Virtual intermediates in photosynthetic electron transfer.

We explore the possibility of virtual transfer in the primary charge separation of photosynthetic bacteria within the context of several types of experimental data. We show that the peak that might be expected in the virtual rate as electric fields vary the intermediate state energy is severely broadened by coupling to high-frequency modes. The Stark absorption kinetics data are thus consistent with virtual transfer in the primary charge separation. High-frequency coupling also makes the temperature dependence weak over a wide range of parameters. We demonstrate that Stark fluorescence anisotropy data, usually taken as evidence of virtual transfer, can in fact be consistent with two-step transfer. We suggest a two-pulse excitation experiment to quantify the contributions from two-step and virtual transfer. We show that virtual absorption into a charge transfer state can make a substantial contribution to the Stark absorption spectrum in a way that is not related to any derivative of the absorption spectrum.