Vibrational Coupling Pathways in Methanol As Revealed by Coherence-Converted Population Transfer Fourier Transform Microwave Infrared Double-Resonance Spectroscopy

Coherence-converted population transfer infrared-microwave double-resonance spectroscopy is used to record the infrared spectra of jet-cooled CH3OH and CH3OD. 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 CH3OH in the ν3 symmetric CH stretch region contains 12 interacting vibrational bands, whereas in CH3OD, 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 ν3 but also contains an admixture of the binary CH bending combinations, particularly 2ν5. The lack of interacting bands in CH3OD confirms that in CH3OH the binary combinations of the OH bend (ν6) and a CH bend (ν4, ν5, ν10) 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.