Resonant four-wave mixing spectroscopy: A new probe of molecular structure and dynamics in regimes of extreme vibrational excitation

A new, folded variant of optical-optical double resonance has been developed for the systematic investigation of polyatomic molecules that contain chemically significant quantities of vibrational excitation. Based upon a novel implementation of phase-conjugate degenerate four-wave mixing (DFWM) spectroscopy, this technique provides a quantum state-specific probe of molecular structure and dynamics that offers substantial advantages over more conventional methods. Despite the third-order nonlinearity inherent to the DFWM process, the tremendous resonant enhancement that accompanies this Doppler-free interaction enables the facile detection of double resonance signals even in ratified (viz., <10-3 Torr) environments. The utility of this spectroscopic tool will be demonstrated by application to both stable and transient molecular species prepared under a variety of experimental conditions (e.g., bulk-gas, supersonic expansion, and precursor photolysis). In particular, the enhanced sensitivity and resolution afforded by the DFWM scheme has permitted the detailed characterization of rovibrational structure for gaseous CS2 molecules having up to 2 eV of energy selectively deposited into their internal degrees of freedom. This regime of vibrational excitation, corresponding to roughly 50% of that required for dissociation of the CS2 ground electronic potential surface, provides a fertile ground for rigorously assessing our present-day understanding of molecules and their behavior