Resonance Polarization and Phase-Mismatched CARS of Pheophytin b Excited in the Qy Band

Resonance polarization and phase-mismatched coherent anti-Stokes Raman scattering (CARS) measurements were performed on pheophytin b dissolved in acetone excited in the Qy absorption band, where strong broad fluorescence makes spontaneous Raman spectroscopy impossible. The phase-mismatching technique was applied to suppress solvent background and used in combination with the polarization-sensitive CARS technique to measure directly the x1111(3) and x1221(3) components to estimate depolarization ratios. The spectra were fitted by a non-linear least-squares procedure yielding vibrational band parameters. Some CARS dispersion information on the vibrational amplitudes was obtained by varying the pump wavelength. CARS excitation profiles based on transform theory were calculated and partly explain the observed amplitude dispersion. The application of the combined phase-mismatched polarization CARS technique may be useful in many other cases of highly fluorescing molecules when resonantly excited.