Resonant Two-Photon Excitation Pathways During Retinal-Isomerization in Bacteriorhodopsin.

Resonant two-photon excitation is observed in Bacteriorhodopsin using transient absorption experiments with hyperspectral probing (440 - 770 nm) at different excitation wavelengths. Signal contributions from ground as well as excited electronic states show distinct dependences on excitation energies and wavelengths during all timescales of population relaxation. An additional photoproduct is observed upon high-energy excitation with an absorption maximum red-shifted with respect to the known K-intermediate, exclusively formed under linear excitation conditions. Spectral signatures of this photoproduct persist on a timescale of tens of nanoseconds after excitation, comparable to the lifetime of the K-intermediate. The observed additional photoproduct is likely to be a precursor state of an eventually forming blue-shifted, thermally stable photoproduct observed under prolonged high-intensity illumination of BR samples. [ABSTRACT FROM AUTHOR]