RAMAN EXCITATION PROFILES OF C-7O IN BENZENE SOLUTION - ASSIGNMENT OFTHE ELECTRONIC-SPECTRUM IN THE 380-510-NM REGION

The resonance Raman (RR) spectrum of C-70 has been studied in benzene using 11 laser excitation energies across the main visible absorption band (MVAB) of C-70 between 514.5 and 406.7 nm. Raman excitation profi les (REPs) were constructed for the 15 most intense RR bands of C-70, and symmetry assignments have been made partly on the basis of polariz ation work. Contrast is made to work performed on thin films where pro blems have arisen from the symmetry-lowering effect of the surface and from neglect of resonance. Assignments for nine other less intense RR bands are suggested. Three electronic transitions under the MVAB are identified and assigned definitively: the HOMO - 4 (e(2) '') --> LUMO + 1 (e(1) '') transition in the 514/501-nm excitation region, the HOMO - 5 (e(1)') --> LUMO + 1 (e(1) '') transition in the 476/472-nm excit ation region, and the HOMO (a(2) '') --> LUMO + 2 (a(1) '') in the 457 /452-nm excitation region. The REPs reveal that these three electronic transitions are vibronically coupled to the strong electronic transit ion at 382 nm which is assigned to the HOMO - 2 (a(2)') --> LUMO + 3 ( e(1)') transition. RR B-term scattering mechanisms are the major sourc e of intensity enhancement for bands of the totally-symmetric A(1)' an d the non-totally-symmetric E-1 '' and E-2' Raman modes. The REPs of t he 15 bands are grouped into four types that provide insight into the change in the electronic distribution upon excitation for each transit ion. Unlike C-60, whose extraordinarily high symmetry makes it very se nsitive to solvent-induced symmetry lowering and whose RR spectrum is rich in forbidden, overtone, and combination bands, C-70 displays a re stricted subset of RR scattering phenomena. The lower symmetry and mor e localized molecular orbitals of C-70 make it a better model for the RR scattering mechanisms and vibronic coupling expected in the higher fullerenes.