Electron stimulated desorption of Xe,Kr, and Ar

Electron stimulated desorption cross sections have been measured for Xe, Kr, and Ar overlayers on the Ag(111) surface. The Xe cross section is less than 10^?4 Ų; the Kr cross section is strongly temperature dependent, rising from 0.1 Ų at 10 K to 0.18 Ų at 50 K; the Ar cross section is 4 Ų and temperature independent. These results are rationalized using a model of the stimulated desorption similar to that proposed by Antoniewicz [Phys. Rev. B21 (1980) 3811], in which an atom is ionized by the incident beam, accelerates towards its image and is neutralized, desorbing only if the kinetic energy gained is greater than the neutral atom binding energy at the neutralization position. Fitting these data requires an exceedingly rapid dependence of the neutralization on distance for these slow ions. Rather than the effect of the mass on the ion velocity, the most important effect in determining the diverse behavior of the different gases is that the equilibrium position for the heavier gases is farther up the overlap repulsive potential and so in a region of more rapid neutralization. The model identifies several contributions to the isotope effect, predicting it to be temperature dependent. The results are extremely sensitive to the anharmonicity of the holding potential.     

Image theory of lateral interactions applied to desorption rate of Xe from W and Ag

The image theory of lateral interactions between rare gas atoms given by Mahanty and March [J. Phys. C9 (1976) 2905] is shown to be consistent with the mean features of desorption rate measurements for Xe on W(111) and W(110). Some results are also presented for both Xe and Ar on Ag(111).     

Ar+ and Xe+ velocities near the presheath-sheath boundary in an Ar/Xe discharge

The velocities of Ar+ and Xe+ ions near the presheath-sheath boundary in an Ar/Xe discharge are studied by particle-in-cell Monte Carlo simulation. For a pure argon discharge the argon ion has almost the same velocity profile as it does in the mixture of argon and xenon. Similarly, for a xenon discharge the xenon ion has almost the same velocity profile as it does in the mixture of argon and xenon. The ion speed at the sheath-presheath boundary is the same for an ion in a pure argon or xenon discharge and for the same ion in a mixture of argon and xenon. We conclude that, in our simulation, each ion reaches its own Bohm speed at the presheath-sheath interface.     

Ion induced desorption of oxygen from platinum

Ion induced desorption has been used to study the initial uptake of oxygen on polycrystalline platinum. Rapid increase is observed in the O^? secondary ion signal with oxygen exposure. The dependence of the O^? signal on sample temperature is observed to exhibit a peak near the temperature corresponding to the main thermal desorption state.     

Electron-hole recombination induced desorption of excimers from solid Ar

Excimer desorption from solid Ar under selective excitation by photons in the range 10–35 eV at different temperatures is studied. An additional mechanism of exciton-induced desorption of excited dimers Ar*₂ is proposed. By combining photo-, cathodo- and thermoluminescence experiments the excimer desorption is shown to be enhanced by electron-hole recombination. The role of both participants of the recombination process is discussed.     

Thermal desorption from metal surfaces: A review

In this review an attempt is made to draw correlations between thermal desorption and structural studies of chemisorption on metal surfaces. Alternative models are discussed for the appearance of multiple peaks and for lineshape analysis in desorption, the first involving multiple binding states and the second lateral interactions within a homogeneous chemisorbed layer. Criteria are discussed for distinguishing between the various possibilities for a particular system, in particular with relation to the adsorption of hydrogen and carbon monoxide on tungsten.     

Pion absorption reactions on N, Ar and Xe

The absorption of π⁺ on Ar was studied at pion energies of 70, 118, 162, 239 and 330MeV, and on N and Xe at 239MeV. Twenty-six absorption reaction channels with at least two energetic charged particles in the final state have been evaluated. Partial cross-sections have been determined according to the number of protons, neutrons and deuterons in the final state. Received: 16 October 2000 / Accepted: 1 December 2000     

Thermal neutron induced charged particle reactions in radioactive targets of37Ar,109Cd,125Xe,127Xe,and132Cs

