Intramolecular energy transfer in highly vibrationally excited methanol. II. Multiple time scales of energy redistribution

Abstract

State-selected spectra of the OH stretch overtones of methanol in the range of v1=3–8 reveal spectral splittings and broadenings that result from vibrational couplings within the molecule. We employ a two-color excitation technique in which an infrared pulse promotes jet-cooled methanol molecules to a single rotational state in v1=1 or 2 and a second visible or near-infrared laser pulse is scanned to record a vibrational overtone spectrum. The final vibrationally excited species are detected by infrared laser assisted photofragment spectroscopy. The implications of the spectra for vibrational dynamics in the time domain can be understood in terms of a hypothetical coherent excitation of relevant portions of the spectrum. The observed splittings and widths correspond to three time scales. The largest splittings imply subpicosecond oscillation of energy between the OH stretch and a combination with the C–H stretch (5ν1⇔4ν1+ν2 and 6ν1⇔5ν1+ν2) or a combination with the COH bend (7ν1⇔6ν1+2ν6). Secondary time scales correspond to finer splittings and are thought to arise from low-order resonances with other vibrational states. We argue that the nonmonotonic energy dependence of the presence and extent of such secondary structure throughout the recorded spectra reflects the requirement of resonance with important zeroth-order states. The third time scale, represented by the widths of the narrowest features at each overtone level, reflects the onset of vibrational energy randomization. These widths increase exponentially with vibrational energy in the range 2ν1 up to 8ν1. At the highest energy (25 000 cm−1) the three time scales begin to converge, implying an irreversible decay of the OH stretch overtone in 300 fs.