Long-lived coherent oscillations of the femtosecond transients in cyanobacterial photosystem I

Abstract

The pulsed excitation of electronic levels coupled to specific nuclear modes by a 26 fs laser pulse at 706 nm creates a wavepacket in the nuclear space of photopystem I (PS I) of Synechocystis sp. strain PCC 6803 both in the ground state and in the one-exciton manifold. Fourier transform of transient decay curves shows several low frequency peaks. The most prominent Power Spectral Density (PSD) peaks are at ω = 49 cm⁻¹ and ω = 88 cm⁻¹. The peculiarity of the coherent wavepacket in the PS I of S. sp. strain PCC 6803 is the unique, long-lived 49 cm⁻¹ and 88 cm⁻¹ oscillations with decay times up to 10 ps. It was suggested that such a long-lived coherence is determined by a contribution of the ground state wavepacket. The dependence of these two PSD peaks on the probe wavelength resembles the profile of the transient absorption spectra of PS I. The pump–probe signal in the Soret region reflects the dynamics of the ground state wavepacket created by pulsed excitation of the Q_y-band. It was shown that the multimode Brownian oscillator model allows a quantitative fit of the oscillatory patterns of the pump–probe signal to be obtained.