Muonium emission into vacuum from mesoporous thin films at cryogenic temperatures

We report on muonium (Mu) emission into vacuum following μ(+) implantation in mesoporous thin SiO(2) films. We obtain a yield of Mu into vacuum of (38±4)% at 250 K and (20±4)% at 100 K for 5 keV μ(+) implantation energy. From the implantation energy dependence of the Mu vacuum yield we determine the Mu diffusion constants in these films: D(Mu)(250 K)=(1.6±0.1)×10(-4) cm(2)/s and D(Mu)(100 K)=(4.2±0.5)×10(-5) cm(2)/s. Describing the diffusion process as quantum mechanical tunneling from pore to pore, we reproduce the measured temperature dependence ～T(3/2) of the diffusion constant. We extract a potential barrier of (-0.3±0.1) eV which is consistent with our computed Mu work function in SiO(2) of [-0.3,-0.9] eV. The high Mu vacuum yield, even at low temperatures, represents an important step toward next generation Mu spectroscopy experiments.     

Field desorption of common gases from iridium and tungsten

The technique of field desorption has been used to study adsorption and desorption of gases from field ion tips. The formal procedure of the experiments is quite similar to thermal flash desorption. Values for the desorption field and their change with coverage have been obtained. For H₂/W a field adsorbed state could be detected. Time-of-flight analysis was applied to determine the desorbing species. The gas supply for field ion tips could be measured; it increases exponentially with the field.     

Interaction of aluminum with iridium surface: Adsorption, desorption, dissolution, and formation of surface compounds

The interaction of aluminum with an iridium (111) surface was studied in ultrahigh vacuum by Auger electron spectroscopy over the broad temperature range 300–2000 K. At room temperature, layer-by-layer growth of an aluminum film was observed, with a monolayer forming in coherent relation to the substrate. Deposition at 1100–1300 K gives rise to the formation of surface aluminide Ir₄Al with an adatom concentration N _Al = (4.20 ± 0.15) × 10¹⁴ cm^?2. It was shown that aluminum escapes out of the surface aluminide by thermal desorption in the 1300–1700-K temperature interval, with the desorption activation energy changing from ～4.5 to ～5.7 eV as the coverage decreases from the value corresponding to the surface aluminide (taken for unity) down to zero.     

Muonium adduct radicals from vinyl-aromatic compounds

During irradiation of liquid samples of 1-vinylimidazole, 2-vinylnaphthalene, 2-vinylpyridine, 4-vinylpyridine and 1,1-diphenylethene with positive muons, radicals of the type MuCH₂ĊHAr (Ar=the appropriate aromatic group), and the MuCH₂ĊPh₂ radical were observed by transverse field μSR; the muon hyperfine couplings are considered in the context of the effectiveness of the aryl groups in delocalising the unpaired electron.     

Electron-stimulated desorption from the surface of alumina ceramics

The changes in the surface composition of UV-46, VK 94-1, and VK 100-1 alumina ceramics and leucosapphire under (1–2)-keV electron irradiation with doses D up to 4 × 10²⁰ electrons cm^?2 have been investigated by low-energy electron spectroscopy. The effect of the bulk composition of the ceramics and some other factors on the surface oxides destruction rate is shown.     

Electron-stimulated desorption of lithium atoms from oxidized molybdenum surface

The yield and energy distributions of lithium atoms upon electron-stimulated desorption from lithium layers adsorbed on the molybdenum surface coated with an oxygen monolayer have been measured as functions of the impact electron energy and lithium coverage. The measurements are performed using the time-of-flight technique and a surface ionization detector. The threshold of the electron-stimulated desorption of lithium atoms is equal to 25 eV, which is close to the ionization energy of the O 2s level. Above a threshold of 25 eV, the yield of lithium atoms linearly increases with an increase in the lithium coverage. In the coverage range from 0 to 0.45, an additional threshold is observed at an energy of 55 eV. This threshold can be associated with the ionization energy of the Li 1s level. At the electron energies above a threshold of 55 eV, as the coverage increases, the yield of lithium atoms passes through a maximum at a coverage of about 0.1. Additional thresholds for the electron-stimulated desorption of the lithium atoms are observed at electron energies of 40 and 70 eV for the coverages larger than 0.6 and 0.75, respectively. These thresholds correlate with the ionization energies of the Mo 4s and Mo 4p levels. Relatively broad peaks in the range of these thresholds indicate the resonance excitation of the bond and can be explained by the excitation of electrons toward the band of free states above the Fermi level. The mean kinetic energy of the lithium atoms is equal to several tenths of an electronvolt. At electron energies less than 55 eV, the energy distributions of lithium atoms involve one peak with a maximum at about 0.18 eV. For the lithium coverages less than 0.45 and electron energies higher than 55 eV, the second peak with a maximum at 0.25 eV appears in the energy distributions of the lithium atoms. The results obtained can be interpreted in the framework of the Auger-stimulated desorption model, in which the adsorbed lithium ions are neutralized after filling holes inside inner shells of the substrate and lithium atoms.     

