VIBRATIONALLY COHERENT PHOTOCHEMISTRY IN THE FEMTOSECOND PRIMARY EVENT OF VISION

Femtosecond pump-probe experiments reveal the impulsive production of photoproduct in the primary event in vision. The retinal chromophore o f rhodopsin was excited with a 35-femtosecond pulse at 500 nanometers, and transient changes in absorption were measured with 10-femtosecond probe pulses. At probe wavelengths within the photoproduct absorption band, oscillatory features with a period of 550 femtoseconds (60 wave numbers) were observed whose phase and amplitude demonstrate that they are the result of nonstationary vibrational motion in the ground stat e of the photoproduct. The observation of coherent vibrational motion of the photoproduct supports the idea that the primary step in vision is a vibrationally coherent process and that the high quantum yield of the cis-->trans isomerization in rhodopsin is a consequence of the ex treme speed of the excited-state torsional motion.