Excited state dynamics of bacteriorhodopsin revealed by transient stimulated emission spectra

The femtosecond photoisomerization of bacteriorhodopsin (bR), pumped at either 550 nm or 405 nm, was probed between 640 nm and 950 nm. The 550 nm pump spectra exhibit net gain signals peaking at > 950 nm. A transient absorption is proposed to partially cancel the emission. The 405 nm pump transients exhibit nonexponential kinetics and display this new absorption, due to the presence of hot molecules. The absence of time dependence of the shape of the stimulated emission spectra is inconsistent 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.