EXCITONS IN C-60 STUDIED BY TEMPERATURE-DEPENDENT OPTICAL 2ND-HARMONIC GENERATION

The electric-dipole-forbidden T-1(1g) excitonic state of solid C-60 at hw=1.81 eV can be probed with a second-harmonic generation (SHG) expe riment. We show that the SHG line shape depends strongly on the degree of rotational order. We observe a splitting into two peaks below the rotational ordering phase-transition temperature of 260 K. The origin of this splitting is discussed in terms of a possible Jahn-Teller effe ct, a possible Davydov splitting due to the four molecules per unit ce ll in the low-temperature phase, and a mixing of the nearly degenerate T-1(1g) and (1)G(g) free molecule states because of the lower symmetr y in the solid. The exciton band structure is calculated with a charge -transfer-mediated propagation mechanism as suggested by Lof et al.(2) and with one-electron (-hole) transfer integrals determined from band -structure calculations. Comparison with our experimental SHG data lea ds to a reasonable agreement and shows that a mixing of (1)T(1)g and ( 1)G(g) states may explain the splitting at low temperature.