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

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 5 nu(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.

State-to-state studies of intramolecular dynamics and unimolecular reactions

Thomas Rizzo

Double-resonance vibrational overtone excitation spectra of small molecules, in combination with laser-induced fluorescence product detection, provide information on both their energetics and intramolecular energy redistribution dynamics. Application of this approach to measure OH stretch overtone spectra of HOOH and NH2OH near their dissociation threshold provides estimates of the O-O and N-O bond energies of 17051.8 +/- 4 cm(-1) and 21 620 +/- 20 cm(-1) respectively. In symmetrical molecules, such as HOOH, the double-resonance excitation technique allows us to prepare molecules in either pure OH stretch overtone levels or combination levels in which the excitation energy is divided between two identical OH stretch oscillators. Comparison of the resulting vibrational overtone linewidths indicates no clear difference between the pure OH stretch and the local-local combination levels. However, comparison of the energy transfer rates from pure OH stretch vibrations and those in which one quantum of energy is put in a low frequency mode reveals faster rates from the latter. This observation clearly indicates that coupling strengths depend markedly on the vibrational character of the coupling partner and not simply on the total energy. Comparison of the OH stretch overtone spectra of the structurally similar molecules HOOH, NH2OH, and CH3OH reveal similar rates of vibrational energy redistribution, despite the difference in the density of states in these molecules. This observation reinforces the notion that energy flow between the OH stretch and dark background states is not statistical but rather is controlled by coupling to a small subset of background states.

1997

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

Thomas Rizzo

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 excitation technique in which an infrared pulse promotes jet-cooled methanol molecules to a single rotational state in upsilon(1) = 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 (5 nu(1) double left right arrow 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 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 2 nu(1) up to 8 nu(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].

1999

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

Thomas Rizzo

1999