Study of lithium adsorption on the TiO2 surface by electron-stimulated desorption

This paper reports on a study of electron-stimulated desorption (ESD) of O⁺ and Li⁺ ions from titanium dioxide as a function of the preheating temperature T and of the concentration of lithium adsorbed at 300 K, which was carried out with a static magnetic mass spectrometer combined with a retarding-field energy analyzer. For T>1500 K, the TiO₂ surface undergoes irreversible rearrangement. At temperatures from 300 to 900 K and at lithium coverages Θ<1, the ESD cross sections of the O⁺ and Li⁺ ions vary in a reversible manner with temperature, while for lithium coverages Θ>1, the changes in the Li⁺ and O⁺ ESD cross sections become irreversible. For θ<1, the appearance threshold of the Li⁺ and O⁺ ions is 25 eV, whereas for θ>1, the ESD threshold of Li⁺ ions shifts to 37 eV.     

Energy distributions of ions produced by electron-stimulated desorption

We have measured the initial kinetic energy distributions of ions produced by electron bombardment of various oxides and halides. The instrument used allows ions directly ejected from the sample surface to be distinguished from ions formed by electron impact in the gas phase. Singly and multiply charged positive ions of species present in the matrix as anions and cations were desorbed by high energy ( 11 keV) electron impact. Directly desorbed positive halogen ions show a narrow, low energy peak, consistent with conventional models of electron stimulated desorption (ESD). In addition, some of the cation species exhibited similar narrow energy spectra. Charge states up to +6 were observed for the halides; with the exception of F²⁺ and Cl²⁺, multiple charge states were due to electron impact ionization of desorbed neutrals. Charge states up to +4 were seen for silicon from electron-bombarded SiO₂; energy distributions of Si⁺, Si²⁺ and Si³⁺ showed that these species were desorbed directly from the surface. The energy distributions of O⁺ and O²⁺ ions ejected from SiO₂ are relatively wide, compared to the energy distribution of Si⁺ ions. In contrast, O⁺ ions ejected from TiO₂ have a much narrower energy distribution, like those observed for the halogen ions.     

Application of electron-stimulated desorption for studying adsorbed layers

After a brief discussion of the main result of the research initiated by N.I. Ionov in his laboratory using electron-stimulated desorption for studying the surface layers of tungsten, we consider in greater detail recent results on layered coatings formed on the tungsten surface upon simultaneous adsorption of sodium (or cesium) and gold atoms on this surface, as well as the effect of sputtering of samarium atoms on the (Cs + Au)/W(100) surface that has already been formed at 300 K.     

Stimulated desorption studies of defect structures on TiO2

The influence of the surface geometric structure on the electron and photon-stimulated desorption (ESD, PSD) ion yield from TiO₂(001) and TiO₂(110) has been studied. For both surfaces, angle-integrated ESD yields have been measured as a function of annealing temperature ranging from room temperature sputtered surfaces to 1200 K annealed surfaces. These measurements imply that the local geometry of the desorption site must be considered in order to adequately explain the observed ion yield variations. The ESD ion angular distribution (ESDIAD) patterns have been obtained after annealing in the temperature range 300–1200 K. These patterns show the presence of short-range structural order, even on highly defective, sputtered surfaces, and they are interpreted in terms of plausible models of surface structure. Changes in both the surface Ti-cation valence state and its coordination number as a function of annealing temperature are proposed to give rise to the observed ESDIAD patterns and the total ion yield behavior.     