Thermal neutron induced charged particle reactions in a radioactive target of³⁷Ar have been studied. Upper limits of the cross-sections for the (n, α) reaction in radioactive targets of¹⁰⁹Cd,^{125, 127}Xe, and¹³²Cs have been obtained. The isotopically pure targets were produced at the ISOLDE facility at CERN and irradiated with thermal neutrons at the high flux reactor of the Institute Laue-Langevin in Grenoble. Charged particles from (n, p) and (n, α) reactions in³⁷Ar were observed with cross-sections of 69±14b and 1970 ±330b, respectively. TheQ-values for these reactions were determined to be 1600 ±12 keV and 4630±7 keV, in agreement with existing mass data. The branching ratio Γ_α/Γ_p of the³⁷Ar capturing state was found to be 28.5±2.7. An upper limit of the cross-section for the (n, γα) reaction in³⁷Ar was obtained.     

The temperature programmed desorption of hydrogen from platinum and platinum-gold films

The thermal desorption of hydrogen from Pt and PtAu films has been measured in an ultra-high vacuum system by means of a mass spectrometer. On the average, hydrogen is more loosely bound on the alloys than on pure Pt. About 50% of the adsorbate is desorbed by pumping at 78 K from the alloys while only a very small percentage is desorbed from Pt at this temperature. After maximum coverage of Pt films by hydrogen adsorption three desorption peaks have been observed: γ (120 K), β₁ (200 K) and β₂ (330 K). The same peaks have been found for the alloys as well but the relative population of the various adsorption types was different. The relative peak heights vary with the alloy composition.     

Thermal desorption of dysprosium from tungsten microcrystal

Activation energy for thermal desorption of dysprosium from a tungsten microcrystal of about 300 nm diameter was determined by means of the field-emission method. The desorption was detected from the whole W emitter surface in the temperature range 1490-1665 K for dysprosium average coverage θ < 0.06 monolayer. The average activation energy was determined to be 4.09 ± 0.06 eV/atom and the frequency factor to be about 10¹¹ s⁻¹. The energy may mainly concern the desorption from the atomically rough regions of the microcrystal.     

Thermal desorption of Ag from oxidized W

Thermal desorption spectra (TDS) of Ag condensates deposited at two substrate temperatures T_s = 300 K and T_s = 779 K have been obtained. A shift of the temperature T_m of the maximum of the desorption flux R_e was observed. It was established that the shift of the maximum depends on the value of the initial coverage σ₀. A significant difference was found to exist between TDS of silver condensates deposited on oxidized and clean W substrates due to differences in the mechanism of condensation. Silver condensates were deposited on oxidized W at different initial conditions (T_s, impingement rates R_i, etc.) but equal σ₀. The corresponding TDS were compared and a conclusion has been drawn that the shift of T_m is due to the different structure of Ag condensates. TDS of Ag condensates deposited at room temperature (T_s = 300 K) were interpreted using the method of Bauer et al. [J. Appl. Phys. 45 (1974) 5164: Surface Sci. 53 (1975) 87]. The dependence of the desorption flux R_e on the substrate temperature T_s and coverage σ was treated on the basis of the Polanyi-Wigner equation and some parameters of the condensation process were evaluated.     

Electron impact desorption of Xe from the tungsten (110) plane

The electron stimulated desorption of Xe adsorbed on the clean and on oxygen and CO covered tungsten (110) surfaces has been investigated. Only neutral Xe desorption was observed; for Xe on clean W a very small initial regime with cross section 10^?17cm² is followed by a slow decay with cross section 3×10^?19cm². The Xe yield varies nonlinearly with coverage, suggesting desorption from edges of islands or from sites with less than their full complement of nearest neighbor Xe atoms. Desorption from oxygen or CO covered surfaces results in an apparent desorption cross section identical to that of the underlying adsorbate. This results from a kicking off of Xe by electron desorbed O or CO. The true cross sections for these processes are ～10^?14cm² for Xe-0 and ～10^?15 cm² for Xe-CO. Some speculations about the mechanism, particularly the absence of ions are presented.     

