Molecular, multiresonant coherent four-wave mixing spectroscopy

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Abstract
Recent research has expanded the capabilities of four-wave mixing by providing it with component selectivity, site selectivity, and mode selectivity. The selectivity is achieved by taking advantage of the three resonance enhancements that occur in a four-wave mixing process. New spectral scanning strategies allow one to scan a single resonance while maintaining the other two resonances at constant values. The constant resonances can be used to select a specific component, a specific site within an inhomogeneously broadened envelope of a component, and/or a specific vibrational or vibronic mode of that site. The scanned resonance will then contain enhanced features corresponding to the particular component, site, and/or mode that was chosen by the constant resonances. These component and site selective capabilities of the four-wave mixing complement the single vibronic level fluorescence methods. The relative transition intensities from a specific component or site reflect the mode coupling between the vibrational and vibronic modes. Since the four-wave mixing is fully resonant in these experiments, saturation effects play a dominant role and cannot be ignored. This review presents a complete discussion of the theoretical and experimental factors that control the fully resonant four-wave mixing. It shows examples of the component selectivity and the line narrowing capabilities. It also provides examples of how mode selection can identify mode coupling through the relative intensities of different vibrational features.