Electron-stimulated desorption of positive ions from hexagonal α-SiC

Electron-stimulated desorption (ESD) of positive ions (H⁺, O⁺, ⁺OH, F⁺ and Cl⁺) from the Si- and C-terminated surfaces of hexagonal α-SiC has been observed for electron energies in the 5–105 eV range. Comparison of these results with those for the ESD of the same ions from the surfaces of Si and condensed hydrocarbons leads to a model for the H⁺, ⁺OH, F⁺ and Cl⁺ threshold desorption process based on transitions from deep valence Si and C “s-like” levels to states in the conduction band, followed by Auger decay to form a localized multiple valence-hole configuration. O⁺ desorption, on the other hand, is initiated by O 2s ionization. Evidence is found for a strong dependence of the F⁺ threshold on the local chemical bonding. The results indicate that the thresholds for ESD of these ions from SiC are determined more by the electronic excitation of the substrate than by direct excitation of the adsorbate bond.     

Electron and photon stimulated desorption of positive ions from alkali halide surfaces

We have observed desorption of positive ions from alkali halides stimulated by low energy electron and photon bombardment. Our experiments include the first measurements of electron stimulated desorption (ESD) of Na⁺ from NaCl and the first measurements of photon stimulated desorption (PSD) of positive ions from NaCl and LiF. The energy dependence data indicate that the initial onset for Na⁺ ejection by ESD occurs at the excitation threshold of the Cl(3s) core level. Similarly for the PSD of positive ions from NaCl and LiF we can relate incident photon beam energy dependent ion yields with the production of substrate core holes. The data provide insight into the mode of initial energy transfer to the solid which leads to desorption. ESD and PSD ion yields were measured to be on the order of 10^?7 ions per incident electron or photon.     

Ion angular distributions in electron stimulated desorption: Oxygen and CO on W(111)

The ion angular distributions resulting from electron stimulated desorption (ESD) of oxygen and carbon monoxide chemisorbed on a tungsten (111) crystal have been determined. The O⁺ ions released during ESD of adsorbed oxygen exhibit three-fold symmetric angular distributions in orientational registry with the W(111) substrate. The CO⁺ and O⁺ ions released during ESD of a monolayer of CO are desorbed normal to the (111) surface. Models for both oxygen and CO adsorption are discussed. The data for CO are consistent with adsorption of CO in “standing up” carbonyl structures in the virgin and α-CO binding states.     

Desorption methods as probes of kinetics and bonding at surfaces

This paper is divided into two parts. Firstly, a review of desorption methods is presented, with emphasis on the use of temperature programmed desorption (TPD) and electron stimulated desorption (ESD) for understanding the bonding of adsorbed species to surfaces. Secondly, recent studies of the angular distribution of ESD ions from adsorbed layers on W(011) are discussed. The ESD of O⁺ ions from oxygen adsorbed on a stepped W(011) surface is shown to be sensitive to the presence of atom steps.     

多晶镍表面上氧的电子诱导脱附研究

长期暴露于大气的多晶镍样品,经700℃2小时真空退火及常温和500℃暴露氧20—60L后,发现其表面将产生两种电子诱导脱附(ESD)氧正离子。所对应的电子能量阈值分别为29eV和23eV。我们认为,两种氧正离子的产生机制为俄歇诱导脱附。29eV阈能者对应于镍表面化学吸附氧的2s能级电子向真空中电离;而23eV阈能者则对应于表面层中未与镍化合的自由氧的2s电子向体内费密能级跃迁。 关键词：     

Kinetic theory description of electron stimulated desorption

A first principles kinetic theory is developed, applicable to angular dependent emission in Electron Stimulated Desorption. Expressions for the ionic and neutral atom ESD cross sections are formulated and applied in a model calculation of O⁺ emission from W(111). Strong focussing of the outgoing ions was found, with the use of a model ion-solid potential in which the substrate was free of excitation. Off-axis spot groups were simulated. The peak ion energies obtained with this model are, however, small compared to experimental energies for the high coverage case. The need to introduce substrate excitations to describe this case is discussed.     

The effect of potassium on the molecular orientation of CO chemisorbed on Ni(111): An esdiad study

Using electron stimulated desorption (ESD) and electron stimulated desorption ion angular distribution (ESDIAD) techniques, we have determined that coadsorbed potassium systematically quenches the O⁺ ion yield from CO on the Ni(111) surface for 1000 eV electron excitation energies. The quenching appears to be a short range K-CO interaction; 3 or 4 CO molecules are affected for each K atom adsorbed on the surface. The quenching effect of K on CO indicates that a significant electronic perturbation of CO is caused by its local interaction with K. This effect prevents ESDIAD observation of the K-quenched CO species. In addition, the CO molecules that are not quenched at a potassium coverage of 0.02 K/Ni exhibit a normally oriented C-O bond similar to that found for CO adsorbed on a K-free Ni(111) surface.     

Study of lithium adsorption on the TiO2 surface by electron-stimulated desorption

This paper reports on a study of electron-stimulated desorption (ESD) of O⁺ and Li⁺ ions from titanium dioxide as a function of the preheating temperature T and of the concentration of lithium adsorbed at 300 K, which was carried out with a static magnetic mass spectrometer combined with a retarding-field energy analyzer. For T>1500 K, the TiO₂ surface undergoes irreversible rearrangement. At temperatures from 300 to 900 K and at lithium coverages Θ<1, the ESD cross sections of the O⁺ and Li⁺ ions vary in a reversible manner with temperature, while for lithium coverages Θ>1, the changes in the Li⁺ and O⁺ ESD cross sections become irreversible. For θ<1, the appearance threshold of the Li⁺ and O⁺ ions is 25 eV, whereas for θ>1, the ESD threshold of Li⁺ ions shifts to 37 eV.     

