Vibrational echoes for classical and quantum solutes

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Abstract
The infrared vibrational echo measurement has the capacity to discriminate among spectral line-broadening processes according to time scale. Modelling infrared vibrational echoes in condensed phases at the molecular level requires computing the dynamics of large, anharmonic systems, a task far more tractable in classical than in quantum mechanics. The use of classical trajectory data to compute vibrational echoes raises the issue of the quantitative relationship between classical and quantum vibrational echo calculations. This relationship is addressed here in classical and quantum calculations of the vibrational echo for an anharmonic oscillator coupled to a harmonic solvent. Our calculations demonstrate that for a high-frequency solute coupled to a low-frequency solvent, in which the echo is dominated by pure dephasing dynamics, a fully classical calculation can represent a reasonable approximation to the quantum mechanical result.