Journal of Physical Chemistry A, Vol.108, No.26, 5564-5572, 2004
The unusual intensity behavior of the 281-cm(-1) resonance Raman band of C-60: A complex tale of vibronic coupling, symmetry reduction, solvatochromism, and Jahn - Teller activity
Resonance Raman (RR) Stokes and anti-Stokes experiments were performed on C-60 in benzene, toluene, and CS2 solutions in the low energy region 240-300 cm(-1) with the laser probing the first-allowed electronic transition, "A". Two bands were evident at 265 and 281 cm(-1), which are both assigned to E modes that originate from the H-g(1) mode of IhC60. The 16 cm(-1) energy separation suggests that the ground-state geometry of C-60 in solution is significantly distorted. The solvatochromism of C-60 was used to "tune" the laser relative to the A transition. The band at 281 cm(-1) displays unusual intensity behavior in that it has greater intensity in the anti-Stokes region of some RR experiments relative to the Stokes region, even though thermal populations of the first vibrational excited state do not exceed 20%. Further, the 281 cm(-1) band appears to have intensity when the incident laser is able to probe the 1(1)A(g) --> [1T(lu) + H-g(1)] vibronic transition (0-1) of C-60, but not when the laser is in resonance with the pure electronic transition, 1(1)A(g) --> 1T(lu) (0-0). As such, the 281 cm(-1) band appears to have a Raman excitation profile that follows the 0-1 absorption envelope. The RR intensity of the 281 cm(-1) band is proposed to derive from the intramanifold coupling of the A and E states, which arise from the T-lu electronic state under I-h symmetry, further strengthening the case that D-5d is a good approximation for the symmetry of C-60 in solution.