Excited state relaxations of phytochrome studied by femtosecond spectroscopy

Femtosecond time-resolved spectroscopic studies with phytochrome in its red light absorbing form (Pr) were performed. Upon excitation of Pr with 75 fs pulses at 618 nm bleaching of the Pr ground state absorption accompanied by a broad transient absorption centered near 725 nm was observed. The stimulated emission of Pr was detected with a delay of about 200 fs. The appearance of stimulated emission coincided with a strong decrease of the transient absorption near 725 nm. From these results it is suggested that Pr is initially excited into a vibrationally unrelaxed state that undergoes relaxation within about 200 fs towards the fluorescing state. From 1 ps after excitation the initially fast relaxation processes were followed by comparatively slower ones. From decay kinetics of bleaching and fluorescence in the first 6 ps after excitation a decay time of about 15 ps was estimated.