Thermal desorption of metals from tungsten single crystal surfaces

After a short review of the experimental methods used to determine desorption energies E and frequencies v the assumptions underlying the theoretical analysis of the experimental data are discussed. Then recent experimental results on the flash desorption of Cu, Ag and Au from clean, well characterized W {110} and {100} surfaces are presented and analysed in detail, in particular with respect to the coverage dependence. The results obtained this way clearly reveal the limitations of previous analysis methods and of the experimental technique per se (such as structure and phase changes below and in the temperature region in which desorption occurs). Furthermore the need for more theoretical work to understand the large changes of v and E with coverage and the so-called “compensation effect”, i.e. the relation between In v and E, becomes evident.     

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.     

Spontaneous emission of positronium negative ions from polycrystalline tungsten surfaces

Recently, the spontaneous emission of positronium negative ions from polycrystalline tungsten surfaces was observed. In the present work, the emission of these ions in ultra-high vacuum has been studied and the long-term stability of the emission efficiency has been investigated.     

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 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.     

Thermal desorption of N2 from (N2 + O2) coadsorbed on polycrystalline rhenium surfaces

The coadsorption of ¹⁵N₂ and O₂ on polycrystalline rhenium filament has been studied by thermal desorption mass spectroscopy. The sample was exposed to a mixture of ¹⁵N₂ and O₂ with various concentrations of ¹⁵N₂ at room temperature. It is suggested that NO on Re at low coverage is dissociative and the bonding strength of nitrogen on Re is weakened by the coadsorption with oxygen atoms.     

Adsorption and desorption of oxygen on stepped tungsten surfaces

The adsorption and desorption of oxygen on stepped tungsten surfaces with orientations close to the (110) orientation and steps parallel to the most densely packed crystal direction ([111]) is studied with low energy electron diffraction, Auger electron spectroscopy, work function measurements and thermal desorption spectroscopy. With increasing deviation from the (110) orientation, an increasing preference for the formation of the p(2 × 1) domain with the densely packed direction parallel to the steps is noted. The adsorption kinetics does not differ markedly from that on the flat (110) surface, however the desorption behaviour at low coverages (θ < 0.3) is quite different. The results are interpreted in terms of the dissociation of a mobile precursor at terrace and step sites, the competition between the two domains during their growth and a step-induced premature transition to the complex structure observed on flat (110) surfaces at $\text{θ ? 8}$. The steps are believed to play also a significant role in desorption.     

Laser-induced thermal desorption mass spectrometry of functionalized silicon surfaces

Functionalization of materials and laser patterning of chemisorbed layers play an increasing role in tailoring and structuring surface properties on the nanoscale. An attractive method of investigating organic functionalizations is laser-induced thermal desorption (LITD). The analysis of well-defined H- and D-terminated Si(1 1 1)-(1 × 1):H(D) surfaces was used to quantify the LITD technique. Moreover, oxidized silicon surfaces were functionalized with trimethylsilyl (TMS) and (3,3,3-trifluoropropyl)-dimethylsilyl (TFP) hydrophobic end groups. The samples were irradiated normal to the surface with focused XeCl laser pulses. The desorbed species were monitored at an oblique angle and their time-of-flight (TOF) distributions were measured with a quadrupole mass analyzer. The TOF temperatures of silicon were calibrated for different laser pulse energies by desorption of H₂ and D₂. In the LITD experiments, the desorption of trimethylsilanol groups was observed for TMS terminations, indicating that essentially the whole molecule desorbs from the surface. The TOF data could be fitted to Maxwellian distributions, providing the desorption yield of the emitted species, their mass, and temperature. On the other hand, several characteristic fragments were found for the TFP-terminated surface. The TOF distributions indicate that the fragments detected with the analyzer derived from different desorbed species.     

Diffusion and desorption of barium and oxygen on tungsten dispenser cathode surfaces

Auger measurements have been made on the concentration of barium and oxygen diffusing out of a pore and slot onto the surface of a simulated tungsten dispenser cathode. Profiles of concentration vs distance from the source were obtained at different temperatures. It is found that at cathode operating temperatures very little gradient of oxygen concentration exists on the surface, and under steady state conditions Ba is the main diffusing species. Ba diffusion distances derived are consistent with previous measurements. However it is found that this distance increases with concentration in spite of the fact that the Ba surface lifetime decreases with concentration. Time dependent measurements made on a clean surface show that the rate of Ba and O build up is limited by the supply rate of oxygen to the surface. This supply rate is not diffusion limited but seems to be limited by the mechanism generating free oxygen. A comparison of emission microscope measurements on a tungsten matrix dispenser cathode with the simulated cathode data indicates that similar oxygen generation processes may be controlling the activation of these cathodes.     

