Two-photon spectroscopy allows the assignment of the 1Ag^*--like states in protein-bound chromophores.

Sumie Shima adjusting the sample chamber of the two-photon spectrometer.

The locations and properties of these so-called “forbidden” states are critical to the photochemical properties of both carotenoids and retinal chromophores, and in the visual pigments, bR, sRII and proteorhodopsin are predicted to be responsible for creating barrierless excited state photo-isomerization surfaces. The spectra at right show results obtained for the carotenoid peridinin.

The two-photon spectra identify three excited singlet states, while normal UV-vis observes only one.

J. Phys. Chem. A. 107, 8052 - 8066 (2003)