Electron-stimulated desorption of sodium atoms from oxidized molybdenum

The time-of-flight technique combined with a surface-ionization-based detector has been used to investigate the yield and energy distribution of sodium atoms escaping in electron-stimulated desorption (ESD) from adlayers on the surface of molybdenum oxidized to various degrees and maintained at T=300 K as functions of incident electron energy and surface coverage by sodium. The sodium-atom ESD threshold is about 25 eV, irrespective of sodium coverage and extent of molybdenum oxidation. Molybdenum covered by an oxygen monolayer exhibits secondary thresholds at ∼40 eV and ∼70 eV, as well as low-energy tailing of the energy distributions, its extent increasing with surface coverage by sodium Θ. The most probable kinetic energies of sodium atoms are about 0.23 eV, irrespective of the degree of molybdenum oxidation and incident electron energy at Θ=0.125, and decrease to 0.17 eV as the coverage grows to Θ=0.75. The results obtained are interpreted within a model of Augerstimulated desorption, in which adsorbed sodium ions are neutralized by Auger electrons appearing as the core holes in the 2sO, 4sMo, and 4pMo levels are filled. It has been found that the appearance of secondary thresholds in ESD of neutrals, as well as the extent of their energy distributions, depend on surface coverage by the adsorbate. Fiz. Tverd. Tela (St. Petersburg) 40, 768–772 (April 1998)     

Electron-stimulated desorption of potassium and cesium atoms from oxidized molybdenum surfaces

The electron-stimulated desorption (ESD) yields and energy distributions for potassium (K) and cesium (Cs) atoms have been measured from K and Cs layers adsorbed at 300 K on oxidized molybdenum surfaces with various degrees of oxidation. The measurements were carried out using a time-of-flight method and surface ionization detector. The ESD appearance threshold for K and Cs atoms is independent of the molybdenum oxidation state and is close to the oxygen 2s level ionization energy of 25 eV. Additional thresholds for both K and Cs atoms are observed at about 40 and 70 eV in ESD from layers adsorbed on an oxygen monolayer-covered molybdenum surface; they are associated with resonance processes involving Mo 4p and 4s excitations. The ESD energy distributions for K and Cs atoms consist of single peaks. The most probable kinetic energy of atoms decreases in going from cesium to potassium and with increasing adsorbed metal concentration; it lies in the energy range around 0.35 eV. The K and Cs atom ESD energy distributions from adlayers on an oxygen monolayer-covered molybdenum surface are extended toward very low kinetic energies. The data can be interpreted by means of the Auger stimulated desorption model, in which neutralization of adsorbed alkali-metal ions occurs after filling of holes created by incident electrons in the O 2s, Mo 4s or Mo 4p levels.     

Electron stimulated desorption of cesium atoms from germanium-covered tungsten

The electron stimulated desorption (ESD) yield and energy distributions for Cs atoms from cesium layers adsorbed on germanium-covered tungsten have been measured for different Ge film thicknesses, 0.25-4.75 ML (monolayer), as a function of electron energy and cesium coverage Θ. The measurements have been carried out using a time-of-flight method and surface ionization detector. In the majority of measurements Cs is adsorbed at 300 K. The appearance threshold for Cs atoms is about 30 eV, which correlates well with the Ge 3d ionization energy. As the electron energy increases the Cs atom ESD yield passes through a wide maximum at an electron energy of about 120 eV. In the Ge film thickness range from 0.5 to 2 ML, resonant Cs atom yield peaks are observed at electron energies of 50 and 80 eV that can be associated with W 5p and W 5s level excitations. As the cesium coverage increases the Cs atom yield passes through a smooth maximum at 1 ML coverage. The Cs atom ESD energy distributions are bell-shaped; they shift toward higher energies with increasing cesium coverage for thin germanium films and shift toward lower energies with increasing cesium coverage for thick germanium films. The energy distributions for ESD of Cs from a 1 ML Ge film exhibit a strong temperature dependence; at T = 160 K they consist of two bell-shaped curves: a narrow peak with a maximum at a kinetic energy of 0.35 eV and a wider peak with a maximum at a kinetic energy of 0.5 eV. The former is associated with W level excitations and the latter with a Ge 3d level excitation. These results can be interpreted in terms of the Auger stimulated desorption model.     