Ion-stimulated desorption from a semiconductor surface containing metal nanodots

A model of the mechanism of ion-stimulated desorption of atoms and molecules pre-adsorbed from the surface of wide-band gap samples with a system of metal nanodots is developed. The relaxation processes of vibration-exciting states in adsorbed layer via electronic channel are considered. It is found in the model that the presence of nanosized metal clusters can affect the relaxation rate of the vibration excitations on the surface and the velocity of ion-stimulated desorption.     

Hydrogen desorption from tantalum with segregated oxide surface layers

The combination of thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES) was used to examine the correlation between elementary processes on an oxidized tantalum surface and the acceleration of the dehydriding rates at increasing temperatures. We present the TDS of TaD_x foils (foil thickness ~ 25μm) for various initial concentrations (0.038 ⩽ x ⩽ 0.108; α-phase), various heating rates and for high and ultra high vacuum conditions. Activation energies for desorption were derived from the TDS spectra on the assumption that the rate determining process is the recombination of hydrogen atoms to hydrogen molecules at the surface. We find two different activation energies for the low and for the high temperature region. The AES measurements show that the corresponding change in the desorption process is correlated with the dissolution of the segregated oxide surface layer.     

A method for assessing the coverage dependence of kinetic parameters: Application to carbon monoxide desorption from iridium (110)

A method for analyzing thermal desorption mass spectra has been developed for determining the coverage dependence of the pre-exponential factor and the desorption energy in the Arrhenius equation for a one-step desorption process. The method, which involves variable heating schedules, is applicable to spectra in which several features appear. However, if the desorption process involves multiple steps, or if substantial desorption from multiple sites occurs for any one coverage, this method cannot be used. The method is applied to CO desorption from the (110) surface of Ir. Three features can be resolved in the desorption spectrum. Both the preexponential factor and the desorption energy vary strongly with coverage, and a compensation effect occurs.     

Samarium atom yield under electron-stimulated desorption from oxidized tungsten surface

The yield of samarium (Sm) atoms under electron stimulated desorption from Sm layers adsorbed on the surface of oxidized tungsten was studied as a function of incident electron energy, surface coverage by samarium, degree of tungsten oxidation, and substrate temperature. The measurements were conducted by the time-of-flight technique with a surface ionization detector in the substrate temperature interval from 140 to 600 K. The yield vs. incident electron energy function has a resonance character. Overlapping resonance peaks of Sm atoms are observed at electron energies of 34 and 46 eV, which may be related to excitation of the Sm 5p and 5s levels. The Sm yield is a complex function of samarium coverage and substrate temperature. Sm atom peaks occur only in the Sm coverage range from 0 to 0.2 monolayers (ML), in which the yield passes through a maximum. The shape of the yield temperature dependence is a function of Sm coverage. For low Sm cover-ages (<0.07 ML), the yield decreases slowly with the temperature increasing to 270 K, after which it drops to zero at temperatures above 360 K. At higher coverages, the Sm yield passes through a maximum with increasing temperature and additional peaks appear at electron energies of 42, 54, and 84 eV, which can be assigned to excitation of the tungsten 5p and 5s levels. These peaks are most likely associated with desorption of SmO molecules, whose yield reaches a maximum at an Sm coverage of about 1 ML.     

