Non-Resonant Background Suppression in Preresonance CARS Spectra of Flavin Adenine Dinucleotide : Demonstration of a Background Suppression Technique using Phase Mismatching and Comparison with the Polarization-Sensitive CARS Technique

Polarization-sensitive CARS spectra of a 5.7 x lo-’ mol dm-’ flavin adenine dinucleotide (FAD) solution were recorded under preresonance conditions at a pump wavelength of 532 nm. The depolarization ratios of the vibrations are shown to be close to the depolarization ratio of the non-resonant background. This results in a severe reduction of the vibration resonant signal (a factor of 700-900) in the polarization CARS spectrum, and a poor improvement in the ratio of the resonant signal and the non-resonant background (< 10). In this context, a non-resonant background suppression technique is discussed and demonstrated for 5.7 x lo-’ and 1.4 x lo-’ mol dm-’ F A D solutions excited at 532 nm; ihe non-resonant sweptibility of the walls of the cuvette, which contains the F A D solution, is used to compensate the non-resonant signal contribution of the solution. An improvement in the signal-to-noise ratio of ca. 50 is achieved at the cost of a factor of 30 in the resonant signal strength. Lorentzian-shaped spectral bands are obtained, facilitating the determination of band position, width and intensity. Line shape parameters and depolarization ratios for F A D are extracted from the presented spectra by curve fitting. The signal strength and background suppression achieved with these techniques and the resonance CARS technique (at a pump wavelength of 480 om) are compared and discussed.