Abstract

Visual excitation is initiated by the absorption of a photon by the 11-ds retinal chromophore bound within the pigment called rhodopsin. The photochemical cisterns isomerization in rhodopsin is complete in only 200 fs. The extreme speed of this process suggests that the photochemistry involves nonstationary states or vibrational coherence. Recent femtosecond studies have observed coherent vibrational oscillations of the photoproduct ground state having a period of 550 fs (60 cm-¹ mode).² This provides proof that the isomerization in rhodopsin is a vibrationally coherent process, and indicates that the 60 cm^-1 mode may play a significant role in tire isomerization reaction coordinate. These observations further suggest that the isomerization quantum yield is directly dependent on the excited-state torsional velocity as would be expected for a Landau-Zener tunneling process.

© 1996 Optical Society of America