Classical quantal method for multistate dynamics: A computational study

We discuss a classically-motivated method for modeling ultrashort laser pulse optical excitation. The very same method can be used to treat the breakdown of the Born-Oppenheimer approximation. The results are compared to numerically-exact quantum mechanics for a model problem representing excitation from the X (ground) state to the B (excited) state of molecular iodine. Expectation values and final B state populations are predicted quantitatively. The method provides a new way to simulate pump-probe experiments in particular and multistate dynamics in general. The method appears extendible to multidimensional problems. We argue that the increase of effort with dimensionality will be similar to that encountered in classical mechanical simulations as opposed to the exponential scaling of numerically-exact quantum mechanical propagation techniques. (C) 1996 American Institute of Physics.