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

Abstract

Relaxation of vibrationally excited C 6 F 6 * in a thermalized bath of N 2 molecules is studied by condensed-phase chemical dynamics simulations. The average energy of C 6 F 6 as a function of time, ⟨E(t)⟩, was determined using two different models for the N 2 −C 6 F 6 intermolecular potential, and both gave statistically the same result. A simulation with a N 2 bath density of 20 kg/m 3 was performed to check the convergence and validate the results obtained previously with a higher bath density of 40 kg/m 3 . The initial ensemble of C 6 F 6 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 ⟨ΔE 2 ⟩ 1/2 of P(E), versus time, shows that P(E) first broadens and then narrows. Simulations with the C 6 F 6 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 C 6 F 6 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 C 6 F 6 initial vibrational energy were compared, and the initial relaxation rate was slower for the quantum ensemble.

Non-statistical intermolecular energy transfer from vibrationally excited benzene in a mixed nitrogen-benzene bath

“…IV. Consistent with previous simulations, [27][28][29] there is negligible energy transfer to N 2 vibration. The center-of-mass translation and rotation energies of the N 2 molecules increase with time, via V → R/T (vibration to rotation and translation) energy transfer.…”

“…2. As found for previous simulations [27][28][29][30] and as shown in Fig. 2, E(t) is well fit by the bi-exponential…”

“…To achieve the binary/single collision limit for comparison with experiments, the bath density was chosen as 40 kg/m 3 or 32.5 atm which was found to be the binary/single collision limiting density for C 6 F 6 + N 2 simulations. 27,28 For the smaller C 6 H 6 molecule as compared to C 6 F 6 and a very small percentage of C 6 H 6 in the bath, the binary/single collision limiting density is expected to be achieved at 40 kg/m 3 or higher density. Performing the simulations in the binary/single collision limit allows extrapolation of the simulation results to lower densities/pressures.…”

“…The simulations were performed with the same methodology as described for previous intermolecular energy transfer bath simulations. [27][28][29][30][31] A vibrational energy of 148.1 kcal/mol was first added to the one excited C 6 H 6 * molecule to model the experimental 193 nm laser excitation. This energy was added randomly with classical microcanonical normal mode sampling, 38,39 as implemented in a modified version 27 of the general chemical dynamics computer code VENUS.…”

“…However, simulations of energy transfer in collisions with organic surfaces have shown that statistical-like intermolecular energy transfer may occur with direct collisions and without formation of a collision intermediate. 25,26 In recent work, a unified protocol for simulating liquid and gas phase intermolecular energy transfer was developed 27 and applied to both vibrationally excited C 6 F 6 and azulene in a N 2 bath, [27][28][29] vibrationally excited propyl benzene cation in a helium bath, 30 and cold C 6 F 6 in a warmer N 2 bath. 31 Here, this simulation method is used to model intermolecular energy from vibrationally excited benzene to compare with experiments in which a fraction of benzene molecules are excited within a N 2 -benzene bath.…”