Vibrational energy transfer from highly excited anharmonic oscillators. Dependence on quantum state and interaction potential

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Abstract
In order to elucidate the general features of vibrational deactivation of highly excited anharmonic oscillators, we present quasiclassical trajectory calculations on prototype collinear I2 (v)‐inert gas collision systems. The results for vibrational‐translational energy transfer reveal several interesting trends as a function of initial vibrational quantum state, projectile mass, and projectile–oscillator interaction potential. (1) Vibrational deactivation is inefficient from a l l quantum levels and for all projectile masses. The average energy transfer per collision ΔE is strongly peaked at intermediate vibrational levels (v≊80) and is observed to be at most ≊−k b T. Furthermore, when scaled to h/ω(E), the ’’local’’ oscillatorenergy spacing, ΔE can be accurately represented by a simple power law in vibrational quantum number over a wide range of bound states. (2) Energy transfer is progressively l e s s efficient from levels in the neighborhood of and approaching dissociation. (3) Vibrational energy loss for high levels of initial vibrational excitation (v≳90) is rather insensitive to the nature of the interaction potential. Smooth exponential and hard‐sphere interaction results differ by less than an order of magnitude. This observed insensitivity motivates the development of an analytic collision model, in which simple hard‐sphere geometry and dynamics are used to calculate ΔE. The model results are in qualitatively good agreement with trajectory calculations and also indicate that nonuniform sampling of the anharmonic oscillator velocity and phase are responsible for decreased energy transfer efficiency from high vibrational states.

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