MOLECULAR COLLISION DYNAMICS ON SEVERAL ELECTRONIC STATES

A time-dependent quantum mechanical method for propagating the wave fu nction on several electronic states is discussed for the polyatomic ca se and illustrated by the quenching collision of a Na (3p P-2) atom by H-2. The specification of method is governed by the need to have a cl ear physical interpretation of the results, by the recognition that th e motion on a given electronic state can often (but not always) be wel l approximated by classical mechanics, and by the need for a computati onal procedure that is simple enough to handle polyatomic systems. The se desiderata are realized by the spawning technique which is discusse d in detail. One more feature of the method is that it allows for a sm ooth interface with the methodologies of quantum chemistry so that the electronic structure problem can be solved simultaneously with the ti me propagation of the nuclear dynamics. The method is derived from a v ariational principle and so can yield quantum mechanically numerically converged results. The parameters that govern the numerical accuracy of the method are explicitly discussed with special reference to their physical significance. The quenching of a Na (3p P-2) atom by H-2 due to a conical intersection of two potential energy surfaces is used as a computational example since it illustrates many of the features of the method. This collision is found to be sticky and exhibits many seq uential nonadiabatic couplings, each of which is localized in time, wh ere the quenching probability per traversal of the conical intersectio n region is small. However, the accumulated transfer of population to the ground state can be significant since the duration of the overall transfer is spread over many vibrational periods of H-2.