Collision partner and level dependence of vibrational relaxation in S p-difluorobenzene. Stimulated emission pumping combined with single vibronic level fluorescence spectroscopy

Abstract

Rate coefficients have been measured for vibrational relaxation in S0 p-difluorobenzene vapor under assault from eight collision partners. The dissipation of vibrational energy is monitored from each of four vibrational states with energies in the range ∼1500–3300 cm−1. Rate coefficients are found to vary between 1.5×10−10 cm3 molecule−1 s−1 for helium colliding with p-difluorobenzene in the lowest state explored, to 12×10−10 cm3 molecule−1 s−1 for p-difluorobenzene self-relaxation from the highest level studied. The dependence of collision partner on the vibrational relaxation efficiency for each initially prepared state is studied, revealing a dependence on the reduced mass of the collision pair, and/or the well depth of the system. In addition the dependence of the efficiency on the initially prepared state is investigated for any one collision partner using a modified version of the Parmenter–Tang propensity rule model. A comparison between these data for vibrational relaxation in the S0 state of p-difluorobenzene and previous data for vibrational relaxation in the S1 state of the same molecule is discussed.