Electron-stimulated desorption of europium atoms from an oxidized tungsten surface

The yield of europium atoms in electron-stimulated desorption from Eu layers adsorbed on the surface of oxidized tungsten was studied with a surface-ionization detector as a function of the incident-electron energy, surface coverage by europium, and degree of tungsten oxidation. The yield of Eu atoms measured as a function of electron energy exhibits a distinct resonant character with peaks at electron energies corresponding to europium and tungsten core-level ionization energies. The peaks associated with the europium ionization reach a maximum intensity at europium coverages less than 0.1 and decrease subsequently to zero with increasing coverage, while the peaks due to tungsten ionization pass through the maximum intensity at a monolayer europium coverage. The coverage corresponding to the maximum europium atom yield grows with increasing tungsten oxidation. The results obtained are accounted for by the formation of the europium and tungsten core excitons. In the first case, the particles desorb in the reverse motion toward the surface of the oxidized tungsten; in the second, they desorb as a result of repulsion between the tungsten core exciton and the EuO molecule.     

Simulation of desorption effects in vacuum by parallel processing

Summary We propose an approach for the simulation of desorption processes, which is particularly well suited for parallel processing. The approach is applied to the study of desorption in vacuum for several models of initial density distribution and to the solution of the inverse problem.     

Interaction of silver atoms with iridium and with a two-dimensional graphite film on iridium: Adsorption,desorption, and dissolution

The initial stages in the interaction of silver with the (111)Ir surface and with a two-dimensional graphite film (2D GF) on (111)Ir were studied by high-resolution electron Auger spectroscopy in ultrahigh vacuum. The growth mechanisms of silver films and the desorption fluxes of Ag atoms were determined, and their desorption energies estimated. It was found that the Ag desorption fluxes from a 2D GF on Ir and from a thick silver film on the pure metal are similar and considerably (an order of magnitude) smaller than the sublimation fluxes from bulk silver at the same temperatures. The activation energy for desorption from a submonolayer film varies from 3.2 eV for coverage θ=1 to 3.7 eV at θ ～ 0. It was shown that silver atoms do not penetrate into the substrate bulk throughout the temperature range covered (300–1800 K).     

Mechanism of associative oxygen desorption from Pt(111) surface

Mechanism of the associative desorption of oxygen from the Pt(111) surface has been studied on atomic level by means of DFT/GGA calculations and kinetic Monte Carlo simulations. It has been found that two oxygen adatoms can occur, with sufficient probability, in neighboring on-top sites, which is essential for formation and subsequent evaporation of the oxygen molecule. Monte Carlo simulations have demonstrated effectiveness of this channel for O₂ formation on Pt(111) and strongly support the suggested model of associative desorption from transition metal surfaces.     

Electron-stimulated desorption of europium atoms from the surface of oxidized tungsten

An analysis of the yield q of europium atoms is made, and scenarios of electron-stimulated desorption are put forward. Expressions are obtained for the dependence of q on the coverage of oxidized tungsten by europium atoms.     

Effect of phase transitions on thermal desorption from a solid surface

Thermal desorption from the surfaces of films and polycrystals undergoing semiconductor-metal (VO₂), ferroelectric-paraelectric (Ba_0.9Sr_0.1TiO₃ and TsTS-19), ferromagnet-paramagnet (Ni) phase transitions has been investigated. A sharp increase in the desorption activity of a surface was observed near a phase transition. The increase in the thermal desorption signal is caused by local deformations which arise in a solid at a phase transition. Fiz. Tverd. Tela (St. Petersburg) 39, 573–576 (March 1997)     

Surface photochemistry: from hot reactions to hot materials

The field of surface photochemistry has been rather fruitful for two major reasons: its contribution to the understanding of surface dynamics and its importance in the fabrication of surface materials. On the fundamental side, instead of addressing electronic transitions, it is my attempt here to illustrate the importance of surface photochemistry in understanding dynamics of the ground electronic states, those which are generally associated with thermal reactions. In the world of materials fabrication, I would like to point out the unique advantages of surface photochemistry over gas-phase photochemistry or thermal chemistry in the stringent control of ultra-thin films and ultra-small dimensions, i.e. nanotechnology. Both aspects of surface photochemistry are demonstrated using selected examples from my laboratory.