Wave packet dynamics of IBr predissociation

Abstract

The delayed photofragmentation of IBr following perturbative excitation to the B(0+) state by a sub-100 fs laser pulse has been studied theoretically within a time-dependent framework. The principal aim of this work is to provide an interpretation of time-resolved experiments of IBr predissociation over a range of initial energies [M. J. J. Vrakking, D. M. Villeneuve, and A. Stolow, J. Chem. Phys. 105, 5647 (1996)]. Calculations of the time dependence of individual quasistationary vibrational levels of the B(0+) diabatic potential and B′(0+) adiabatic potential, and coherent superposition states of the diabatic vibrational levels, have been carried out to determine the quantized molecular evolution over intersecting bound and repulsive diabats. It is found that the dissociation probability varies as a function of energy within the B(0+) well, giving vibrational state-specific decays that range from below 1 ps to greater than 12 ps. The vibrational lifetimes are interpreted in terms of the degree of resonance between B(0+) diabatic levels and those of the excited B′(0+) Born–Oppenheimer state that arises from the diabatic curve crossing, expressed via the shapes of the diabatic and adiabatic wave functions in the region of the crossing point. To connect the vibrational dynamics with experiments, 1+2 pump–probe transient ionization signals and the frequency-resolved absorption cross sections have been computed. The former are interpreted in terms of their corresponding power spectra calculated by the maximum entropy method, which reveal the vibrational beat processes responsible for the quasibound time evolution monitored experimentally. An iterative comparison of these calculations with experiment in principle allows the shapes of the diabatic and adiabatic potential curves to be mapped out over a wide energy range from the dissociation asymptote to the diabatic crossing point, and provides a strong indication of the distance variation of the off-diagonal elements of the Hamiltonian matrix that couple the two diabatic excited states.