2-PHOTON-INDUCED ANISOTROPIC TRANSIENT ABSORPTION IN BACTERIORHODOPSIN

Two-photon excitation (TPE) of the photocycle of bacteriorhodopsin (BR ) with 1064 nm pulses was observed, and the subsequent transient absor ption and linear dichroism changes measured at 568 nm. An anisotropy o f 0.55 was induced in the ground-state transition of BR. A theoretical formalism is described to treat the isotropic and anisotropic compone nt of the two-photon absorptivity measured by a secondary transition d ipole that is induced by TPE and probed by polarized fluorescence or t ransient-absorption methods. The two-photon tensor for BR is found to contain components that are not parallel to the one-photon transition dipole.