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

Abstract

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.

Dynamics of the defect-mediated desorption of alkali halide surfaces

“…It was also demonstrated, that the flux of emitted atoms is anticorrelated with the concentration of F-centers at the surface, or in the subsurface region. Similar process has been observed for the desorption stimulated by UV photons [13,14] and by low energy ions [15]. It has been found, that simultaneous irradiation of the surface with visible light leads to increase of the desorption coefficient, although photons of such energies are unable to cause desorption.…”

Highly charged ion induced nanostructures at surfaces by strong electronic excitations

“…Therefore, both desorption processes preserve the stoichiometry of the surface even on the atomic scale [57]. However, the excited F * -center will only initiate desorption at low coordinated sites, like terrace edges or kinks [63,64]. Thus, depending on the surface roughness, different amounts of excited F * -centers are reflected back to the bulk where they deexcite and stabilize.…”

Water Vapor and Alcohol Vapor Induced Healing of the Nanostructured KBr Surface

“…These pits are different from those formed via electron deposition that have step edges oriented along the nonpolar ⟨100⟩ directions. 7 The step edges of the pits in Figure 2 a are random in shape and orientation and thus have some polar character. While the nanostructured surface is stable in UHV for days, pressure-dependent differences in topography are apparent in AFM images ( Figure 2 b–f): the small pits with monatomic thickness merge into one another, leading to the surface becoming less corrugated with increasing pressure.…”

“…Alkali halide surfaces have been nanostructured by various radiation treatments (e.g., e-beam, ion beam, plasma, etc.) to increase the density of undercoordinated step sites, where the preferential adsorption of molecules takes place, in a controlled manner. − A major obstacle to utilizing alkali halides in molecular electronics is their stability in moist air because these surfaces have a high aqueous solubility and therefore could dissolve or even deliquesce in the presence of water vapor present in air above a certain relative pressure (deliquescence is the process of a solid fully dissolving due to the solvating influence of condensed water vapor). Relative pressure is defined as p / p 0 , where p is the absolute pressure and p 0 is the vapor pressure of the gas, typically at RT.…”