THE HAM BANDS REVISITED - SPECTROSCOPY AND PHOTOPHYSICS OF THE C6H6-CCL4 COMPLEX

The S0-S1 spectroscopy of the C6H6-CCl4 complex formed in a supersonic expansion has been investigated using both laser-induced fluorescence and resonant two-photon ionization (R2PI) methods. The ability of mass-selected R2PI to record spectra of the complex essentially free from interference from other sources allows the observation of several transitions which are forbidden in C6H6 but induced by the presence of CCl4. In particular, the S0-S1 origin is present at an intensity about 20% of the intensity of the vibronically allowed 6(0)1 transition. 1(0)1 and 16(0)1 are also induced by the CCl4. The latter transition can be induced only if the complex possesses at most C(s) symmetry. Transitions involving degenerate vibrations (e.g., 6(0)1 and 16(0)1 are split by 2-3 cm-1 due to the presence of the CCl4 molecule. The van der Waals' structure of the transitions is analyzed in terms of progressions involving the van der Waals' bend and stretch. Dispersed fluorescence spectra provide bounds on the ground and excited state binding energies of the complex of 2.44 less-than-or-equal-to D0" less-than-or-equal-to 3.25 kcal/mol and 2.63 less-than-or-equal-to D0' less-than-or-equal-to 3.44 kcal/mol, respectively. Below the dissociation threshold, the fluorescence lifetimes of the complex are 4-10 times shorter than the corresponding levels of free benzene. A comparison of these lifetimes with those from C6H6-CFCl3 suggests that the source of the reduced fluorescence lifetime is coupling of the S1 state with a charge-transfer state of the complex. Finally, two-color photoionization efficiency scans through the 6(0)1 transition of C6H6-CCl4 show a distinct threshold at a total energy of 9.094(-0.014)+0.007 eV. Combining these with the bounds on the ground-state binding energy constrains the binding energy of the ionized complex to be between 5.65 and 7.0 kcal/mol.