IMPULSIVE EXCITATION OF COHERENT VIBRATIONAL MOTION GROUND SURFACE DYNAMICS INDUCED BY INTENSE SHORT PULSES

A framework for understanding impulsively photoinduced vibrational coherent motion on the ground electronic surface is presented. In particular strong resonant excitation to a directly dissociative electronic surface is considered. Three distinct approaches are employed. A two surface Fourier wavepacket method explicitly including the field explores this process in isolated molecules. A coordinate dependent two-level system is employed to develop a novel analytical approximation to the photoinduced quantum dynamics. The negligible computational requirements make it a powerful interactive tool for reconstructing the impulsive photoexcitation stage. Its analytical nature provides closed form expressions for the photoinduced changes in the material. Finally the full simulation of the process including the solvent effects is carried out by a numerical propagation of the density operator. In all three techniques the excitation field is treated to all orders, allowing an analysis of current experiments using strong fields, resulting in substantial photoconversion. The emerging picture is that the impulsive excitation carves a coherent dynamical "hole" out of the ground surface density. A rigorous definition of the dynamical "hole" is constructed and used to define a measure of its coherence. In particular all photoinduced time dependence in the system can be directly related to the dynamical "hole." All three techniques are used to simulate the pump probe experiment on the symmetric stretch mode of I3-, including electronic and vibrational dephasing. © 1994 American Institute of Physics.