POLARIZED RAMAN-SPECTRA OF ORIENTED ALL-TRANS-BETA-CAROTENE IN POLYETHYLENE FILMS

Polarized Raman spectra at 77 K of all-trans-β-carotene stretch-oriented in polyethylene films are reported for the first time. The measurements were carried out either in the 90° and back-scattering geometries at excitation wavelengths in resonance with the lowest allowed 1Bu«1Ag electronic transition, and indicate a rather good orientation of the polyenic chains along the stretching direction. By changing the excitation wavelength, a significant dispersion of the Raman shift relative to the CC stretching mode is observed. One interpretation of this effect, based on the existence of two (or more) species (rotational conformers, aggregates, etc.) whose resolved absorption structures are displaced by one half the CC vibrational frequency, is presented and discussed. © 1990.