Photon-stimulated desorption of neutral sodium from alkali halides observed by laser-induced fluorescence

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Abstract
We have used laser-induced fluorescence detection of neutral sodium atoms to study photon-stimulated desorption from NaF, NaCl, NaBr, and NaI. Monochromatic synchrotron radiation was used to measure the neutral Na yield as a function of photon energy in the range 39--160 eV. Velocity distributions of the desorbed Na atoms were obtained from the Doppler shift of the resonant fluorescent scattering of 589-nm photons from a tunable dye laser. The velocity distributions show that a thermal mechanism accounts for virtually all of the neutral Na desorption in all four halides for the range of temperatures studied (25--250 ^\circ\else\textdegree\fi{}C). A weak dependence of the desorption upon photon energy is found near the thresholds for core-hole excitation. These results are consistent with current models in which the fundamental desorption process is the ejection of halogen species induced by valence excitations, followed by thermal evaporation of excess surface sodium.