Resonance Raman and optical dephasing study of tricarbocyanine dyes

Fluorescence lineshape analysis based on resonance Raman spectra of the dye HITCI was used to determine the details and magnitude of the vibrational part of the line broadening function. Forced light scattering (FLS) was applied to measure optical dephasing of HITCI in ethylene glycol, pumping at 770 nm with 20 fs pulses from a Ti:sapphire laser and probing with continuous-wave light from an argon ion laser. The observed response is well described by the line broadening function derived from the fluorescence lineshape. High-frequency vibrational modes not covered directly by the laser spectrum account for 20% of the vibration-induced broadening in the FLS experiment. This effect is explained by difference frequency mixing with thermally populated low-frequency vibrations.