Collisional effects on the fluorescence of p-fluorotoluene vapour

Abstract

Bimolecular processes occuring upon excitation of p-fluorotoluene vapour to the first excited singlet state have been followed using fluorescence spectral, quantum yield and decay time measurements. Below a pressure of 0.5 torr the molecules behave independently of collisions. Above this pressure of p-fluorotoluene (PFT), or of added pentane, vibrational relaxation is in competition with electronic relaxation, resulting in fluorescence spectra changes, fluorescence quantum yield changes, and non-exponential fluorescence decay curves. These have been analysed according to a stochastic model in which the molecule is represented as having one effective oscillator and relaxes by one quantum jumps. A best fit to the experimental data was obtained for an effective six-fold degenerate oscillator of frequency 500 cm −1, assuming 90–100% collision efficiency. Addition of 2-pentanone does not apparently cause significant vibrational relaxation, but causes electronic quenching with unit collision efficiency at the zero-point level by electronic energy transfer. Quenching by 2-pentanone is slightly less efficient for excitation of PFT to higher vibrational levels and a tentative suggestion for the cause of this is offered.