Resonance-enhanced multiphoton ionisation spectroscopy of methanethiol

Abstract

We report the first multiphoton study of excited electronic states of the methanethiol molecule. Members of four Rydberg series are identified via the two- and three-photon resonance enhancements they provide in the multiphoton ionisation spectrum of this molecule. Spectral analysis is aided by parallel measurements of the kinetic energies of the accompanying photoelectrons. Such studies also yield a refined value for the first ionisation limit of methanethiol: E_i= 76 260 ± 40 cm^–1. Detailed inspection of the photoelectron spectra obtained following REMPI via all but the lowest energy resonances reveal the presence of ‘slower’ photoelectrons, the energies of which can be interpreted if we invoke near-resonant inter-Rydberg state vibronic mixing. It is suggested that such inter-Rydberg state vibronic mixing must be rather commonplace in polyatomic systems, especially in molecules like methanethiol where symmetry imposes few restrictions on the allowed state couplings.