Energy gap dependence of vibrational dephasing rates in a bath: a semigroup description

The quantum dynamical semigroup formalism provides an appealing general fra mework for discussing factors that affect pure vibrational dephasing rates in chemical systems. Within this framework, we formulate a Poisson model of pure vibrational dephasing which is more generally applicable than the com monly employed stochastic Gaussian dephasing model. In the limit of small a nd frequent phase changes, the Poisson model reduces to the stochastic Gaus sian form. We find that for certain vibrational states of the lithium dimer in argon, the stochastic Gaussian model is valid, while for other states l arge and abrupt phase changes clearly require application of the Poisson mo del. In the former case, dephasing rates increase with the difference in qu antum number between constituent vibrational states, while in the latter ca se, the dependence on quantum number difference or energy gap can become ne gligible. Recent experimental advances described by Amitay and Leone are ex pected to permit experimental tests of our theoretical predictions. (C) 200 1 Elsevier Science B.V. All rights reserved.