New synchronous scanning methods are developed for fully resonant four-wave mixing experiments with three input frequencies. Enhanced sensitivity and selectivity are observed for the vibrational and vibronic resonances that are coupled, consequently simplifying the spectral information obtained. The factors affecting selectivity and spectral simplification are discussed. Spectra obtained with the use of two-laser and three-laser CARS reveal various schemes of multiple resonance. Under resonant excitation at moderate power, saturation and power broadening reduce the effective electronic resonance enhancement. Resonances based on coupling between vibrational and vibronic levels are less susceptible to such power-induced phenomena. It is demonstrated that four-wave mixing with three input lasers provides the capabilities of measuring intermode vibronic coupling terms of the third-order susceptibility and performing fully resonant nonparametric four-wave mixing experiments that have the same characteristics as CSRS but are free from molecular fluorescence.

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