Variational upper bounds are derived for the escape rate of a particle trapped in a metastable well and interacting wth a dissipative medium. The theory leads to a reduced two-degrees-of-freedom Hamiltonian involving the unstable normal mode and a newly defined collective bath mode. Explicit treatment of strong nonlinearities or low-barrier systems present no special problem.

• Received 23 October 1989

DOI:https://doi.org/10.1103/PhysRevLett.65.1399