Cross sections for electron-stimulated desorption of alkali-metal atoms from adlayers on oxidized tungsten

Earlier cross-section measurements of electron-stimulated desorption (ESD) of Li, Na, K, and Cs atoms from adlayers on oxidized tungsten are analyzed with respect to substrate temperature and degree of oxidation. It is conjectured that the ESD cross sections are determined by the ratio of the rates of neutral alkali-metal re-ionization on the surface and of relaxation of the O⁺ charge in the substrate. A comparison with experiment revealed the dependence of the re-ionization rate of an alkali-metal adatom on its size and mass, as well as the dependence of the O⁺ charge relaxation rate on substrate temperature and degree of oxidation. The relation of the charge relaxation time to the substrate band structure is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1491–1497 (August 1997)     

On the isotope effect in the relaxation model of electron-stimulated desorption

The dependence of the ESD yield of ions on their mass has been studied. The influence of short-range electronic-shell repulsion on the isotope effect is discussed. The isotope effect is shown to be depressed with increasing non-Coulombic initial velocity. Fiz. Tverd. Tela (St. Petersburg) 40, 764–767 (April 1998)     

Effect of surface roughness on the electron-stimulated desorption of D from microporous D2O ice

The electron-stimulated desorption (ESD) of D⁺ from microporous D₂O ice films condensed on Pt(111) has been investigated. The total D⁺ yield as a function of temperature from 90–180 K depends sensitively on the film roughness, surface temperature and ice phase. In particular, we observe an irreversible increase in the cation yield as the microporous thin film is heated from 90–120 K, which we associate with a decrease in surface roughness as the micropores collapse. We present evidence which suggests that the number of surface sites available for emission, the surface roughness, and reneutralization or reactive scattering of the D⁺ desorbate play major roles in determining the ion yield. A simple model which qualitatively addresses the role of surface roughening on ESD ion yields shows good agreement with the data.     

Investigation of interaction between lithium and tantalum under a silicon film coating by electron-stimulated desorption technique

The electron-stimulated desorption of Li⁺ ions from lithium layers adsorbed on the tantalum surface coated with a silicon film has been investigated. The measurements are performed using a static magnetic mass spectrometer equipped with an electric field-retarding energy analyzer. The threshold of the electron-stimulated desorption of lithium ions is close to the ionization energy of the Li 1s level. The secondary thresholds are observed at energies of about 130 and 150 eV. The threshold at an energy of 130 eV is approximately 30 eV higher than the ionization energy of the Si 2p level and can be associated with the double ionization. The threshold at 150 eV can be caused by the ionization of the Si 2s level. It is demonstrated that the yield of Li⁺ ions does not correlate with the silicon amount in near-the-surface region of the tantalum ribbon and drastically increases at high annealing temperatures. The dependence of the current of Li⁺ desorption on the lithium concentration upon annealing of the tantalum ribbon at T>1800 K exhibits two maxima. The ions desorbed by electrons with energies higher than 130 and 150 eV make the largest contribution to the current of lithium ions after the annealing. The yield of lithium ions upon ionization of the Li 1s level at an energy of 55 eV is considerably lesser, but it is observed at higher concentrations of deposited lithium. The results obtained can be interpreted in the framework of the Auger-stimulated desorption model with allowance made for relaxation of the local surface field.     

On the appearance threshold and the reverse motion of ions in electron-stimulated desorption

We report here the first observation of the ESD appearance threshold energy exceeding the energy of the inner shell ionization, which is interpreted as a consequence of a shake-off process. For a tightly bound chemisorbed system we discovered the possibility of reverse ion motion due to an interatomic Auger process. The observed expansion of the ion retardation curves and the weakening of the isotope effect in the case of reverse ion motion is discussed in terms of an improved ESD model considering local surface field relaxation.     

The temperature dependence of Eu atom yield resonances in electron-stimulated desorption from oxidized tungsten

This paper reports on the first measurement of the intensity of the resonances in the yield of europium neutrals as a function of temperature observed in electron-stimulated desorption from tungsten surfaces oxidized to different degrees and having different europium coverages. The measurements were carried out by the time-of-flight method with a surface ionization detector. The temperature dependences obtained for resonances due to europium and tungsten core level ionization differ qualitatively. The relation is reversible for temperatures below the onset of europium thermal desorption.     

