The collisional flow of vibrational energy into surrounding vibrational fields within S1 benzene

Abstract

Vapor phase fluorescence spectra are used to determine the absolute rate constants for the collisional transfer of vibrational energy from initial single vibronic levels of S₁ benzene into the surrounding S₁ vibronic field. 11 initial levels are probed with vibrational energies ranging to 2368 cm⁻¹ where the level density is about 10 per cm⁻¹. CO, isopentane, and S₀ benzene are the collision partners. Benzene rate constants are three to four times gas kinetic for all levels, and electronic energy switching between the initial S₁ molecule and the S₀ collision partner probably makes important contributions. Isopentane efficiencies range from one to two times gas kinetic. Most of the transfer from low S₁ levels occurs with excitation of vibrational energy within isopentane. These V–V contributions decline to only about 10% for the high transfer. CO‐induced transfer is by V‐T,R processes for all levels. The CO efficiency rises from about 0.1 for low regions to about unity for levels above 1500 cm⁻¹. The CO efficiencies retain significant sensitivity to initial level identity even in the higher regions. Propensity rules derived from collisional mode‐to‐mode transfer among lower levels of S₁ benzene are used to calculate the relative CO efficiencies. The calculated efficiencies agree well enough with the data to suggest that it may be meaningful to model vibrational equilibration with the use of propensity rules. The rules suggest that only a small number of levels among the thousands surrounding a high initial level contribute significantly to the total relaxation cross section and that this number is rather independent of the level density. While CO and isopentane have comparable efficiencies for relaxation of a high S₁ level, CO is much less efficient for inducing full thermal equilibration. CO, which uses V‐T,R processes only, becomes a relatively poor transfer partner once low levels are reached during the equilibration, whereas V–V transfer sustains isopentane’s large efficiency for all levels.