Two-photon evanescent-volume wave spectroscopy: a new account of gas-solid dynamics in the boundary layer.

A novel type of laser spectroscopy for the investigation of the collisional dynamics of atoms in the close vicinity of a surface has been developed. The technique utilizes excitation of the vapor atoms in two crossed laser fields, one of which is directed normally to the surface, whereas the other one excites an evanescent wave propagating along the surface. The results, obtained for sodium atoms near a dielectric prism surface, are quantitatively reproduced by a rigorous theoretical approach. This new, nonintrusive method allows one to distinguish by pure optical means between different groups of atoms and to extract their two-dimensional velocity distributions.