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

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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.