Picosecond studies of the intramolecular relaxation processes in isolated C-60 and C-70 molecules

Time-resolved fluorescence spectra for C-60 molecules embedded in Ne and Ar matrices and C-70 in Ne matrices were recorded using a synchroscan streak camera. In the case of C-60, the decay times of the S-1 state are identical for all fluorescence bands confirming that it is a state of mixed T-1g, T- 2g, and G(g) characters. Its lifetime is determined by the intersystem cros sing rate to the triplet manifold which increases in going from Ar to Ne ma trices. This is attributed to a change of the dominant symmetry character o f S-1 in the two matrices. A transient, short-lived (similar to 170 ps in N e, similar to 70 ps in Ar), S-3 fluorescence of dominant G(g) symmetry is a lso observed, which feeds the S-2/S-1 population (of dominant T-2g and T-1g symmetry). Using the Energy Gap Law, a change of the S-3 --> S-2/S-1 decay rate from Ne to Ar matrices is attributed to the different number of phono ns needed to bridge the gap between these states and/or to an environment-d ependent change of the coupling matrix element. In C-70, the fluorescence f rom both S-1 and S-2 are populated at the same rate despite the 165 cm(-1) energy gap between them. This is explained in terms of an intersystem cross ing from the triplet state manifold. Finally, S-2 decays predominantly to S -1, while the latter decays to the triplet states by reversible intersystem crossing. These conclusions are confirmed by a simple kinetic model. (C) 1 999 American Institute of Physics. [S0021-9606(99)01226-X].