Vibrational coupling pathways in methanol as revealed by coherence-converted population transfer Fourier transform microwave infrared double-resonance spectroscopy

摘要

Coherence-converted population transfer infrared-microwave double-resonance spectroscopy is used to record the infrared spectra of jet-cooled CH _3OH and CH_3OD. Population transfer induced by a pulsed IR laser is detected by Fourier transform microwave spectroscopy background-free using a two-MW pulse sequence. The observed spectrum of CH_3OH in the ν_3 symmetric CH stretch region contains 12 interacting vibrational bands, whereas in CH_3OD, only one vibrational band is observed in the same interval (2750-2900 cm~(-1)). The bright state, responsible for the transitions observed in this region, is not just ν_3 but also contains an admixture of the binary CH bending combinations, particularly 2ν_5. The lack of interacting bands in CH_3OD confirms that in CH_3OH the binary combinations of the OH bend (ν_6) and a CH bend (ν_4, ν_5, ν_(10)) act as doorway states linking the bright state to higher order combination vibrations involving torsional excitation. A time-dependent interpretation of the frequency-resolved spectra reveals a fast (～200 fs) initial decay of the bright state followed by a slower (1-2 ps) redistribution among the lower frequency modes.