Resonance CARS (coherent anti-Stokes Raman scattering) line shapes via Frank–Condon scattering: Cytochrome c and β-carotene

Abstract

The CARS (coherent anti-Stokes Raman scattering) line shapes of the 1362 cm−1 Raman transition of dilute aqueous FeII cytochrome c have been studied with excitation in the region of the Soret band. In contrast to previous results for flavin, for which a Lorentzian CARS peak was observed at the electronic origin, the Lorentzian peak for cytochrome c occurs on the low energy side of the absorption band. The relative positions of the dispersive and negative line shapes are also red-shifted compared to flavin. When the scattering model previously developed for flavin is extended to include higher vibrational levels of the excited state, the cytochrome c pattern emerges as a consequence of a small origin shift parameter for the 1362 cm−1 mode. A line shape analysis for β-carotene indicates large origin shift parameters, near unity, for the vibrational modes at 1525, 1155, and 1005 cm−1, in agreement with previous analyses of the spontaneous Raman and CARS excitation profiles. Inclusion of multimode terms in the scattering model, however, decreases the estimated origin shifts substantially.