EXCITED-STATE DYNAMICS OF BACTERIORHODOPSIN REVEALED BY TRANSIENT STIMULATED-EMISSION SPECTRA

The femtosecond photoisomerization of bacteriorhodopsin (bR), pumped a l either 550 nm or 405 nm, was probed between 640 nm and 950 nm. The 5 50 nm pump spectra exhibit net gain signals peaking at > 950 nm. A tra nsient absorption is proposed to partially cancel the emission. The 40 5 nm pump transients exhibit nonexponential kinetics and display this new absorption, due to the presence of hot molecules. The absence of t ime dependence of the shape of the stimulated emission spectra is inco nsistent with conventional models invoking low-frequency motion out of the Franck-Condon region following excitation. The time independence is reminiscent of the photoisomerization of cis-stilbene.