Alkali halide decomposition and desorption by photons—the role of excited point defectsand surface topographies

Abstract

Our recent work on photon-stimulated desorption of alkali halide surfaces has beenreviewed. Most of the experimental data are presented for the first time. This new materialis supplemented with two examples of our work published previously (see Szymonski et al 1996 Surf. Sci. 363 229; 2002 Acta Phys. Pol. 33 2237). The results are discussed andcompared with relevant experimental and theoretical work by other authors. In particular,we focus on two aspects of the studies: high resolution AFM imaging of desorbed surfaces,and measurements of mass selected fluxes of the desorbing atoms. It is foundthat, at initial stages of the UV-photon stimulated desorption, the process isoccurring in a ‘layer-by-layer’ mode as a result of growth and linking of monatomicpits on the surface. This results in periodic changes of surface topography fromatomically flat to rough with a period equal to the time needed for desorption of1 ML. Such periodic changes of the surface topography affect the efficiency of thedesorption and result in oscillatory dependence of the emitted particle yieldsversus photon fluence. Careful analysis of the yields reveals that during irradiationthere is a substantial number of stable ground state F-centres accumulated underthe surface. The number of accumulated defects is changing periodically withthe photon fluence in anti-correlation with the desorption yields. Based on thementioned results, we argue that the production of F-centres and their interaction withthe surface are, in fact, the limiting factor for PSD. To test the validity of thisstatement a series of desorption experiments for the crystal co-irradiated withvisible light (with a wavelength corresponding to the F-centre absorption band)and UV photons have been performed. A dramatic increase in the desorptionyield, as well as pronounced changes in the surface erosion process, have beenfound.