Dissipation, tunneling, and adiabaticity criteria for curve crossing problems in the condensed phase

The role of dissipation in curve crossing phenomena in condensed phases is discussed. Adiabaticity criteria are found in two regimes via path integral arguments. In the high temperature regime a stochastic quantum Langevin approach is developed. In the low temperature region an instanton method is used. In both cases the curve crossing time scale competes with the time scale of molecular motion. Dissipation causes the process to be more likely to be adiabatic.