Curve crossing beyond semiclassical approximations.

Nonsemiclassical effects in two-state and multistate curve crossing are considered. In the two-state case such effects appear in the crossing of nonlinear potentials and in systems described by nonlinear equations. Multistate curve crossing in linear potential grids involving two sets of parallel potentials is considered here using a quasidegeneracy approximation. The approximation describes phenomena, such as counterintuitive transitions and saturation (incomplete population transfer), not predictable by the assumption of independent crossings. Also, a new kind of oscillations due to quantum interference (different from the well-known Stückelberg oscillations) is disclosed, and its nature discussed.