Wave packet motion and decoherence in an anharmonic potential

The time evolution of wave packets in the Morse potential is investigated under the influence of the environment consisting of harmonic oscillators. We obtain a Markovian master equation that incorporates the fact that due to the nonequidistant spectrum, the relaxation rate of each energy eigenstate is different. The nonunitary time evolution is visualized by the aid of the Wigner function related to the reduced density operator of the Morse system. The time scale of decoherence is much shorter than that of dissipation, and gives rise to states which are mixtures of localized states along the phase space orbit of the corresponding classical particle. This behaviour is to a large extent independent of the coupling strength, the temperature of the environment and also of the initial state.