Quantum dissipation in a spin bath; Applications to chemical dynamics

Dissipation is an integral part of a quantum system coupled to its environment. Traditionally in physical and chemical sciences, the environment is modeled by a reservoir of harmonic oscillators. However, there arise situations where the localized modes dominate in the dynamics of dissipation. Spin-bath has proven to be a successful alternative in such cases. In this review we have outlined a scheme for a universal description of an environment in terms of a generalized spin-bath. Making use of Holestein-Primakoff transformation one may realize the two limits of the reservoir, the spin-1/2 or fermionic bath and the harmonic or bosonic bath. The basis of the present analysis is the coherent state representation of the noise operators and thermal canonical distributions of Gaussian form. The appropriate quantum Langevin equation and its variant in c-numbers have been derived for spatial and momentum coupling between the system and the bath. The scheme has been applied to the problems of rate theory of chemical reactions, tunneling and spectroscopy with quantum dots.