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

A chemical dynamics simulation was performed to model experiments [N. A. West et al., J. Chem. Phys. 145, 014308 (2016)] in which benzene molecules are vibrationally excited to 148.1 kcal/mol within a N₂-benzene bath. A significant fraction of the benzene molecules are excited, resulting in heating of the bath, which is accurately represented by the simulation. The interesting finding from the simulations is the non-statistical collisional energy transfer from the vibrationally excited benzene C₆H₆ ^∗ molecules to the bath. The simulations find that at ∼10^-7 s and 1 atm pressure there are four different final temperatures for C₆H₆ ^∗ and the bath. N₂ vibration is not excited and remains at the original bath temperature of 300 K. Rotation and translation degrees of freedom of both N₂ and C₆H₆ in the bath are excited to a final temperature of ∼340 K. Energy transfer from the excited C₆H₆ ^∗ molecules is more efficient to vibration of the C₆H₆ bath than its rotation and translation degrees of freedom, and the final vibrational temperature of the C₆H₆ bath is ∼453 K, if the average energy of each C₆H₆ vibration mode is assumed to be RT. There is no vibrational equilibration between C₆H₆ ^∗ and the C₆H₆ bath molecules. When the simulations are terminated, the vibrational temperatures of the C₆H₆ ^∗ and C₆H₆ bath molecules are ∼537 K and ∼453 K, respectively. An important question is the time scale for complete energy equilibration of the C₆H₆ ^∗ and N₂ and C₆H₆ bath system. At 1 atm and 300 K, the experimental V-T (vibration-translation) relaxation time for N₂ is ∼10^-4 s. The simulation time was too short for equilibrium to be attained, and the time for complete equilibration of C₆H₆ ^∗ vibration with translation, rotation, and vibration of the bath was not determined.