Five Wave Mixing: Surface-Specific Transient Grating Spectroscopy as a Probe of Low Frequency Intermolecular Adsorbate Motion

Surface-specific five wave mixing spectroscopy is used to examine adsorbate dynamics at the fused silica/air interface. Signals whose temporal response is significantly broader than the instantaneous fourth-order electronic polarizability are attributed to low frequency, adsorbate nuclear motion. Based on the dependence of these dynamics on the moment of inertia and their comparison with dynamics observed in liquids, the five wave mixing temporal response has been assigned to adsorbate intermolecular librational motion. Demonstration of this new fourth-order technique allows one, in principle, to extend all third-order nonlinear bulk-phase spectroscopies to surfaces and interfaces with specificity.