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

State-selected spectra of the OH stretch overtones of methanol in the range of upsilon(1) = 3-8 reveal spectral splittings and broadenings that result from vibrational couplings within the molecule. We employ a two-color exci tation technique in which an infrared pulse promotes jet-cooled methanol mo lecules to a single rotational state in upsilon(1) = 1 or 2 and a second vi sible or near-infrared laser pulse is scanned to record a vibrational overt one spectrum. The final vibrationally excited species are detected by infra red laser assisted photofragment spectroscopy. The implications of the spec tra 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 la rgest splittings imply subpicosecond oscillation of energy between the OH s tretch and a combination with the C-H stretch (5 nu(1) double left right ar row 4 nu(1) + nu(2) and 6 nu(1) double left right arrow 5 nu(1) + nu(2)) or a combination with the COH bend (7 nu(1) double left right arrow 6 nu(1) 2 nu(6)). Secondary time scales correspond to finer splittings and are tho ught to arise from low-order resonances with other vibrational states. We a rgue that the nonmonotonic energy dependence of the presence and extent of such secondary structure throughout the recorded spectra reflects the requi rement of resonance with important zeroth-order states. The third time scal e, represented by the widths of the narrowest features at each overtone lev el, reflects the onset of vibrational energy randomization. These widths in crease exponentially with vibrational energy in the range 2 nu(1) up to 8 n u(1). 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. (C) 1999 American Institute of Physics. [S0021-9606(99)02317-X].