Adsorption and angle-resolved electron-stimulated desorption of CCl4 on Ru(0001)

The adsorption and electron-stimulated desorption of CCl₄ on Ru(0001) have been studied at 100 K using a variety of surface analytical techniques, including thermal desorption spectroscopy, Auger electron spectroscopy, low-energy ion scattering, and mass and angle-resolved electron-stimulated desorption (ESDIAD). CC1₄ is found to dissociate partially for fractional monolayer coverages, and to adsorb molecularly above a coverage of 13%. Upon electron bombardment, Cl⁺ ions are found to desorb with their angular distribution directed along the surface normal; this implies that CC1₄ orients with one C-C1 bond normal to the surface. The total yield of Cl⁺ increases linearly with increasing molecular CC1₄ coverage up to one monolayer, which implies that Cl⁺ stems mainly from molecular CCl₄ and that interadsorbate quenching is not significantly affecting the desorption yield. Beyond a coverage of 1 ML, the Cl⁺ yield continues to increase, and starts leveling off after 2 ML. We estimate the yield from a thick layer of CCl⁴ to be of the order of 3 × 10⁻⁸ ions/electron. The angular distribution of the desorbing Cl⁺ ions widens with increasing CCl₄ exposure. Besides Cl⁺, higher mass fragments of CCl₄, such as CCl⁺, CCl₂⁺ and CCl₃⁺ are also found to desorb from multilayers upon electron bombardment.     

Energy distribution of alkali-metal atoms under electron-stimulated desorption from the surface of oxidized refractory substrates

An analysis is made of experimental data on the dependence of the energy distributions of Li, Na, and Cs atoms emitted in electron-stimulated desorption on their concentration on the surface of oxidized tungsten and molybdenum substrates.     

Influence of Au and TiO2 structures on hydrogen dissociation over TiO2/Au(100)

We performed H₂–D₂ exchange reactions over TiO_x/Au(100) and compared the observed reaction kinetics with those reported for TiO_x/Au(111) in order to clarify the influence of the Au and TiO₂ structures on dissociation of H₂ molecules. Low energy electron diffraction observations showed that the TiO₂ produced on Au(100) was disordered, in contrast to the comparatively ordered TiO₂ structure formed on Au(111). The activation energies and the turnover frequencies for HD formation over TiO₂/Au(100) agreed well with those for TiO₂/Au(111), clearly indicating that the hydrogen dissociation sites created over TiO₂/Au(100) were the perimeter interface between stoichiometric TiO₂ and Au, as was previously concluded for TiO₂/Au(111). We concluded that the creation of active sites for hydrogen dissociation was independent of the Au and TiO₂ structures consisting perimeter interface, and that local bonds that formed between Au and O atoms of stoichiometric TiO₂ were essential for the creation of active sites.     

Mechanisms of electron-stimulated desorption of protons from water: Gas,chemisorbed and ice phases

The stimulated desorption of ions from gas phase and condensed phase H₂O on Ni(111) has been examined theoretically and experimentally for the near threshold excitation region, 15 to 40 eV. The excited state potential energy curves have been calculated using configuration interaction for H₂O and a restricted Hartree-Fock (RHF) approach for a variety of small clusters including (H₂O)₅ and NiH₂O. Both proton yield and kinetic energy distributions have been measured for chemisorbed, ice phase, and gas phase water and are discussed in terms of specific electronic excitations corresponding to possible desorption pathways. For condensed phase water, the major proton desorption threshold occurs at 20–21 eV and is due to surface predissociation. The final state potential energy curves reached in this process are, in general, described by two electron excitations from the ground state and are thus not dipole allowed. At threshold, these potential energy curves correspond to the excited states of the neutral rather than the ionized molecule. Above 28–29 eV, predissociation or shake-up involving excitations from the O 2s orbital contributes to the ion yield and can give rise to protons of high (7–8 eV) kinetic energy.