J . Phys. Chem. 1991, 95,9699-9707
9699
The Ham Bands Revisited: Spectroscopy and Photophysics of the C6H6-CCI, Complex Albert J. Gotch, Aaron W. Garrett, and Timothy S.Zwier*Yt Department of Chemistry, Purdue University, West Lafayette, Indiana 47907- 1393 (Received: April 29, 1991)
The So-SI spectroscopy of the C,&-CC14 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 R2P1 to record spectra of the complex essentially free from interferencefrom other sources allows the observation of several transitions which are forbidden in C6H6 but induced by the presence of CCI,. In particular, the So-Sl origin is present at an intensity about 20%of the intensity of the vibronically allowed 6; transition. 1; and 16; are also induced by the CCl+ The latter transition can be induced only if the complex possesses at most C, symmetry. Transitions involving degenerate vibrations (e&, 6; and 16;) are split by 2-3 cm-’ due to the presence of the CCI4 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 IDd’ I3.25 kcal/mol and 2.63 ID,,’ I3.44 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-CFCI3suggests that the source of the reduced fluorescence lifetime is coupling of the SIstate with a charge-transfer state of the complex. Finally, two-color photoionization efficiency scans through the 6; transition of C&,
E 40
a2000
0
-2000
-4000
Relative Frequency (cm-'1
20
Figure 6. Dispersed fluorescence scans from (a) the zero-point level of the SI state of free benzene, (b) 6'1' of C6H6
