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The Journal of Chemical Physics

Quasidecay of harmonic oscillator coherent states in nonharmonic potentials

Juergen Brickmann, Peter Russegger

Published Dec 15, 1981• 42 citations

Abstract

The time evolution of harmonic oscillator coherent states (HOCS’s) (displaced ground state wave functions) in symmetrical and nonsymmetrical nonharmonic potentials is studied numerically. The potentials were modeled with an ansatz V(ε, q) = (1−ε) VHO+ε VNH, where VHO is the harmonic oscillator potential and VNH is chosen as a convenient Morse oscillator (nonsymmetrical) or as a negative Gaussian (symmetrical) so that the potential near the minimum is not distorted. The initially well located HOCS’s decay within a medium time scale of 5–100 classical oscillations to a wave packet which is delocalized in the position area of the potential well between the classical turning points. The time period up to this delocalization is designed as quasidecay time τQ, and it is demonstrated with some examples that in the symmetrical potentials the HOCS is refocused after a period of τR = 8τQ, while, as a rule, in the nonsymmetrical potential the recurrences need much longer time and could not be observed in the time scale under investigation. It is shown that the absolute values of the quasidecay times cannot be related either to the energy moments 〈H〉 and ΔH nor to the contribution of a quasicontinuum to the initial HOCS but that there is a strong correlation of ΔH and τQ for the nonsymmetrical potential considering only initial wave packets of the same energy.

DOI: 10.1063/1.442012

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The time evolution of harmonic oscillator coherent states (HOCS’s) (displaced ground state wave functions) in symmetrical and nonsymmetrical nonharmonic potentials is studied numerically. The potentials were modeled with an ansatz V(ε, q) = (1−ε) VHO+ε VNH, where VHO is the harmonic oscillator potential and VNH is chosen as a convenient Morse oscillator (nonsymmetrical) or as a negative Gaussian (symmetrical) so that the potential near the minimum is not distorted. The initially well located HOCS’s decay within a medium time scale of 5–100 classical oscillations to a wave packet which is delocalized in the position area of the potential well between the classical turning points. The time period up to this delocalization is designed as quasidecay time τQ, and it is demonstrated with some examples that in the symmetrical potentials the HOCS is refocused after a period of τR = 8τQ, while, as a rule, in the nonsymmetrical potential the recurrences need much longer time and could not be observed in the time scale under investigation. It is shown that the absolute values of the quasidecay times cannot be related either to the energy moments 〈H〉 and ΔH nor to the contribution of a quasicontinuum to the initial HOCS but that there is a strong correlation of ΔH and τQ for the nonsymmetrical potential considering only initial wave packets of the same energy.

The dynamics of an initially excital Gaussian wave packet (harmonic oscillator coherent state, HOCS) in a general one-dimensional anharmonic system is studied analytically with the aid of a second quantization formalism for times shorter than a classical oscillation period. The initial state population probability PQ(t), its classical counterpart PC(t), and the overlap probability PQC(t) between a time evolved wave packet and a classically moving Gaussian with stationary shape are calculated in order to analyze the spreading of the wave packet. It was found that, in general, PQ(t) and PC(t) agree very well if the potential energy function has no strongly changing low order derivatives. This agreement is held even if the wave packet spreads drastically within the time scale considered as can be demonstrated by analyzing PQC(t). The analytical expressions for PQ(t), PC(t), and PQC(t) are applied to a dissociative third-order oscillator and a strongly anharmonic fourth-order oscillator. While for a harmonic system, the initial decay time of PQ(t) and PC(t) (which are identical in this case) are only dependent on the energy expectation value 〈H〉 with respect to the initial HOCS, differences up to an order of magnitude occur for HOCS with the same energy in the model systems.

We investigate how wave functions evolve with time in the harmonic oscillator. We first review the periodicity properties over each multiple of a quarter of the classical oscillation period. Then we show that any wave function can be simply transformed so that its centroid, defined by the expectation values of position and momentum, remains at rest at the center of the oscillator. This implies that we need only consider the evolution of this restricted class of wave functions; the evolution of all others can be reduced to these. The evolution of the spread in position Δx and momentum Δp throws light on energy and uncertainty and on squeezed and coherent states. Finally, we show that any wave function can be transformed so that Δx and Δp do not change with time and that the evolution of all wave functions can easily be found from the evolution of those at rest at the origin with unchanging Δx and Δp.

Abstract 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.

The time evolution of wave packets in the Morse potential is investigated under the influence of the environment consisting of harmonic oscillators. These oscillators represent photon or phonon modes and are assumed to be in thermal equilibrium. Our model explicitly incorporates the fact that in the case of a nonequidistant spectrum the rates of the environment induced transitions are different for each transition. The nonunitary time evolution is visualized by the aid of the corresponding Wigner function. The time scale of decoherence is much shorter than that of dissipation, and gives rise to states that are mixtures of localized states along the phase-space orbit of the corresponding classical particle. This behavior is to a large extent independent of the coupling strength, the temperature of the environment, and also the initial state.

We apply our minimum-uncertainty coherent-states formalism, which is physically motivated by the classical motion, to two confining one-dimensional systems: the harmonic oscillator with centripetal barrier and the symmetric P\"oschl-Teller potentials. The minimum-uncertainty coherent states are discussed in great detail, and the connections to annihilation-operator coherent states and displacement-operator coherent states are given. The first system discussed provides an excellent bridge between the harmonic oscillator and more general potentials because, even though it is a nonharmonic potential, its energy eigenvalues are equally spaced. Thus, its coherent states have many, but not all, of the properties of the harmonic-oscillator coherent states.

In this study, we obtained the quasi-coherent states of the damped harmonic oscillator which satisfy the Hermite differential equation classically. For the general damped oscillator, the Gaussian wave packets were derived in configuration and momentum spaces with minimum uncertainities at t = 0, and the quasi-stationary states also obtained and showed that the expansion coefficients give a time-dependent Poisson distribution. As a special case, we found the displaced Gaussian wave packets for the Hermite oscillator and also discussed the weak coupling limit of the wave packets.