Two-photon laser spectroscopy of the gas boundary layer in crossed evanescent and volume waves

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Abstract
A spectroscopic approach to the scattering dynamics in the gas-surface boundary layer is presented. The technique utilizes excitation of the vapor atoms in two crossed laser fields, one of which is directed normally to the surface while the other one excites an evanescent wave propagating along the surface. We provide a rigorous and quantitative theoretical description of these two-photon evanescent-volume wave spectra and apply it to the model system of sodium atoms colliding with a dielectric prism surface. A comparison with experimental results reveals quantitative agreement between theory and experiment. We demonstrate that one can distinguish by pure optical means between different groups of atoms, namely atoms moving to the surface and apart from it after being specularly or diffusively scattered. We also extract the two-dimensional velocity distributions of both the arriving and departing groups of atoms.