THE WAVE PACKET MOTION AND INTRAMOLECULAR VIBRATIONAL REDISTRIBUTION IN CHX3 MOLECULES UNDER INFRARED MULTIPHOTON EXCITATION

We report results from quantum dynamical simulations of ultrafast vibrational redistribution processes in the CH chromophore of CHX3 molecules (CHD3, CHF3) during and after infrared-multiphoton excitation. The vibrational Hamiltonian is based on results from high resolution spectroscopy and ab initio calculations of the potential hypersurfaces for these molecules. The quantum dynamical calculations involve accurate solutions of the time dependent quantum equations of motion by means of both Floquet and quasiresonant approximations. We find mode selective redistribution between the CH stretching and bending modes on a time scale of 50 to 100 fs. Other modes participate only on much longer time scales (> 1 ps), as was shown previously by analysis of the spectra. For the real, strongly anharmonic systems (k(sbb)' congruent-to 30 to 100 cm-1), the redistribution is nonclassical with fast spreading to a quasimicrocanonical distribution, which is particularly pronounced if a narrow range of energies (for example, the N = 6 polyad) is initially excited. The effect can be interpreted as an intrinsic quantum statistical behavior induced by anharmonicity. In comparison, a weakly anharmonic hypothetical model system (k(sbb)' less-than-or-equal-to 2 cm-1) leads to quasiclassical motion of the wave packet with quasiperiodic exchange between stretching and bending motions. We present an approximate analytical investigation of the Fermi modes underlying the dynamics which provides a semiquantitative understanding of the Fermi-resonance spectra. On the basis of these results, we discuss possibilities of mode selective reaction control in unimolecular processes with laser excitation and some aspects of intramolecular "chaos."