Bath Model for N₂ + C₆F₆ Gas-Phase Collisions. Details of the Intermolecular Energy Transfer Dynamics

Relaxation of vibrationally excited C₆F₆∗ in a thermalized bath of N₂ molecules is studied by condensed-phase chemical dynamics simulations. The average energy of C₆F₆ as a function of time, <E(t)>, was determined using two different models for the N₂-C₆F₆ intermolecular potential, and both gave statistically the same result. A simulation with a N₂ bath density of 20 kg/m³ was performed to check the convergence and validate the results obtained previously with a higher bath density of 40 kg/m³. The initial ensemble of C₆F₆ is nearly monoenergetically excited, but the ensemble acquires as energy distribution P(E) as it is collisionally relaxed. An evaluation of P(E) and the root-mean-square deviation <ΔE²>^1/2 of P(E), versus time, shows that P(E) first broadens and then narrows. Simulations with the C₆F₆ vibrational excitation energy of 85.8 kcal/mol, studied experimentally, show that the width of P(E) does not affect the average collisional deactivation rate. The role of the intramolecular vibrational frequencies on the energy transfer dynamics was studied by changing the mass of the F-atoms of C₆F₆ to that of an H-atom. The resulting increase in the frequencies decreased the efficiency of collisional energy transfer. Simulations with classical and quantum microcanoical ensembles of the C₆F₆ initial vibrational energy were compared, and the initial relaxation rate was slower for the quantum ensemble.