Simulation of quantum decoherence in condensed phases

New computational methods are extended and applied to interpret recent experiments exploring decoherence, the loss of quantum phase, of quantum subsystems excited in condensed phase environments. The experiments of interest involve preparing a quantum subsystem initially in a linear superposition of states. These states have fixed relative phases and are coupled to an environment. Quantum evolution of the coupled system and environment results in an entangled state that is a linear combination of different composite states that can not be separated into system and environment components. The measurements focus on properties of the quantum subsystem and involve integrating over all states of the environment. The amplitude of the quantum entanglement decays at a rate determined by the decoherence time which gives a time scale over which quantum interference effects can be observed in condensed phase measurements. This time is dominated by the overlap of the entangled environment state components.; This thesis focuses on exploring factors which influence and can be used to control the decoherence time. The decoherence of a quantum subsystem, e.g. an intramolecular vibration, coupled to a solid host is investigated. Mixed Quantum Classical methods are developed which involve propagating classical-like trajectories with initial condition sampled from the quantum phase space distribution known as the Wigner transform of the quantum density. A practical many-body Wigner transform method employing a variational approach, originally due to Feynman and Kleinert, which can incorporate quantum zero point energy and fluctuations and is applicable to condensed phase systems, is realized. Formulas are derived by approximating the many-body potential by a locally harmonic form and using the thermal quantum variational principle due to Gibbs, Bogliubov and Feynman. Equilibrium properties are investigated using these approximations. The dynamical properties of a model molecular impurity in condensed phases are investigated by a mixed quantum classical theory which evolves trajectories sampled from Wigner transforms of initial densities and operators. The pure dephasing of vibrationally excited I2 in a low temperature Krypton matrix is studied. The dephasing rates computed with the methods outlined above give excellent agreement with experimental results. Various dephasing mechanisms inferred from simulations are discussed.