measurements of the kinetic energies of the a ccompanying photoelectrons. Such studies also yield a refined value fo r the first ionisation limit of methanethiol: E(i) = 76260 +/- 40 cm(- 1). Detailed inspection of the photoelectron spectra obtained followin g REMPI via all but the lowest energy resonances reveal the presence o f 'slower' photoelectrons, the energies of which can be interpreted if we invoke near-resonant inter-Rydberg state vibronic mixing. It is su ggested that such inter-Rydberg state vibronic mixing must be rather c ommonplace in polyatomic systems, especially in molecules like methane thiol where symmetry imposes few restrictions on the allowed state cou plings.