Intramolecular dynamics in a congested set of states: A reduced description of the quantal long time evolution with application to high Rydberg states

Time evolution when many states are strongly coupled is approximated by a reduced description where nearly degenerate states are taken to be equally populated, on the average. This grouping, valid at longer times, can significantly reduce the dimension of the problem, thereby making tractable computations which include all nearly isoenergetic states. The validity of the approach is examined by comparison with exact results. The application illustrated is to high molecular Rydberg states where a large basis size is required for convergence because, at times of experimental interest for ZEKE spectroscopy, many zero-order states have been accessed.