COMBINED ELECTRON-MICROSCOPE SURFACE-DECORATION AND HELIUM-ATOM-SCATTERING STUDY OF THE LAYER-BY-LAYER PHOTON-STIMULATED DESORPTION FROM NACL CLEAVAGE FACES

The electron-microscope surface-decoration technique and He-atom scattering (HAS) have been used to characterize the topography of NaCl(001) cleavage faces during vacuum ultraviolet (VUV) photon-stimulated desorption (PSD). The experiments were carried out at crystal temperatures below 550 K for which pure thermal desorption can be neglected. In the temperature range from T = 550 to 440 K the decoration experiments reveal that PSD progresses mainly by the creation and subsequent spreading of two-dimensional (2D) holes of monolayer depth in the surface plane. From T = 550 K down to 240 K the HAS experiments reveal periodic oscillations of the specularly scattered He intensity with exposure. These oscillations are attributed to the same process of creation and subsequent spreading of 2D holes which leads to a layer-by-layer removal of the surface. The HAS results indicate that the rate of PSD decreases strongly with decreasing temperature whereas the density of photon-induced 2D holes increases. The evolution with time of the specular He intensity also indicates that isolated adparticles are created during VUV irradiation. Simultaneous mass-spectrometer studies of the desorbing species reveal in addition to Na and Cl atoms a considerable PSD flux of NaCl molecules not predicted by the currently accepted models of PSD. Moreover the photon-induced desorption fluxes of the different species are found to be independent of the actual step density. Thus, the observed temporal alterations of the surface topography (step density) have no effect on the PSD process but solely reflect its progress.