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.     

Influence of x irradiation on internal friction in silicon

An investigation was made of the influence of γ irradiation on the temperature dependences of internal friction in disk-shaped silicon substrates in the kilohertz frequency range. After exposure to doses of 10⁴ and 10⁵ R, two dominant internal friction peaks were observed at ∼330 and ∼450 K with activation energies H ₁=0.6 eV and H ₂=0.9 eV, respectively. These peaks were evidently caused by reorientation of interstitial silicon atoms in dumbbell configurations. Fiz. Tverd. Tela (St. Petersburg) 40, 1257–1258 (July 1998)     

Autoionization cross section of lithium atoms excited by electron impact

We measure the full autoionization cross section of lithium atoms excited by electrons in the energy range from the first autoionization threshold at 56.39 to 600 eV. Data are obtained by determining the total intensity of electron spectrum of autoionization states 1sn ₁ l ₁ n ₂ l ₂ detected at the “magic” observation angle of 54.7°. The cross section behavior is characterized by a sharp increase to a maximum value of 1.7 × 10⁻¹⁸ cm² in the energy interval of 56.4–60 eV and a subsequent monotonic drop to a value of 10⁻¹⁸ cm² at 600 eV. We have discovered a “thin” cross section structure that reflects the presence of strong resonances of Li⁻ ions in the near-threshold area of excitation of the lowest energy autoionization states (1s2s2)2S, (1s2s2p)⁴ P, 1s(2s2p ³ P)² P), and 1s(2s2p ¹ P)² P. We have established that the contribution of autoionization to the absolute cross section of single ionization of lithium atoms does not exceed 4%. We perform a comparative analysis of the data with analogous data for potassium and cesium atoms.     

Electron-stimulated desorption of cesium atoms from cesium layers deposited on a germanium-coated tungsten surface

The yield and energy distribution of Cs atoms from cesium layers adsorbed on germanium-coated tungsten were measured, using the time-of-flight technique with a surface-ionization-based detector, as a function of the energy of bombarding electrons, germanium film thickness, the amount of adsorbed cesium, and substrate temperature. The threshold for the appearance of Cs atoms is ～30 eV, which correlates well with the germanium 3d-level ionization energy. As the electron energy increases, the Cs atom yield passes through a broad maximum at ～120 eV. For germanium film thicknesses from 0.5 to 2 monolayers, resonance Cs yield peaks were observed at electron energies of 50 and 80 eV, which can be related to the tungsten 5p and 5s core-level ionization energies. As the cesium coverage increases, the Cs atom yield passes through a flat maximum at monolayer coverage. The energy distribution of Cs atoms follows a bell-shaped curve. With increasing cesium coverage, this curve shifts to higher energies for thin germanium films and to lower energies for thick films. The Cs energy distribution measured at a substrate temperature T = 160 K exhibits two bell-shaped peaks, namely, a narrow peak with a maximum at ～0.35 eV, associated with tungsten core-level excitation, and a broad peak with a maximum at ～0.5 eV, deriving from the excitation of the germanium 3d core level. The results obtained can be described within a model of Auger-stimulated desorption.     

Intercalation of two-dimensional graphite films on metals by atoms and molecules

An analysis is made of some general laws governing a new physical effect, i.e., the spontaneous penetration of particles (atoms, C₆₀ molecules) adsorbed on a two-dimensional graphite film on a metal (Ir, Re, Pt, Mo,...) to beneath the graphite film (intercalation). It is shown that atoms having low ionization potentials (Cs, K, Na) intercalate a two-dimensional graphite film on iridium at T=300–400K with an efficiency χ≈0.5, accumulating beneath the film to a concentration of up to a monolayer. Atoms having high ionization potentials (Si, Pt, Ni, C, Mo, etc.) intercalate a two-dimensional graphite film on iridium at T≈1000K with an efficiency, χ≈1, forming beneath the film a thick intercalate layer which is strongly bonded chemically to the metal substrate but is probably weakly bonded to the graphite monolayer by van der Waals forces. The presence of a graphite “lid” impeding the escape of atoms from the intercalated state up to record high temperatures T∼2000K leads to superefficient diffusion (with an efficiency close to one) of various atoms (Cs, K) into the bulk of the substrate (Re, Ir). Zh. Tekh. Fiz. 69, 72–75 (September 1999)     

