2-PHONON PROCESSES IN THE 2-STATE DYNAMICS

We study the influence of phonons on the dynamics of a symmetric two-state system. The spectral density of the environmental excitations is explicitly evaluated, taking into account single- and two-phonon processes. The interplay between these two different interactions is studied both in the coherent and incoherent tunneling regime. Attention is focused on the behavior of the crossover temperature as function of the coupling constants. The renormalized tunneling level splitting and the relaxation rates are evaluated as function of temperature in the physically relevant region of the parameters.