Excited-state dynamics of bacteriorhodopsin probed by broadband femtosecond fluorescence spectroscopy

The impact of varying excitation densities (similar to0.3 to similar to40 photons per molecule) on the ultrafast fluorescence dynamics of bacteriorhodopsin has been studied in a wide spectral range (630-900 nm). For low excitation densities. the fluorescence dynamics can be approximated biexponentially with time constants of <0.15 and similar to0.45 ps. The spectrum associated with the fastest time constant peaks at 650 nm, while the 0.45 ps component is most prominent at 750 nm. Superimposed on these kinetics is a shift of the fluorescence maximum with time (dynamic Stokes shift). Higher excitation densities alter the time constants and their amplitudes. These changes are assigned to multi-photon absorptions. (C) 2004 Elsevier B.V. All rights reserved.