Multiple timescales in the intramolecular vibrational energy redistribution of highly excited methanol

Abstract

Highly resolved vibrational overtone spectra of jet-cooled CH₃OH, obtained using selective IRLAPS detection of the vibrational transition, reveal rich vibrational structure that carries detailed information on the intramolecular dynamics. Low-resolution single resonance spectra of the jet-cooled molecules exhibit a 50 cm^–1 splitting resulting from strong coupling between the OH and CH stretch modes at the energy of the 5ν_OH band. Subsequent to coherent excitation, this coupling would lead to energy redistribution between the OH and CH on a 100 fs timescale. High-resolution spectra obtained using double-resonance excitation reveal smaller vibrational splittings resulting from weak vibrational couplings that control the longer time dynamics. The extensive vibrational structure in this overtone spectrum of methanol clearly indicates that it is the presence of low-order resonances rather than the total density of vibrational states that control the first 10 ps of vibrational-energy redistribution.