Wave packet dynamics of IBr predissociation

The delayed photofragmentation of IBr following perturbative excitation to the B(0(+)) state by a sub-100 fs laser pulse has been studied theoreticall y within a time-dependent framework. The principal aim of this work is to p rovide an interpretation of time-resolved experiments of IBr predissociatio n over a range of initial energies [M. J. J. Vrakking, D. M. Villeneuve, an d A. Stolow, J. Chem. Phys. 105, 5647 (1996)]. Calculations of the time dep endence of individual quasistationary vibrational levels of the B(0(+)) dia batic potential and B'(0(+)) adiabatic potential, and coherent superpositio n states of the diabatic vibrational levels, have been carried out to deter mine the quantized molecular evolution over intersecting bound and repulsiv e diabats. It is found that the dissociation probability varies as a functi on of energy within the B(0(+)) well, giving vibrational state-specific dec ays that range from below 1 ps to greater than 12 ps. The vibrational lifet imes are interpreted in terms of the degree of resonance between B(0(+)) di abatic levels and those of the excited B'(0(+)) Born-Oppenheimer state that arises from the diabatic curve crossing, expressed via the shapes of the d iabatic and adiabatic wave functions in the region of the crossing point. T o connect the vibrational dynamics with experiments, 1+2 pump-probe transie nt ionization signals and the frequency-resolved absorption cross sections have been computed. The former are interpreted in terms of their correspond ing power spectra calculated by the maximum entropy method, which reveal th e vibrational beat processes responsible for the quasibound time evolution monitored experimentally. An iterative comparison of these calculations wit h experiment in principle allows the shapes of the diabatic and adiabatic p otential curves to be mapped out over a wide energy range from the dissocia tion asymptote to the diabatic crossing point, and provides a strong indica tion of the distance variation of the off-diagonal elements of the Hamilton ian matrix that couple the two diabatic excited states. (C) 1999 American I nstitute of Physics. [S0021-9606(99)00405-5].