Electron-stimulated desorption of atomic oxygen from polycrystalline Ag

Emission of neutral oxygen atoms from an oxygen-charged polycrystalline Ag wire has been examined by using electron-stimulated desorption (ESD) in conjunction with threshold ionization detection (appearance potential). A quadrupole mass spectrometer (QMS) was used to detect the neutral oxygen atoms, but the ionizer was operated in the appearance potential (AP) mode to make it possible to distinguish the oxygen atoms from products formed by collision of the oxygen atoms with the walls. Loss of the reactive oxygen atoms was also minimized by enclosing the ESD chamber in fused silica. With a primary beam energy of 100 eV, the ESD cross section for oxygen atom desorption was found to be 7 × 10^-19cm² at 100 °C, and an ESD threshold was found to exist below 34 eV. Flash desorption of oxygen allowed estimation of the bulk diffusivity of oxygen through polycrystalline Ag. A value of 2.64 × 10^-6cm²s^-1 at 500 °C was obtained.     

Isothermal desorption of hydrogen from polycrystalline diamond films

The kinetics of isothermal H₂ desorption from polycrystalline diamond are studied in real time. The surface H coverage (θ_H) is measured by mass analyzing the recoiled H₊ ion signal during the desorption. We find that the H₂ desorption is 1st order in θ_H with an activation energy of 69 ± 6 kcal/mol and a prefactor of 10^{10.5 ± 0.9} s⁻¹. We suggest that formation of a C---C π-bond on the clean surface plays a key role in H₂ desorption from diamond, a view consistent with previous theoretical calculations of H₂ desorption from diamond.     

Theoretical analysis of desorption from surfaces

The general theory of the desorption of atoms from surfaces is discussed and a method given for the calculation of the surface residence time. Potential parameters are deduced for a number of experimental systems using a variety of adsorption sites to model the interactions. By using this technique valuable information about the surface-atom potential may be obtained.     

Electron-stimulated desorption of sodium atoms from germanium-coated tungsten

The yield of sodium atoms and energy distribution upon electron-stimulated desorption from sodium layers adsorbed on tungsten coated with a germanium thin film are measured under variations in the electron energy, the sodium coverage, and the surface temperature by the time-of-flight method with the use of a surface ionization detector. It is revealed that the electron-stimulated desorption of sodium atoms occurs via three channels, namely, a channel involving ionization of adsorbed sodium; the most efficient channel, which is produced by the germanium ionization; and a channel associated with the formation of tungsten excitons, which brings about desorption of NaGe molecules.     

Atomic imaging of oxygen desorption from tungsten trioxide

We report direct observations by high-resolution electron microscopy of oxygen desorption from tungsten trioxide. Clear evidence is found for layer epitaxial growth of metallic tungsten with (110)W|(100)WO₃ and [001]W and [001]WO₃ parallel to the electron beam, consistent with low-energy electron diffraction data on the low-temperature epitaxy of WO₃ on W. W[001] was always observed parallel to the electron beam independent of the surface normal. The results suggest that bulk and surface damage, and displacement processes, are similar.     

Hydrogen plasma treatment: desorption of atomic hydrogen from silicon surfaces studied by in-situ spectroscopic ellipsometry

The use of H₂ and Ar plasmas for the cleaning of crystalline Si surfaces has been investigated previously by in-situ spectroscopic ellipsometry, LEED and Auger spectroscopy. H₂ plasma treatment, followed by annealing at 700°C to desorb the adsorbed hydrogen, was found to produce high quality clean crystalline surfaces. Ellipsometry was shown to be sensitive not only to surface oxide layers and micro-roughness but also to hydrogen adsorbed on silicon surface. The desorption of hydrogen as a function of temperature from H₂ plasma treated Si (1 0 0) and (1 1 1) surfaces was investigated. The presence of different phases of adsorbed hydrogen was shown by spectroscopic ellipsometry, in agreement with UPS and EELS studies. The clean (plasma treated and annealed) Si (1 0 0) and Si (1 1 1) surfaces show differences in their apparent dielectric functions, which may be due to the nature of the surface reconstruction.     

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.     

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.