Time-domain formulation of photofragmentation involving nonradiatively coupled excited states, and its implementation via wave packet perturbation theory

A time-domain formulation of photodissociation theory is given for the case that two or more excited states are involved due to nonradiative coupling. We show how to compute the fragment distributions onto the various potential surfaces involved. In addition, we give an alternative prescription for calculating the total absorption cross section onto each surface (‘‘branching ratio’’) as a function of irradiation frequency. The alternative technique avoids the necessity of explicitly summing over all energetically allowed fragment state cross sections; it is therefore expected to be simpler to implement in many-dimensional applications then the fragment summation procedure. Our formalism is successfully implemented via localized (Gaussian) wave packet perturbation theory for a model problem involving two coupled one-dimensional excited state potentials. In particular, rather strong nonradiative coupling effects are accommodated.