V. N. Ageev

THERMAL DESORPTION OF SODIUM ATOMS FROM THIN SiO2 Films

The adsorption and thermal desorption of Na from thin SiO2 films have been studied. X-ray photoelectron spectroscopy (XPS), angle-resolved XPS (ARXPS), low energy ion scattering (LEIS), temperature-programmed desorption (TPD), low energy electron diffraction (LEED) and work function measurements have been used to characterize the growth mechanism and properties of stoichiometric SiO2 films deposited onto a Re (0001) substrate. Upon deposition of Na onto SiO2 at 250 K, the first monolayer of Na exhibits ionic character, and evidence of metallic Na (plasmon features in XPS) is observed for higher coverages. TPD spectra for Na from SiO2 include a monolayer peak at ~700 K, and the multilayer peak due to sublimation of bulk Na at ~330 K. Penetration of Na into SiO2 can be induced by heating, or by He ion bombardment of a Na/SiO2 layer. The sticking probability for Na on SiO2 is ~0.5 at 250 K, and it decreases at higher substrate temperatures.

Electron-stimulated desorption of alkali metal and barium atoms from an oxidized tungsten surface

Electron-stimulated desorption (ESD) of Li, Na, K, Cs, and Ba atoms has been studied from metal layers adsorbed on an oxygen monolayer-covered tungsten and on tungsten oxide surfaces. The ESD yields and energy distributions for metal atoms have been determined as a function of the incident electron energy and the adparticle concentration at various surface temperatures. The ESD threshold for alkali metal atoms is close to the oxygen 2s level ionization energy of 25 eV. At T = 300 K the ESD energy distributions for alkali metal atoms consist of one peak. No ESD yield of Ba atoms has been detected at T = 300 K. At T = 77 K, the ESD energy distributions for Li, Cs and Ba atoms consist of several peaks. The Li, Cs, and Ba ESD yield dependencies on the incident electron energy reveal secondary thresholds corresponding to the inner shell ionization energies of adsorbed metals. The ESD of Li and Ba atoms arises from physisorbed and chemisorbed states, while that of Cs atoms arises only from a chemisorbed state in the position between two neighboring oxygen ions. It is shown that there are several ESD mechanisms for Li, Cs, and Ba atoms from layers adsorbed on oxidized tungsten.

Electron stimulated desorption of alkali metal ions and atoms: Local surface field relaxation

Abstract Electron stimulated desorption (ESD) of neutral alkali metal atoms and positive ions has been studied from alkali metal layers adsorbed on oxidized and silicided tungsten by means of a static magnetic mass spectrometer combined with a retarding field analizer. The desorbed alkali metal atoms were ionized in a surface ionization detector and their energy distributions were measured using a time-of-flight method. The ESD cross sections and energy distributions for alkali metal atoms and ions have been measured as a function of the incident electron energy and the deposited alkali metal concentration. To interpret the results we have extended the Auger stimulated desorption model taking into account the surface Coulomb field relaxation and the ESD of neutrals.

Resonances in electron-stimulated desorption of Eu atoms

The electron-stimulated desorption of europium (Eu) atoms has been detected for the first time from Eu layers adsorbed on an oxygen-covered tungsten surface. The dependence of Eu atom yield on electron energy has a pronounced resonance form. As a function of Eu coverage, there appear to be two different types of resonant processes associated with both Eu core excitations and W core excitations.

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

The yield and energy distributions of potassium and cesium atoms emitted in electron-stimulated desorption (ESD) from a molybdenum surface, oxidized to different extent and maintained at 300 K, have been measured by the time-of-flight technique with a surface ionization detector. The ESD threshold for potassium and cesium atoms lies around 25 eV, irrespective of molybdenum oxidation state. In the case of molybdenum coated by an oxygen monolayer, secondary thresholds at ∼40 and ∼70 eV have been observed, as well as atomic energy distribution tailing down to very low energies. The most probable kinetic energies of the atoms are a few tenths of one eV. The results are explained within a model involving Auger neutralization of the adsorbed alkali metal ions after the filling of the 2s O, 4s Mo, and 4p Mo core holes. The possibility of ESD of a neutral species as a result of oxide-cation core-level ionization has been demonstrated for the first time.

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

Abstract 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.

Electron-stimulated desorption of samarium from oxidized tungsten

Abstract The electron-stimulated desorption (ESD) yields for neutral samarium (Sm) atoms from layers adsorbed on oxidized tungsten surfaces have been measured as a function of electron energy, rare-earth metal coverage, degree of oxidation of tungsten and substrate temperature. The results are compared to previous ESD studies of europium (Eu). The measurements have been carried out using a time-of-flight method and surface ionization detector in the temperature range 160–600 K. The rare-earth metal atom ESD yield has a resonant character as a function of electron energy. Overlapping resonant-like Sm atom yield peaks are seen at electron energies of 34 and 46 eV that may possibly be connected with Sm 5p and 5s level excitations. Similar resonant-like Eu atom peaks are observed at electron energies of 36 and 41 eV that might be associated with Eu 5p and 5s level excitations. The rare-earth metal atom yield is a complicated function of rare-earth metal coverage and substrate temperature. The temperature dependent shape of the Sm atom yield depends on the Sm coverage. At low Sm coverage (below 0.07) the Sm atom yield decreases slightly with temperature up to 270 K and then falls to zero at a temperature above 360 K. At higher Sm coverages the Sm atom yield passes through a maximum with increasing substrate temperature. There are qualitative similarities but quantitative differences seen in ESD of Eu. At higher coverages of both Eu and Sm, additional features (resonant-like peaks) are observed at electron energies of 42, 54 and 84 eV that can be associated with tungsten 5p and 5s level excitations. It is possible that the ESD yield connected with tungsten core level excitations includes desorbed SmO molecules; these yields reach maxima at rare-earth metal coverages of about 1 ML.

Initial stages of the interaction of sodium and cesium with gold

The desorption of Cs and Na atoms from the corresponding layers applied to a gold film deposited on textured tungsten ribbon with a preferred orientation of the (100) surface is studied by thermal desorption spectroscopy with the products of thermal desorption scanned on a pulsed time-of-flight mass spectrometer. The Cs atoms evaporated at T = 300 K are desorbed by two phases, one of which can be identified with the filling of a monolayer and the other can be attributed to the formation of the CsAu compound. The Na atoms evaporated at T = 300 K are desorbed by three phases associated with the formation of a monolayer coating, a sodium compound of with gold, and a multilayer sodium film.

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.

Oxidation kinetics of thin titanium films grown on tungsten

Growth of thin Ti films on (100)W and the kinetics of their oxidation are studied using thermal-desorption spectroscopy and Auger electron spectroscopy. Titanium films grow nearly layer by layer on the (100)W face at room temperature. The activation energy for desorption of Ti atoms decreases from 5.2 eV for coverage θ=0.1 to 4.9 eV in a multilayer film. Oxidation of a thin (θ=6) titanium film starts with dissolution of oxygen atoms in its bulk to the limiting concentration for a given temperature, after which the film oxidizes to TiO, with the TiO2 oxide starting to grow when exposure of the film to oxygen is prolonged. The thermal desorption of oxides follows zero-order kinetics and is characterized by desorption activation energies of 5.1 (TiO) and 5.9 eV (TiO2).