Raman spectrum of beta-carotene using laser lines from green (514.5 nm) tonear-infrared (1064 nm): Implications for the characterization of conjugated polyenes

We have studied the Raman spectrum of beta-carotene using excitation wavele ngths from visible (514.5 nm) to near-infrared (1064 nm), Qualitatively, th e spectrum appears independent of the choice of excitation wavelength, alth ough this is not true in detail: in particular, the ratio of the intensitie s of the nu(C-C) and nu(C=C) fundamentals to their overtones and combinatio ns varies with the excitation wavelength. The inelastic neutron scattering spectrum shows no evidence for unusual dynamics in this system, and the UV/ vis/NIR spectrum shows that there are no allowed electronic transitions fro m the ground state beyond similar to 700 nm. These observations provide str ong evidence for the existence of a different mechanism for the enhancement of the bands. Indirect evidence suggests that, with these longer elicitati on wavelengths, the intensities of the nu(C-C) and nu(C=C) peaks are at lea st two orders of magnitude smaller than those from resonance enhancement wi th the use of excitation into the visible absorption hands. The implication s for the characterization of polyenes in polymers are discussed.