Probability of ionization of sputtered particles as a function of their energy: Part I: Negative Si ions

In this study we have investigated how the probability of ionization of sputtered Si atoms to form negative ions depends on the energy of the atoms. We have determined the ionization probability from experimental SIMS energy distributions using a special experimental technique, which included de-convolution of the energy distribution with an instrumental transmission function, found by separate measurements.We found that the ionization probability increases as a power law ∼E^0.677 for particles sputtered with energies of 0-10 eV, then becomes a constant value (within the limits of experimental error) for particles sputtered with energies of 30-100 eV. The energy distributions of Si⁻ ions, measured under argon and cesium ion sputtering, confirmed this radical difference between the yields from low and high-energy ions.To explain these results we have considered ionization mechanisms that are different for the low energy atoms (<10 eV) and for the atoms emitted with higher energy (>30 eV).     

Carbon diffusion between the volume and surface of (100) molybdenum

The diffusion of carbon atoms between the volume and the surface of (100) molybdenum is directly studied at temperatures between 1400 and 2000 K (i.e., at process temperatures) for the first time. The balance of carbon atoms in the system is determined. The difference in the activation energies of carbon dissolution and precipitation, ΔE=E s 1-E1s, is found for the case when the diffusion fluxes of dissolved and precipitated carbon atoms are in equilibrium. This difference defines the enrichment of the surface by carbon relative to the bulk. The experimentally found activation energy of carbon dissolution is Es1=3.9 eV. The activation energy of carbon precipitation is estimated at E 1 s=1.9 eV. The latter value is close to the energy of bulk diffusion of carbon in molybdenum.     

Experimental determination of the rate constant for spin exchange in collisions of polarized metastable helium atoms with ground-state cesium atoms

The rate constant for spin exchange in a system consisting of a metastable helium atom and an alkali-metal atom is determined. An experiment on optical orientation of atoms established that the rate constant for spin exchange in a collision of a metastable 2³ S ₁ helium atom with a cesium atom in the 6² S _1/2 ground state equals (2.8±0.8)×10⁻⁹ cm³ s⁻¹. The rate constant for chemoionization of cesium atoms by metastable helium atoms was determined at the same time to be (1.0±0.3)×10⁹ cm³s⁻¹. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 145–148 (10 August 1997)     

Parameters of the discharge plasmas of surface-plasma sources of negative hydrogen ions

The main parameters of the plasma of high-current hydrogen-cesium glow discharges of surface-plasma (planotron and Penning) sources of negative hydrogen ions are determined using contact-free spectroscopic methods and compared for identical discharge current densities. The elemental and charge composition of the plasma is established. The temperature of the hydrogen atoms and the energy of the visible-range radiation of the plasma discharge are measured and estimates of the electron density in the plasma are made. The dynamics of the change in the parameters of the discharge plasma of a Penning source — the densities of hydrogen atoms, cesium atoms and ions, and molybdenum atoms — is tracked during a discharge pulse with spatial resolution along two coordinates. It is observed that cesium atoms and ions and molybdenum atoms are pent up near the cathode surface. Zh. Tekh. Fiz. 68, 32–38 (October 1998)     

Anisotropy of permittivity of the hexagonal multiferroic HoMnO<Subscript>3</Subscript> in the spectral range 0.6–5.0 eV

The anisotropy of the optical properties of a single crystal of the hexagonal manganite HoMnO₃ has been investigated by spectroscopic ellipsometry in the spectral range 0.6–5.0 eV. It has been demonstrated that the optical absorption edge for the polarization E ⊥ c is determined by the intense narrow transition O(2p) → Mn(3d) centered at 1.5 eV, whereas this transition for the polarization E ‖ c is strongly suppressed and shifted toward higher energies by 0.2 eV. It has been revealed that, at the temperature T = 293 K, the spectra for both polarizations E ‖ c and E ⊥ c exhibit a broad absorption band centered at ∼2.4 eV, which was earlier observed in nonlinear spectra during optical second harmonic generation.     

