Vibrational relaxation cross sections and rates of equilibration in electronically excited benzene (2537 Å excitation)

The fluorescence spectrum of benzene vapor, excited to a vibrational level 6211 of the 1B2u state by Hg 2537 Å, has been studied in the presence of up to 600 Torr of 16 different added gases. Benzene-d6 has been similarly studied in the presence of N2 and CO2. Low pressure data yield cross sections (which are of the order of hard sphere) for the process 6211→ other vibrational states which conform with the expectation of H. M. Lin, M. Seaver, K. Y. Tang, A. E. W. Knight, and C. S. Parmenter, [J. Chem. Phys. 70, 5442(1979)] that such cross sections should be determined by the intermolecular well depths. High pressure data yield efficiencies in respect of bringing about vibrational equilibration. Estimated average energies lost per hard sphere collision are broadly determined by the number of degrees of vibrational freedom possessed by the colliding molecules. Quantum yields decline to a limiting value as the 6111 state is depopulated.