Rotational analysis of à ²A₁↔X̃ ²E electronic transition of the jet-cooled methylthio radical

A set of molecular parameters describing both the X̃ and à states of CH₃S, has been obtained by a joint fitting of the rotationally resolved electronic transitions observed in a free-jet-cooled laser-induced fluorescence study of CH₃S and an earlier microwave study of its X̃ state. The present work shows that because of incomplete information, nearly all of the previously reported molecular parameters for CH₃S must be significantly revised. The present observations show an unusual electronic structure for the radical, characterized by a short C-S bond distance and peculiar methyl group geometry in the ground state. The C-S bond is observed to lengthen markedly in the excited à state.