By using femtosecond time-resolved stimulated emission pumping fluorescence depletion (FS-TR-SEP-FD), we present a fluorescence depletion study of chlorophyll a (Ch1 a) in various solvent environments. Internal conversion times (IC) in different solvents are observed. It is found that all the S-3 and S-2 to S-1 transition processes are extremely fast, and that the time constants of these processes range from 100 to 260 fs. In aprotic solvent, the rate of IC decreases with an increasing of solvent polarity. (c) 2005 Elsevier B.V. All rights reserved.