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The Journal of Chemical Physics

Linear and nonlinear response functions of the Morse oscillator: Classical divergence and the uncertainty principle

Jianlan Wu, Jianshu Cao

Published Sep 22, 2001• 69 citations

Abstract

The algebraic structure of the quantum Morse oscillator is explored to formulate the coherent state, the phase-space representations of the annihilation and creation operators, and their classical limits. The formulation allows us to calculate the linear and nonlinear quantum response functions for microcanonical Morse systems and to demonstrate the linear divergence in the corresponding classical response function. On the basis of the uncertainty principle, the classical divergence is removed by phase-space averaging around the microcanonical energy surface. For the Morse oscillator, the classical response function averaged over quantized phase space agrees exactly with the quantum response function for a given eigenstate. Thus, phase-space averaging and quantization provide a useful way to establish the classical-quantum correspondence of anharmonic systems.

DOI: 10.1063/1.1389840

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The algebraic structure of the quantum Morse oscillator is explored to formulate the coherent state, the phase-space representations of the annihilation and creation operators, and their classical limits. The formulation allows us to calculate the linear and nonlinear quantum response functions for microcanonical Morse systems and to demonstrate the linear divergence in the corresponding classical response function. On the basis of the uncertainty principle, the classical divergence is removed by phase-space averaging around the microcanonical energy surface. For the Morse oscillator, the classical response function averaged over quantized phase space agrees exactly with the quantum response function for a given eigenstate. Thus, phase-space averaging and quantization provide a useful way to establish the classical-quantum correspondence of anharmonic systems.

In Planck's model of the harmonic oscillator (HO) a century ago, both the energy and the phase space were quantized according to epsilon(n) = nh nu, n = 0, 1, 2..., and integral integral dp(x) dx = h. By referring to just these two relations, we show how the adoption of cycle-averaged phase-space states (CAPSSs) leads to the quantum mechanical energy spectrum of the HO, < E(n)> = (n + 1/2)h nu, n = 0, 1, 2,..., where < E(n)> are the average energies, and to < J(n)> = (n + 1/2)h/2 pi, where < J(n)> are the average actions. When anharmonicity to all orders is added in the form of the Morse oscillator (MO), the concept of CAPSS is implemented in terms of action-angle variables and it is shown that the use of < J(n)> of each MO CAPSS also produces the correct discrete spectrum of the MO, again without applying quantum mechanics (QM). In addition, the concept of CAPSS leads to two well-known post-QM relations which are obtained in terms of time averages of the classical trajectories and of < J(n)> : (1) closed integral p dx = 2 pi < J(n)> = (n + 1/2)h, which is the quantum condition of the old quantum theory, albeit with half-integers (i.e. the result of the WKB approximation), and (2) (Delta p Delta x)(n) >= h/4 pi, which is the Heisenberg uncertainty principle. It is shown via numerical computations that, for two MOs, one with intermediate anharmonicity, supporting 22 levels, and another with strong anharmonicity, with 5 levels, the quantities(Delta p Delta x)(n), n = 0, 1,..., 4, which are computed classically for the appropriately chosen trajectories agree very well with the results of computations that apply QM. The introduction of the CAPSS and the concomitant results underline the significance of the concept of the state of the system in physics, both classical and quantum.

We construct a deformed oscillator whose energy spectra is similar to that of a Morse potential. We obtain a convenient algebraic representation of the displacement and the momentum of a Morse oscillator by expanding them in terms of deformed creation and annihilation operators and we compute their average values between approximate coherent states of the deformed oscillator, and we compare them to the results obtained using the exact Morse coordinate and momenta. Finally we evaluate the temporal evolution of the dispersion (Δx)(Δp) and show that these states are not minimum uncertainty states.

For the one-dimensional Morse oscillator we have built a set of creation and annihilation operators and established the corresponding coherent states. The average values of some observables in these coherent states have been calculated and their time evolution established. In order to study the strong vibrations of di-atomic molecules we use a three-dimensional Morse oscillator model and its dynamic group. For this system we have built the coherent states and calculated the dynamic modifications of the refractive index of the molecules, as the result of interaction with a laser beam.

We obtain the exact, normalized, closed-form eigenfunctions for the one-dimensional Morse oscillator, as well as the raising and lowering operators. We next review a new method for obtaining the coherent states for arbitrary potentials, it being based on the classical motion of a particle. We apply this method to the Morse oscillator. After demonstrating that in the appropriate limit our results reduce to those for the harmonic oscillator, we obtain analytic insights into published numerical results on uncertainty products for Morse-oscillator wave packets.