FEMTOSECOND TRANSIENT ABSORPTION-SPECTROSCOPY ON THE LIGHT-HARVESTINGCHL A B PROTEIN COMPLEX OF PHOTOSYSTEM-II AT ROOM-TEMPERATURE AND 12 K/

The kinetics of excitation energy transfer processes within the light- harvesting Chi a/b protein complex of Photosystem II (LHC II) from spi nach was investigated using femtosecond transient absorption differenc e spectroscopy at room temperature and 12 K. Upon excitation of Chi b with 640 nm pulses of 120 fs duration, time-resolved absorption change s were measured at different wavelengths within the Q(y)(0-0) transiti on band of Chi a. At room temperature the transient spectra showed a s ingle negative band which is due to photobleaching and/or stimulated e mission (PB/SE). The peak position of this band was found to be time i ndependent (greater than or equal to 400 fs) at about 680 nm. In contr ast, at 12 K the maximum of the PB/SE band shifted from 676 nm at 400 fs to about 680 nm at later times (greater than or equal to 100 ps). T he absorption difference spectra at 12 K showed a bipolarity at times greater than or equal to 7.7 ps, where excited state absorption and PB /SE predominate at shorter and longer wavelengths, respectively. The d ynamic spectral evolution processes at 12 K could be rationalized in t erms of excitation energy transfer between excitonically coupled anten na pigments in isolated LHC II complexes. Based on a global analysis o f the flash induced absorption changes at 12 K several components have been obtained with lifetimes in the subpicosecond and picosecond time domain. The results of these fits are discussed with respect to the o bserved time dependent spectral changes.