Stimulation of secondary negative ion emission by implantation of alkaline ions into the surface layer of a solid followed by heating

A new method of stimulating secondary negative ion emission is suggested that is based on implantation of alkaline ions into the surface layer of a solid with subsequent heating to a temperature providing optimal coverage of the surface (about half a monolayer) by activator (alkaline) ions. It is shown that, by appropriately selecting the implantation dose (10¹⁸–10¹⁹ cm⁻³) and surface temperature (500–900°C), one can reach such a degree of coverage of the sample surface by activator ions that its work function eφ becomes minimal: 1.9 eV for molybdenum and 2.1 eV for copper. It is found that, with the implantation (irradiation) dose and surface temperature chosen properly, one can, by means of outdiffusion of cesium atoms, achieve such a degree of surface coverage that remains unchanged during the continuous sputtering of the surface by a cesium ion beam.     

Resonances in the cesium atom yield in electron-stimulated desorption from tungsten coated with a germanium monolayer

The yield and energy distributions of Cs atoms emerging from cesium layers, which are adsorbed on tungsten coated with a thin germanium film (1-to 2-monolayers thick), have been measured as a function of the incident electron energy, the amount of adsorbed cesium, and the substrate temperature. The measurements were performed by the time-of-flight technique with a surface ionization detector. At low cesium coverages (Θ < 0.1), the Cs atom appearance threshold at a substrate temperature T = 160 K is ～24 eV, which correlates with the Cs 5s-level ionization energy. As the electron energy is increased, the yield passes through a broad plateau and reaches saturation. The signal intensity in the plateau region decreases gradually with increasing cesium coverage and tends to zero for Θ > 0.14. For Θ ≥ 0.15, the cesium atom appearance threshold shifts to ～30 eV, which corresponds to the Ge 3d-level ionization energy and the plateau is replaced by a resonance peak at ～38 eV, which can be identified with the ionization energy of the W 5p _3/2 level. This peak is observed only for Θ < 0.3 and T = 160 K. For Θ ≥ 0.3, there appears a resonance peak at ～50 eV, and for Θ ≥ 0.5, another resonance peak appears at ～80 eV. These peak positions correlate with the ionization energies of the W 5p _1/2 and W 5s levels, and their intensity is maximum at Θ = 1. The Cs atom energy distributions for Θ < 0.15 consist of a bell-shaped peak with a maximum at ～0.55 eV, and those for Θ ≥ 0.15 contain two nearly resolved maxima, a broad one peaking at ～0.5 eV and a narrow one at ～0.35 eV. The above results argue for the existence of three channels of Cs atom desorption. One channel involves reverse motion of the Cs²⁺ ion; another channel, neutralization of the adsorbed Cs⁺ ion following the Auger decay of a vacancy in the Ge atom; and the third channel involves desorption of a CsGe molecule as it is repelled from a W core exciton.     

Spectroscopic investigation of thallium 6<Emphasis Type="Italic">s</Emphasis>6<Emphasis Type="Italic">p</Emphasis><Superscript>2</Superscript> configuration at electron-atom collisions: I. Spectra

Thallium emission spectra in the 115–300 nm range excited by electron-atom collisions at electron energies of 12–300 eV are investigated. A number of weak lines that cannot be unidentified using spectroscopic tables are found in the 140–170 nm range. Two of them (144.9 and 148.0 nm) are attributed to the radiative decay of levels that belong to the 6s6p ² configuration and that lie above the ionization potential. A weak emission with an excitation threshold of about 9 eV is revealed in the vicinity of the Tl II resonance line at 132.2 nm. The excitation function of the emission is measured for electron energies below 15.5 eV. It is found that the emission consists of two lines, which also appear due to the excitation of the 6s6p ² configuration and correspond to the radiative decay of levels that are common with the lines at 144.9 and 148.0 nm mentioned above. Their calculated wavelengths are 130.2 and 132.7 nm, respectively.     

