COLLISIONAL DEACTIVATION OF HIGHLY VIBRATIONALLY EXCITED BENZENE PUMPED AT 248-NM

Highly vibrationally excited gas-phase benzene was prepared with a pulsed excimer laser operating at 248 nm, and the subsequent collisional deactivation was monitored with time-resolved infrared fluorescence (IRF) from the C-H stretch modes near 3050 cm-1. Very low pressure photolysis mass spectrometric experiments were carried out to determine whether reaction of the benzene occurs at this laser wavelength and can complicate the energy-transfer investigation. The quantum yield for benzene loss was determined to be 3 ± 1%, consistent with previous experiments (Nakashima, N.; Yoshihara, K. J. Chem. Phys. 1982, 77, 6040) that found photoproducts as the result of multiphoton excitation. The only detectable gas-phase product was H2, which had a quantum yield of 0.8 ± 0.3%. Energy-transfer data were obtained for 19 collider gases, including unexcited benzene. The inversion technique for converting the observed IRF decay to 〈〈E(t)〉〉, the bulk average energy, was examined in detail, and a careful propagation of errors analysis performed. For most colliders, 〈〈ΔE〉〉, the bulk average energy transferred per collision, exhibited an approximately linear dependence on vibrational energy for energies <25 000 cm-1 and a tendency to have a reduced dependence on energy at higher energies. This tendency is not statistically significant at the 95% confidence level for most colliders, although it may be a real effect. Some of the limitations of the IRF method are explored with the aid of master equation simulations, and the results are discussed in the context of other data. © 1990 American Chemical Society.