Semiclassical theory of multi-channel curve crossing problems: Landau-Zener case

A two-by-two diabatic propagation method is developed to deal with general one-dimensional multi-channel curve crossing problems. Each crossing is treated within the newly completed two-state semiclassical theory in the diabatic representation and is represented by a nonadiabatic transition matrix (I-matrix). A product of all the I-matrices yields the reduced scattering matrix for the entire system. This theory can handle the multi-channel curve crossing problems as simply as a two-state problem. An analysis of the complex crossing points in the momentum space can provide a comprehensive illustrative criterion of this method. A detailed severe numerical test is made by taking a seven-state system with 6 and 12 nonzero diabatic couplings. It is demonstrated that dominant processes among the state-to-state transitions are well reproduced by the present semiclassical theory.