Energy redistribution in large molecules. Direct study of intramolecular relaxation in the gas phase with picosecond gating

Abstract

Results are presented on the picosecond time evolution of the fluorescence spectrum of p-difluorobenzene under collision-free conditions. The spectra were obtained by a novel method of time gating involving frequency down-conversion of fluorescence by non-linear mixing with a picosecond laser pulse. A single vibronic level (SVL) fluorescence spectrum of the 3¹₂30³₃ and 3¹₁5⁰₁6⁰₁30³₃ transitions was obtained with 10 ps gating. The spectrum also displays an unstructured portion whose time evolution was found to differ from that of the structured part. The observed kinetics are consistent with intramolecular rovibrational energy redistributions occurring on a timescale of ca. 6 ps.