Two-photon evanescent-wave spectroscopy of alkali-metal atoms

Abstract
The two-photon spectrum of sodium atoms, excited by two counterpropagating evanescent waves, is investigated both experimentally and theoretically. Sub-Doppler linewidths are observed, which are quantitatively reproduced by a rigorous theoretical approach that takes into account optical pumping and transit time broadening due to the translational motion of the arriving and departing atoms through the evanescent-wave fields. The velocity distribution of the atoms desorbing from the surface is found to obey a cosine law, which has a significant influence on the observed line shapes.