Saturation and nonlinear electromagnetic field effects in the picosecond resonance Raman spectra of beta-Carotene.

Spectroscopic studies of chemical kinetics, on picosecond and shorter time scales, involve short optical pulses, often of high field strength. In order to understand resonance Raman scattering under these conditions, we investigate a test case where the scattering process at low field is well understood. Beta-carotene is an intensely colored, naturally occurring conjugated aromatic molecule. We observe that the ground state resonance Raman spectra of beta-carotene, in fluid isopentane at 293K and 119K, show saturation and near Lorentzian broadening as a function of fluence, using intense = 30 ps visible laser pulses. There are no lines assignable to transient species. A two pulse, two color pump-and-probe Raman experiment shows that the broadening is due to high optical field, and not due to unrelaxed internal excitation in the molecule.