A first comparison of vibrational relaxation in ground and excited electronic states of polyatomic molecules. S₀ and S₁ p-difluorobenzene

Rate constants are obtained for vibrational relaxation from selected vibrational levels in S₀ and S₁ p-difluorobenzene (pDFB), induced by collisions with foreign gas, M. The levels studied are 5₂30₂ (∈_vib = 2036 cm^-1) in the S₀ ground electronic state, and the pair of levels 3¹ (∈_vib = 1251 cm^-1), 3¹30¹ (∈_vib = 1373 cm^-1) in the S₁ excited electronic state. The S₀ level is explored by using stimulated emission pumping, single vibronic level fluorescence probing (SEP-SVLF), while S₁ levels are studied by using the more conventional approach of monitoring the evolution of dispersed fluorescence as a function of added pressure of foreign gas. Collision efficiencies for relaxation of the S₁ levels with M = Ar, CO₂, C₆H₁₂ are in accord with expectations based on related studies of levels in S₁ benzene. S₀ p-difluorobenzene displays efficiencies for vibrational relaxation that are completely consistent with these S₁ studies. We conclude that vibrational relaxation in polyatomics such as benzene and p-difluorobenzene should normally proceed with closely comparable efficiencies in both the excited and ground electronic states of the same molecule.