ACTIVATED RATE-PROCESSES - ANHARMONIC CORRECTIONS TO THE QUANTUM RATE

Escape of a particle from a metastable potential, whose motion is gove rned by the generalized Langevin equation, is a common model of many c hemical and physical activated processes in condensed phase. In the in termediate-to-strong damping regime the rate of escape is controlled b y the particle dynamics near the barrier top. Since Kramers, the parab olic barrier approximation is commonly used to get the expression for the rate in this regime. We consider the influence of anharmonic corre ctions to the potential barrier on the quantum rate and get leading or der corrections in terms of the inverse barrier height. New terms appe aring in the quantum expression for the rate are associated with tunne ling through the barrier and become important at low temperatures. The analytic theory is compared with recent numerically exact quantum sim ulations [M. Topaler and N. Makri, J. Chem. Phys. 101, 7500 (1994)]. ( C) 1995 American Institute of Physics.