Reflected electron energy loss microscopy and scanning Auger microscopy studies of electron irradiated alkali halide surfaces

Reflected electron energy loss microscopy (REELM) and scanning Auger microscopy (SAM) have been used to study the chemical changes produced by electron irradiation at the surface of: LiF, NaF, NaCl and KI. Auger spectra suggest that electron bombardment induces alkali metal accumulation and halogen depletion at the surface. Heating electron-irradiated KI up to 460 K makes the K metal layer evaporate and nearly restores the original surface stoichiometry. While SAM experiments on NaF are hindered by surface charging phenomena, REELM experiments are not. Indeed, the latter microscopy provides much clearer evidence than does SAM as to the formation of a surface metal layer in all the halides, as manifested by the presence of characteristic surface and bulk plasmon loss signals. Besides these, a series of unexpected signals is revealed, combining surface plasmon excitations with bulk plasmon excitations, and tentatively interpreted as arising from the inhomogeneous morphology of the alkali metal layer. A submicron spatial resolution is achieved in both SAM and REELM imaging of the surface alkali metal layer. The differences found between SAM and REELM images are discussed in terms of topographic effects, electron backscattering and surface sensitivity