Saturation and nonlinear electromagnetic field effects in the picosecond resonance Raman spectra of β-carotene

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Abstract
The ground state resonance Raman spectra of β‐carotene, in fluid isopentane at 293 and 119 K, 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. The broadening is a manifestation of nonlinear resonance Raman scattering, previously predicted (but not observed) in molecules when the Rabi energy becomes larger than the vibrational dephasing linewidth. Our data can be semiquantitatively explained using a model by Dick and Hochstrasser. The saturation represents population loss from the vibrationally relaxed ground electronic state, and is consistent with lowest excited singlet lifetime on the order of 10 ps.