N. Seifert

The influence of defects and defect clusters on alkali atom desorption stimulated by low energy electron bombardment of alkali halides

Abstract An important challenge in studies of electronically induced desorption on alkali halides is to determine the degree to which the desorption rate of alkali atoms as a function of dose and temperature is influenced by alkali island formation on the surface, alkali colloid formation in the bulk, and individual defect diffusion from the bulk to the surface. To address this problem, we report measurements of transmission optical absorption which gives direct information on defect, cluster and surface island concentrations, and of electron bombardment induced alkali desorption yields performed simultaneously on NaCl, NaF, and LiF at temperatures between 27 and 400°C. These experiments provide insight into the formation of surface and bulk agglomerates which in turn act as sources of desorbing alkali atoms. Our data support a physical picture where bombardment induced F-centers lead to the formation of F-center aggregates in the bulk and alkali metal clusters on the surface at temperatures around room temperature. At higher temperatures alkali metal desorption during electron bombardment is dominated by the emission of weakly bound single alkali atoms, and alkali atoms from alkali metal clusters on the surface of the crystals. After the cessation of the electron beam, the desorption yield is controlled by the thermal stability of metallic colloids which have been formed in the bulk during electron bombardment due to the temperature dependent higher mobility of the F-centers.

Optical absorption spectroscopy of defects in halides

Desorption of alkali atoms from, and defects formed in, alkali halide crystals stimulated by low-energy electron bombardment were investigated simultaneously by optical absorption-, mass spectroscopy-, and depth-profile surface sputtering- techniques. These techniques not only provide important information about the type, amount, and the spatial distribution of the defects formed in the alkali halide crystals, but also indicate which processes govern the emission rate of neutral alkali atoms during and after bombardment. The results show that at temperatures near and lower than room temperature, F-centers, small F-center clusters, and alkali metal clusters are formed during electron bombardment of the crystals

Simultaneous measurements of transmission optical absorption and electron stimulated Li desorption on LiF crystals

Abstract We report the first simultaneously performed transmission optical absorption and neutral lithium desorption yield experiments. Electron-stimulated desorption (ESD) of lithium atoms from lithium fluoride crystals was investigated with quadrupole mass spectroscopy. Correlations have been established between the desorption kinetics of lithium atoms and transmission optical absorption data (200–600 nm) obtained in situ as a function of irradiation time and temperature. The investigated temperature regime covered temperatures as low as room temperature up to 660 K. For temperatures lower than 640 K the data can be consistently explained by the assumption that lithium islands form on the surface of the crystals during bombardment and disintegrate after bombardment. The formation of lithium agglomerates is characterized by the occurrence of a very broad absorption band (maximum at 500 nm or higher). At higher temperatures the shape of the absorption spectrum changes in a way which can no longer be explained by simply assuming lithium islands on the surface.

Photon and electron-stimulated desorption of excited alkali-metal atoms and CN molecules from alkali halide surfaces

Abstract Recent advances in photon- and electron-stimulated desorption of excited alkali-metal atoms and CN molecules are discussed. The role of defects created by photon and electron irradiation leading to surface metalization is particularly emphasized in the desorption mechanisms. Two mechanisms are proposed for the creation of excited alkali-metal atoms: (1) appropriate to the low-temperature regime, the first mechanism assumes that surface exoergic reactions between alkali-metal dimers and halogen atoms produce desorbed excited alkali-metal atoms, the surface reactants being formed by radiation-initiated defect processes, and (2) the second mechanism assumes that the gas-phase excitation of ground-state alkali-metal atoms by primary electrons produce the excited alkali-metal atoms. The mechanism responsible for CN desorption may be described in three steps: (1) pre-irradiation produces alkali-metal rich surfaces via defect-mediated processes, (2) when the surface is exposed to CO2 and N2, surface rea...

Electronically induced sputtering of ground and excited state alkali atoms from alkali halide crystals

Abstract The following processes lead to electronic desorption from alkali halides during low-energy electron or photon bombardment: a) direct bond breaking, and b) collision induced formation of defects in the crystals. The former process may depend on defect creation but involves primarily the desorption of adsorbates bonded on the surface of the crystal and also may be resonant for the case of photon bombardment. The latter process generally leads to the desorption of the constituents of the alkali halide crystals (alkali and halogen atoms). In this paper the authors concentrate on processes leading to the desorption of alkali atoms in either the ground state, or excited state, therefore b)-type processes. Experimental methods used to investigate the desorbing species were quadrupole mass spectroscopy, laser induced fluorescence spectroscopy, and fluorescence spectroscopy. Transmission optical absorption spectroscopy performed simultaneously with desorption measurements of neutral ground state atoms was used to characterize the mobility and stability of defect clusters created in the crystals and to correlate them with the desorption yield of the alkali atoms. Fluorescence spectroscopy measurements show that a significant part of the alkali atoms desorb in an excited state. We discuss a new model which explains the yield of excited alkali atoms at moderate temperatures by means of a surface chemistry process arising from defect migration to the surface.

Anomalous temperature dependence of the desorption of excited sodium atomsinduced by ion bombardment of sodium fluoride crystals

We have observed anomalous temperature dependence in the ion-induced desorption of excited sodium atoms from sodium fluoride crystals. The fluorescence yield of the 3p-3d transition at 8195 {Angstrom}, and the 3p-4d transition at 5688 {Angstrom} increases steadily with temperature; however the yield of sodium doublet 3s-3p transition displays a pronounced maximum in the region of 160{degree}C. Fluorescence yields are also found to be nonlinear as a function of beam current at room temperature. To explain these experimental results, we propose a mechanism that involves electron transfer between the desorbing atoms and the bombarded surface. {copyright} {ital 1997} {ital The American Physical Society}

Simultaneous Measurements of Optical Absorption and Electron Stimulated Li Desorption on LiF Crystals

It has been known for some time that ionizing radiation incident on alkali halide crystals lead to the formation of F- and H-centers [1]. When an F-center reaches the surface of the crystal, it results in the neutralization of an alkali ion. If the temperature of the crystal is high enough the neutral alkali atoms desorb thermally from the surface [2]. It has been observed in many experiments that large doses of energetic neutrons, ions, or X-rays lead to the formation of colloids (alkali agglomerations) in the bulk of the crystals [3,4]. However, agglomeration processes at or close to the surface of the crystals have only been investigated recently [5,6]. Electron energy loss spectroscopy, and Auger electron spectroscopy investigations have supplied evidence for the formation of alkali islands on the surface of alkali halides during electron bombardment [7]. In previous publications [5,6] the desorption kinetics of lithium atom delayed emission (i.e. emission following the cessation of electron bombardment) has been investigated. The results show the occurrence of a prompt and a delayed decay. The delayed decay takes seconds, whereas the prompt decay is faster than a few msec. Under certain circumstances the Li desorption rate even increases after the cessation of electron bombardment (“delayed maximum”). In all these publications conclusions have been drawn about the processes occuring during and after electron bombardment by monitoring the ground state yield of lithium desorbing from the surface. Clearly, this method alone is not capable of differentiating in which manner F-center clusters, lithium islands on the surface, or lithium clusters individually contribute to the observed desorption phenomena. Simultaneous correlated transmission optical absorption spectroscopy (which provides information about defect densities in the crystal) combined with measurements of the ESD of alkali atoms from alkali halides provides a new approach to this problem.