Investigation of the internal conversion time of the chlorophyll a from S3, S2 to S1

By using femtosecond time-resolved stimulated emission pumping fluorescence depletion (FS-TR-SEP-FD), we present a fluorescence depletion study of chlorophyll a (Chl a) in various solvent environments. Internal conversion times (IC) in different solvents are observed. It is found that all the S3 and S2 to S1 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.