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

Our recent work on photon-stimulated desorption of alkali halide surfaces has been reviewed. Most of the experimental data are presented for the first time. This new material is 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 and compared 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 found that, at initial stages of the UV-photon stimulated desorption, the process is occurring in a 'layer-by-layer' mode as a result of growth and linking of monatomic pits on the surface. This results in periodic changes of surface topography from atomically flat to rough with a period equal to the time needed for desorption of 1 ML. Such periodic changes of the surface topography affect the efficiency of the desorption and result in oscillatory dependence of the emitted particle yields versus photon fluence. Careful analysis of the yields reveals that during irradiation there is a substantial number of stable ground state F-centres accumulated under the surface. The number of accumulated defects is changing periodically with the photon fluence in anti-correlation with the desorption yields. Based on the mentioned results, we argue that the production of F-centres and their interaction with the surface are, in fact, the limiting factor for PSD. To test the validity of this statement a series of desorption experiments for the crystal co-irradiated with visible light (with a wavelength corresponding to the F-centre absorption band) and UV photons have been performed. A dramatic increase in the desorption yield, as well as pronounced changes in the surface erosion process, have been found.