Quantum versus classical dynamics in the treatment of multiple photon excitation of the anharmonic oscillator

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Abstract
The response of a one-dimensional anharmonic Morse oscillator to an intense electromagnetic field has been investigated using both a quasiclassical and quantum mechanical description of the oscillator. The anharmonic nature of the Morse potential reduces the coherence of the quantum excitation process after only a few quanta have been absorbed. The classical and quantum behavior of averaged quantities such as the energy absorbed and the oscillator displacement as a function of time are in good agreement; however, the classical description cannot reproduce the multiphoton resonances. We are led to the conclusion that classical mechanics provides an adequate description of the response of a molecule in an intense laser field provided that multiphoton resonances do not individually play a fundamental role in the process.