Photodynamic control using field optimized initial state: A mechanistic investigation of selective control with application to IBr and HI photodissociation

The probability density profiles from the optimal superpositions of the field free vibrational eigenstates which maximize flux out of the desired photodissociation channels are examined for IBr and HI molecules. Analysis of the structure in these optimal superposition states obtained by applying the Rayleigh-Ritz variational procedure to the time integrated flux operator shows that the transfer of probability density to appropriate areas of the Franck-Condon region on the excited surfaces is responsible for selective flux maximization out of different channels. Localizing the wave packet on the more repulsive part of the higher curve facilitates fast diabatic exit out of the upper channel and transition to the less repulsive part promotes slow adiabatic exit out of the lower channel. This mechanism is further probed by utilizing time dependent wave packet dynamics to obtain absorption spectra and branching ratios using full Fourier transform of the autocorrelation functions for these field optimized initial states. The results corroborate the central role of altered spatial profile of the initial state in selective control of photodissociation. (C) 1999 [S0021-9606(99)00610-8].