The Calculations of Ar and Xe Conductivity Under High Pressures

The conductivity of Ar and Xe plasma is calculated for wide range of densities and temperatures. The chemical composition necessary for the conductivity calculation was obtained by means of the generalized chemical model (GCM). The feature of this GCM is the account for neutral‐neutral interaction within the integral equation approach of the theory of liquids. The results of calculations are in agreement with available data of measurements (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Atom scattering from disordered surfaces: calculations for a model of a Xe + Ar mixed overlayer

Exact quantum-mechanical calculations are present for He scattering from one-dimensional models of disordered, mixed Xe + Ar overlayers. A time-dependent wavepacket approach is used with a recent technique for solving the Schrödinger equation. Results are given for several overlayers of different Xe : Ar concentration ratios. The dependence of scattering intensities on the disordered structures is discussed. The results provide a reference for testing approximations for scattering from disordered surfaces.     

Thermal desorption of interacting molecules from mixed adlayers

A Monte Carlo simulation method is used to study thermal desorption of gas molecules from mixed adlayers containing two species. The effects of lateral interactions among adatoms and initial surface coverage on the desorption spectra are examined. It is shown that attractive lateral interactions lead to sharper peaks and that desorption occurs at high temperatures, whereas repulsive interactions lead to multiple peak spectra. It is found that interactions between unlike molecules affect the spectra for species with lower desorption energy only, and that the two species desorb together only in certain cases. Lateral interactions also affect the desorption kinetics significantly and very different behavior for the two species may be obtained.     

kinetic energy reflection from polycrystals bombarded with Ar+, Kr+, and Xe+ ions

When the surface of a solid is bombarded with ions a fraction of the primary energy is reemitted by ion reflection and sputtering. The contribution of ion reflection or sputtering to energy reflection is determined by the mass ratio of the bombarding ions to the target atoms.^1,2 In the case of light ions the contribution of reflected ions is dominant. Results for He⁺ and Ne⁺ bombardment were described in a previous paper.³ The present paper deals with results for Ar⁺, Kr⁺, and Xe⁺ bombardment of the same targets as investigated before.³ The energies of the mass selected bombarding ions range from 9 to 16 keV. The measurements were carried out by means of the thermic detector described in a separate paper.⁴ For the given mass ratios most of the reemitted energy is related to sputtering.     

Thermal desorption of large molecules from solid surfaces

We use molecular-dynamics simulations and importance sampling to obtain transition-state-theory rate constants for thermal desorption of an n-alkane series from Au(111). We find that the binding of a large molecule to a solid surface involves different types of local minima. The preexponential factors increase with increasing chain length and can be substantially larger than typical estimates for small molecules. Our results match recent experimental studies and indicate that a proper treatment of conformational isomerism and entropy, heretofore not found in coarse-grained models, is essential to quantitatively describe the thermal desorption of large molecules from solid surfaces.     

Thermal desorption of silicon from polycrystalline tungsten surfaces

The adsorption of Si on polycrystalline tungsten surfaces was studied for the first time by means of thermal desorption. Instead of the main peak of Si ($\text{m}\text{e}\text{=28}$), the isotope ($\text{m}\text{e}\text{=30}$) contained naturally (～3%) was monitored by mass spectrometry. This method can reduce the contribution from CO to the mass signal of Si. Two desorption peaks were observed at 1480 and 1820 K. The activation energies for the desorption were estimated to be about 90 and 110 kcal/mol, respectively.     

Thermal desorption spectra of chemisorbed atoms and molecules with lateral interactions and finite mobility: Nondissociatively adsorbed molecules

A model of thermal flash desorption of interacting chemisorbed atoms and molecules from a crystal surface is developed. The possibility of adatom diffusion along the surface is taken into account, but thermal equilibrium of the adatom system during the desorption process is not assumed. The whole range of adsorbate mobilities, including absolutely immobile species as well as adsorbates in thermal equilibrium, is investigated. The result is obtained in the form of a chain of differential equations for correlation functions of occupation numbers which is solved in various approximations. The numerical results differ from the results of previous calculations and are in better agreement with the experiments. The experimental thermal desorption spectrum of CO on W(210) is satisfactorily reproduced.