Four-wave mixing spectroscopy of gas-surface scattering

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Abstract
A detailed theory of four-wave mixing (FWM) at a gas-solid interface is developed. Geometry of excitation, where two electromagnetic waves propagate perpendicularly to the interface and the third one is evanescent traveling along it, is considered. Assuming that the evanescent wave penetration depth into the gas is much less than the mean free path of gas molecules, it is shown that FWM spectroscopy allows to distinguish between the contributions from gas molecules moving to the surface, desorbed from it and directly scattered into the gas. It is concluded that by scanning the wave frequencies across the molecular transitions it is possible to determine the parameters of the scattering kernel for direct gas-surface scattering and the velocity-dependent sticking probability. A method based on this principle could open opportunity for studying adsorption, desorption and gas-surface scattering dynamics not only under thermodynamically nonequilibrium conditions but as well as in an equilibrium gas phase.