Observation of the Molecular Quenching of μp(2S) Atoms

Kinetic energy distributions of muonic hydrogen atoms μp(1S) have been obtained by means of a time-of-flight technique for hydrogen gas pressures between 4 and 64 hPa. A high energy component of ∼900 eV observed in the data is interpreted as the signature of long-lived μp(2S) atoms, which are quenched in a non-radiative process leading to the observed high energy: the collision of a thermalized μp(2S) atom with a hydrogen molecule H₂ results in the resonant formation of a {[(ppμ)⁺]^*pee}^* molecule. Then the (ppμ)⁺ complex undergoes Coulomb de-excitation and the ∼1.9 keV excitation energy is shared between a μp(1S) atom and one proton. The preliminary analysis of the time spectra gives a long-lived μp(2S) population of ∼1% of all stopped muons, and a quenching rate of ∼4⋅10¹¹ s⁻¹. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electron-stimulated cesium atom desorption from the Cs/CsAu/Au/W system

This paper reports on a continuation of the investigation of electron-stimulated Cs-atom desorption from a tungsten surface on which cesium and gold films had been adsorbed at T = 300 K. Earlier studies revealed that Cs atoms start to desorb only after more than one monolayer of gold and more than one monolayer of cesium had been deposited on the tungsten surface. In this case, a coating consisting of a gold adlayer on tungsten, a CsAu compound possessing semiconducting properties, and a cesium monolayer capping CsAu (Cs/CsAu/Au/W) is formed on the tungsten surface at 300 K. The yield of atoms from this system exhibits a resonant dependence on the incident electron energy E _e , with an appearance threshold of 57 eV and a maximum at 64 eV. In this case, Cs atoms desorb in two channels, with one of them involving Cs desorption out of the cesium monolayer, and the other, from the CsAu monolayer. The Cs yield at E _e = 64 eV has been investigated in both desorption channels, with an additional cesium coating deposited on the already formed Cs/CsAu/Au/W layered system, as well as of the effect annealing produces on the yield and energy distributions of Cs atoms. It has been demonstrated that Cs atoms evaporated at 300 K on a layered coating with a cesium monolayer atop the CsAu layer on tungsten capped with a gold adlayer, rather than reflected from the cesium monolayer or adsorbing on it, penetrate through the cesium monolayer into the bulk of CsAu even with one CsAu layer present. The desorption yield does not vary with increasing cesium concentration at 300 K, but falls off gradually at 160 K. Annealing within the temperature range 320 K ≤ T _H ≤ 400 K destroys the cesium monolayer and the one-layer CsAu coating, but the multilayer CsAu compound does not break up in this temperature range even after evaporation of the cesium monolayer. It is shown that Cs atoms escape from the multilayer CsAu compound primarily out of the top CsAu layer.     

Entrapment of inert gas ions near molybdenum surface

The thermal desorption spectra of inert gas ions (neon, argon and krypton) injected with various energies (430–1950 eV) into a polycrystalline molybdenum target with various dosages (6.4×10¹²−3.9×10¹⁴ ions/cm²) are reported. At least four different states of binding of the trapped atoms corresponding to the activation energies for desorption have been observed from the spectra. The activation energies are found to be relatively insensitive to the species of the bombarding ion, incident ion energy and the dosage. The shapes of the spectra are strongly influenced by the depth of penetration of the ions into the solid. The activation energies deduced are in good agreement with those reported for the migration of atoms and defects in molybdenum.     

Characteristics of thermally stimulated depolarization phenomena in zinc-oxide ceramics for varistors

The results of experimental investigations and an analysis of the temperature dependences of the thermally stimulated depolarization current are presented for zinc-oxide ceramics suitable for use in high-voltage varistors. A model for the depolarization phenomena is proposed that takes into account charge exchange on localized electronic states on both sides of the intercrystallite potential barrier. The model is used to obtain estimates of the ionization energy and density of shallow (∼0.07 eV, 1×10¹⁷ cm⁻³) and deeper (∼0.2 eV, 1×10¹⁸ cm⁻³) bulk levels and surface-localized levels (∼0.1 eV, 1×10¹³ cm⁻²). Zh. Tekh. Fiz. 67, 60–63 (October 1997)