Dynamics and Relaxation in Condensed Media

Relaxation theory provides a framework for the investigation of the role of the environment in the dynamics of small quantum open systems. Provided that we are only interested in its role as a heat reservoir, the environment affords a simplified description in terms of a collection of independent modes. The main consequence of the coupling between the small system and the large number of degrees of freedom of the macroscopic environment is that the expectation values of observables associated with the former relax to their equilibrium values. If the environment equilibrates in a faster time-scale than the system itself and after short-lived transients, the relaxation is in general markovian. The assumptions underlying this model for relaxation are examined critically. In particular, we study the behavior of a bath consisting of independent modes and discuss the relation between the markovian approximation and the positivity of the system's reduced density matrix. This general theory is also applied to the analysis of experiments of current interest. Specifically, we investigate the reaction dynamics of hydrogen tunneling in condensed media and the phenomenon of spectral diffusion in glasses. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253 -1690.).