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Physical Review A

Time evolution of the average energy of a relaxing molecule

V. M. Kenkre, V. Seshadri

Published Jan 1, 1977• 19 citations
Publisher

Abstract

Evolution equations for the average energy of a harmonic oscillator in interaction with a bath are derived and solved for two cases: (a) when the probability evolution obeys a master equation with a true decay representing a sink-process like radiative emission occurring simultaneously with vibrational relaxation, and (b) when the Markoffian approximation in the Heisenberg-equation analysis of relaxation is not made, allowing for short-time effects.

DOI
10.1103/physreva.15.197

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Evolution equations for the average energy of a harmonic oscillator in interaction with a bath are derived and solved for two cases: (a) when the probability evolution obeys a master equation with a true decay representing a sink-process like radiative emission occurring simultaneously with vibrational relaxation, and (b) when the Markoffian approximation in the Heisenberg-equation analysis of relaxation is not made, allowing for short-time effects.
A theory is developed to describe the interplay of vibrational relaxation and luminescence occurring simultaneously in a molecule, in terms of a master equation involving true sinks of probability. Specifically, the basic equation is the Montroll-Shuler equation augmented by the addition of sink terms which can be nonlinear as well as linear in the vibrational energy. These terms describe radiative and nonradiative decay and expressions for the former are derived explicitly in terms of Franck-Condon factors. Exact solutions in terms of known functions are obtained in the linear case for several physically motivated initial vibrational distributions, viz., $\ensuremath{\delta}$-function, Boltzmann, Poisson, and Laguerre. Two perturbation schemes are developed to analyze the nonlinear case, one of which is useful for small nonlinearities and the other for arbitrarily large nonlinearities but for low temperatures. Illustrative applications of the theory include the exact calculation of time- and frequency-resolved emission spectra (linear decay calculation) and the perturbation analysis of quantum yields in the presence of strongly energy-dependent nonradiative transitions (nonlinear decay calculation).
The approach to thermal equilibrium of a free particle and a harmonic oscillator interacting with a heat bath can be understood from a semiclassical point of view in terms of the time evolution of a quantum mechanical state function appropriate to each system. The state functions are parameterized such that expectation values comply with the predictions of the classical theory of Brownian motion, with the consequence that the Fokker-Planck equation for a free particle in velocity space, the Montroll-Shuler equation for a quantum oscillator in discrete energy space, and their solutions are arrived at by a common procedure. These relaxation processes become visualizable as the progressive development from a coherent, relatively ordered state to that of equilibrium, so that the loss of information as equilibrium is reached and the consequences of random phasing can be traced dynamically in some detail. The unity of approach is reinforced by the demonstration that since both equations represent Markoffian processes, the Montroll-Shuler equation predicts distributions in coordinate space and momentum space which satisfy a Fokker-Planck equation.
The approach to equilibrium of a quantum mechanical system in interaction with a bath is studied from a practical as well as a conceptual point of view. Explicit memory functions are derived for given models of bath couplings. If the system is a harmonic oscillator representing a molecule in interaction with a reservoir, the generalized master equation derived becomes an extension into the coherent domain of the well-known Montroll–Shuler equation for vibrational relaxation and unimolecular dissociation. A generalization of the Bethe–Teller result regarding energy relaxation is found for short times. The theory has obvious applications to relaxation dynamics at ultra-short times as in observations on the femtosecond time scale and to the investigation of quantum coherence at those short times. While vibrational relaxation in chemical physics is a primary target of the study, another system of interest in condensed matter physics, an electron or hole in a lattice subjected to a strong DC electric field that gives rise to well-known Wannier–Stark ladders, is naturally addressed with the theory. Specific system–bath interactions are explored to obtain explicit details of the dynamics. General phenomenological descriptions of the reservoir are considered rather than specific microscopic realizations.
We consider a classical point particle bilinearly coupled to a harmonic bath. Assuming that the evolution of the particle is monitored on a timescale which is longer than the characteristic bath correlation time, we derive the Markovian master equation for the probability density of the particle. The relaxation operator of this master equation is evaluated analytically, without invoking the perturbation theory and the approximation of weak system-bath coupling. When the bath correlation time tends to zero, the Fokker-Planck equation is recovered. For a finite bath correlation time, the relaxation operator contains contributions of all orders in the system-bath coupling.
Time evolution of a harmonic oscillator linearly coupled to a heat bath is compared for three classes of initial states for the bath modes-grand canonical ensemble, number states, and coherent states. It is shown that for a wide class of number states the behavior of the oscillator is similar to the case of the equilibrium bath. If the bath modes are initially in coherent states, then the variances of the oscillator coordinate and momentum, as well as its entanglement to the bath, asymptotically approach the same values as for the oscillator at zero temperature and the average coordinate and momentum show a Brownian-like behavior. We derive an exact master equation for the characteristic function of the oscillator valid for arbitrary factorized initial conditions. In the case of the equilibrium bath this equation reduces to an equation of the Hu-Paz-Zhang type, while for the coherent states bath it leads to an exact stochastic master equation with a multiplicative noise.
We investigate how to model Markovian evolution of coupled harmonic oscillators, each of them interacting with a local environment. When the coupling between the oscillators is weak, dissipation may be modeled using local Lindblad terms for each of the oscillators in the master equation, as is commonly done. When the coupling between oscillators is strong, this model may become invalid. We derive a master equation for two coupled harmonic oscillators that are subject to individual heat baths modeled by a collection of harmonic oscillators and show that this master equation in general contains nonlocal Lindblad terms. We compare the resulting time evolution with that obtained for dissipation through local Lindblad terms for each individual oscillator and show that the evolution is different in the two cases. In particular, the two descriptions give different predictions for the steady state and for the entanglement between strongly coupled oscillators. This shows that when describing strongly coupled harmonic oscillators, one must take great care in how dissipation is modeled and that a description using local Lindblad terms may fail. This may be particularly relevant when attempting to generate entangled states of strongly coupled quantum systems.