Formation of alkali metal layers at the surface of electron-irradiated alkali halides

Scanning Auger microscopy (SAM) and reflected electron energy loss microscopy (REELM) have been used to study the chemical modifications induced by 10-keV electron bombardment at the surface of the alkali halides: UF, NaF, NaCl and KI. Auger spectra show that the alkali-to-halogen surface ratio increases with irradiation time, although they provide no conclusive evidence as to the chemical nature of the damage layer. Conversely, REELM analysis clearly reveals the formation of alkali metal through the occurrence of characteristic bulk plasmon peaks and surface plasmon peaks, which can also be used to image the lateral distribution of the damage layer to nearly submicron spatial resolution. Some unexpected signals are observed, deriving from combination of surface plasmon excitations with bulk plasmon excitations, and tentatively interpreted as arising from the inhomogeneous morphology of the alkali metal layer.