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 Fe^II 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.