FEMTOSECOND QUANTUM DYNAMICS OF FUNCTIONAL-GROUPS UNDER COHERENT INFRARED MULTIPHOTON EXCITATION AS DERIVED FROM THE ANALYSIS OF HIGH-RESOLUTION SPECTRA

We report quantum dynamical trajectory results for the femtosecond vibrational motion of various functional groups in organic molecules, associated with the CH infrared chromophore. The dynamics is derived from our previous experimental analysis of high-resolution spectra of this chromophore from fundamental to high-overtone absorption, which allowed us to obtain a ''spectroscopic'' Hamiltonian for a variety of molecules (also including some ab initio information). The molecular motion is studied in terms of observables during and after coherent, IR-multiphoton excitation. The acetylenic=CH group in (CF3)3C-C=C-H and related molecules shows coherent one-dimensional motion (period 10 fs) for thousands of vibrational periods well into the picosecond time range, without substantial coupling to other modes. The aldehyde, O=C-H, CH chromophore (in CD3-CHO) shows two-dimensional vibrational motion on the 100-500-fs time scale, with fast redistribution of vibrational excitation from the initially excited CH stretching mode to the in-plane CH bending mode with little or no participation of the out-of-plane CH bending mode. The alkyl CH group shows fast redistribution between CH stretching and both in-plane and out-of-plane (C(s) symmetry) bending in CHD2F on the 100-fs time scale. The frame modes participate in none of these cases appreciably in the highly mode-selective redistribution on the subpicosecond time scale. All the results are illustrated in terms of time-dependent quantum wave-packet results for observable structural parameters of the molecules. Further aspects of the dynamics considered concern the influence of laser pulse shapes on IR-multiphoton excitation, average energy absorbed (E(t)), and entropy S(t) both during and after coherent excitation. Microcanonical equilibration and time reversal are discussed briefly. Some aspects of numerical quantum trajectory calculations involving the split operator, Floquet, or quasiresonant approximations are discussed. Benefits and disadvantages of the spectroscopic approach to femtosecond molecular dynamics as compared to real time pump-probe experiments are mentioned as well.