By using the technique of optical second-harmonic generation, we have succeeded in obtaining the orientation, spectrum, and the change in dipole moment upon optical excitation for monolayers of retinal chromophores, which are of relevance for visual excitation. The change in dipole moment of the chromophore embedded in the purple membrane of Halobacterium halobium is found to be identical to that of the free chromophore. The spectrum of all-trans retinal monolayers exhibits a subsidiary one-photon forbidden peak in addition to the main absorption peak.

• Received 27 February 1989

DOI:https://doi.org/10.1103/PhysRevA.40.1684