Absolute resonance Raman intensities demonstrate that the spectral broadening induced by the β-ionone ring in retinal is homogeneous

Abstract

The sensitivity of absolute resonance Raman cross sections to the excited state homogeneous width is used to examine whether the electronic spectral breadth induced by the β-ionone ring of retinal is primarily a homogeneous or an inhomogeneous effect. Resonance Raman intensities have been measured for both all-trans retinal and its 5,6-dihydro analog. The dihydro analog has a structured absorption spectrum because the broadening effect of the C6–C7 bond is lost when the π system does not extend into the ionone ring. While the two molecules have similar relative Raman intensities, the absolute cross sections are about a factor of two lower in all-trans retinal, indicating that the ‘‘effective homogeneous linewidth’’ is significantly larger in all-trans retinal than in the dihydro analog. Calculations carried out to model both the absorption spectra and the resonance Raman intensities require an effective homogeneous linewidth that is more than twice as large for all-trans retinal (∼1200 cm−1) as for the 5,6-dihydro analog (∼500 cm−1). The additional width is most simply attributed to Franck–Condon activity in the C6–C7 torsional degree of freedom; however, no intense resolved lines are observed in the Raman spectrum between 20 and 800 cm−1. The torsional modes involved must have very low ground-state frequencies and may be observable only through their contribution to the apparent Rayleigh and Raman linewidths.