Abstract

Transient absorption spectroscopy has been used to study sub-picosecond energy transfer processes in isolated photosystem II (PS II) reaction centres. As reported previously [1], using long wavelength (694 nm) excitation, spectral evolution of the isotropic Q_y band bleach / stimulated emission is dominated by energy transfer processes with a 100 ± 50 fs time constant. In contrast, depolarisation of this signal occurs with a time constant of 400 ± 30 fs, from an initial anisotropy of ~0.4 to a value of ~0.15 at 1.5 ps. This is illustrated in figure 1 which shows the isotropic absorption and anisotropy transients at a probe wavelength of 683 nm. This decay of the anisotropy is attributed to energy transfer between at least two degenerate states contributing to reaction centre absorption circa 680 nm, with these states having approximately orthogonal transition dipoles. The transient anisotropy barely changes between 1.5 ps and 60 ps, indicating that under these excitation conditions equilibration of the excitation energy between reaction centre excited states occurs on a sub-picosecond timescale. These excited state equilibration processes preceed charge separation, which occurs primarily with a 21 ps time constant [3]. Transient data collected for pheophytin Q_x absorption bands indicate that pheophytin molecules are included in the 100 fs equilibration process.

© 1996 Optical Society of America