Escape from a Metastable State

Many important processes in science involve the escape of a particle over a barrier. In this review, we report, extend, and interpret various theories of noiseactivated escape. We discuss the connection between many-body transition state theory and Kramers' original diffusive Brownian motion approach (both in oneand multidimensional potential fields) and emphasize the physical situation inherent in Kramers' rate for weak friction. A rate theory accounting for memory friction is presented together with a set of criteria which test its validity. The complications and peculiarities of noise-activated escape in driven systems exhibiting multiple, locally stable stationary nonequilibrium states are identified and illustrated. At lower temperatures, quantum tunneling effects begin to play an increasingly important role. Early approaches and more recent developments of the quantum version of Kramers approach are discussed, thereby providing a description for dissipative escape at all temperatures.