How Fast is Excitation Energy Transfer in the Photosystem II Reaction Center in the Low Temperature Limit? Hole Burning vs Photon Echo

The Q_y(S₁) excitonic structure, excitation energy transfer (EET), and primary charge-transfer separation processes of the isolated photosystem II reaction center (PS II RC) have proven to be formidable problems due, in part, to the severe spectral congestion of the S_o → Q_y absorption spectrum. Recently, Prokhorenko and Holzwarth (J. Phys. Chem. B 2000, 104, 11563) reported interesting femtosecond 2-pulse photon echo data on the RC at 1.3 K for excitation wavelengths between 676 and 686 nm. At times longer than ∼1 ps and λ ≳ 678 nm, the echo decay curves are highly dispersive, which was attributed to a distribution of primary charge separation rates ranging from 2 ps to several hundred ps. A prompt subpicosecond component of the echo decay curves was also observed and suggested to be due to EET occurring in ∼100-200 fs. We present here persistent nonphotochemical hole burned spectra and transient triplet bottleneck hole spectra obtained with burn wavelengths between 680 and 686 nm, which show that the EET time in that wavelength region is no shorter than ∼5-10 ps. It is argued that the prompt component of the echo decay curves is due to relaxation of low-frequency phonons excited by the pump pulse. The argument is based on hole burning spectroscopy being the frequency domain equivalent of 2-photon echo spectroscopy, as well as on published photon echo data for chromophores in amorphous hosts.