Vibrational deactiviation of highly excited hexafluorobenzene

Abstract

Highly excited hexafluorobenzene (HFB) molecules in the electronic ground state were prepared by infrared multiphoton absorption by CO₂ laser pumping using the P(38) line at 1029.43 cm⁻¹. The vibrational deactivation of the highly excited HFB by monatomic collider gases was monitored by time-resolved infrared fluorescence (IRF). Deactivation measurements were made for the noble gas colliders He, Ne, Ar, Kr, and Xe. The bulk average energy transferred per collision, <<ΔΕ>>, for these colliders was found to increase from He through to Ar; however it subsequently decreased from Ar through to Xe. This is different than the trend found in the quasiclassical trajectory calculations by Lenzer et al., which predict a decrease in the average energy transferred per collision from He to Ar to Xe. (Calculations were not reported for Ne and Kr.) However, similar trends in related energy-transfer parameters have been reported for the deactivation of C₆F₁₄ and C₈F₁₈ by the same series of five noble gases. A comparison is made with previous experimental measurements for the colliders He and Ar which were obtained using UV excitation of the HFB. For the same monatomic colliders, the <<ΔΕ>> values for HFB are much greater than those for the closely related aromatics, benzene, toluene, and pyrazine.