Adsorption-desorption studies using ESD of CCl2F2, C2H2F2 and C2F6 on tungsten

The behavior of the desorbing F⁺ ion current from electron bombarded CCl₂F₂, C₂H₂F₂ and C₂F₆ adsorbed on tungsten has been used to investigate the processes of adsorption and desorption of these gases. For tungsten near room temperature, measurements of the F⁺ ion current as a function of electron bombardment time indicated very similar or even identical F⁺-yielding adsorbed species resulting from adsorption of either CCl₂F₂ or C₂H₂F₂ and widely different species from C₂F₆. Cl⁺ ions were also observed to desorb from CCl₂F₂ ad-layers. The behavior of the Cl⁺ ion current with time during electron bombardment indicated electronic conversion between adsorbed binding modes. Complementary investigations on the interaction of CCl₂F₂, C₂H₂F₂ and C₂F₆ with tungsten were carried out by thermal desorption experiments in which the F⁺ ion signal was used to observe the coverage decrease of the F⁺-yielding species. The experiments were performed at tungsten temperatures in the 1200–1600 K range. It was concluded that the F⁺-yielding adsorbed species from CCl₂F₂ and C₂H₂F₂ were strongly bound to the tungsten surface. The F⁺-yielding species from C₂F₆ were found to be weakly bound. From a comparison of the ESD and thermal desorption results, the possibility of dissociative adsorption as well as the nature of the adsorbed species is discussed.     

Electron stimulated desorption of carbon monoxide from a (111) niobium surface

Electron stimulated desorption of CO from the (111) face of a Nb single crystal produced both CO⁺ and O⁺ ions after adsorption at 150°K on a clean surface. When the surface was heated to above 250 °K only O⁺ ions were observed, and this current disappeared as the temperature was increased to 700 °K. Readsorption (at 150 °K) was inhibited following the 700 °K heating. These data indicate the formation on heating of a tightly bound surface phase with very low ionic desorption cross section. Threshold energies for CO⁺ and O⁺ ion production were 10.0 ± 0.5 eV and 19.0 ± 0.5 eV, respectively. The cross section for electron stimulated depopulation of the O⁺ producing phase was (4 ± 1) × 10^?18 cm² for 100 eV electrons.     

Electron stimulated desorption of O+ ions: Energy dependence of the desorption efficiency from adsorbed oxygen on metal surfaces and its bearing on the mechanism

Measurements of the structure in the curve of desorption yield versus electron energy for ESD of O⁺ from β₁-oxygen on polycrystalline W and W(100), and from oxidized V are reported. They show definite structure around the core ionization thresholds of the metal atoms. This may be taken as evidence for the existence of Auger mechanisms of ESD, e.g. that proposed recently by Knotek and Feibelman for oxidic oxygen.     

Electron-stimulated desorption of rare-earth metal atoms

The yield of europium and samarium atoms in electron-stimulated desorption from layers of rare-earth metals (REMs) adsorbed on the surface of oxidized tungsten has been measured as a function of the incident electron energy, surface coverage by REMs, degree of tungsten oxidation, and substrate temperature. The measurements were performed using the time-of-flight method with a surface-ionization-based detector within the substrate temperature interval 140–600 K. The yield studied as a function of electron energy has a resonance character. Overlapping resonance peaks of Sm atoms are observed at electron energies of 34 and 46 eV, and those of Eu atoms, at 36 and 41 eV. These energies correlate well with the REM 5p and 5s core-level excitation energies. The REM yield is a complex function of the REM coverage and substrate temperature. The peaks due to REM atoms are seen at low REM coverages only, and their intensity usually passes through a maximum with increasing coverage and substrate temperature. The concentration dependence of the REM atom yield is affected by the deposition of slow Ba⁺ ions, but only if they are deposited after the REM adsorption. At higher REM coverages, additional peaks are observed at electron energies of 42, 54, and 84 eV, which originate from excitation of the 5p and 5s tungsten levels and result from desorption of SmO and EuO molecules. The temperature dependence of the intensity of these peaks is explained to be due to the order-disorder phase transition. The desorption of REM atoms is the result of their reversed motion through the adsorbed REM layer, and the SmO and EuO molecules desorb due to the formation of an antibonding state between the REM oxide molecules and the tungsten ions.     

Chemisorption of CO on tungsten (100): Combined flash desorption and electron stimulated desorption study. II

The chemisorption of CO on W(100) at ~ 100K has been studied using a combination of flash desorption and electron stimulated desorption (ESD) techniques. This is an extension of a similar study made for CO adsorption on W(100) at temperatures in the range 200–300K. As in the 200–300 K CO layer, both α₁-CO and α₂-CO are formed in addition to more strongly bound CO species upon adsorption at ~ 100K; the α-CO states yield CO⁺ and O⁺ respectively upon ESD. At low CO coverages, the α₁ and α₂-CO states are postulated to convert to β-CO or other strongly bound CO species upon heating. At higher CO coverages, α₁-CO converts to α₂-CO upon thermal desorption or electron stimulated desorption. There is evidence for the presence of other weakly-bound states in the low temperature CO layer having low surface concentration at saturation. The ESD behavior of the CO layer coadsorbed with hydrogen on W(100) is reported, and it is found that H(ads) suppresses either the concentration or the ionic cross section for α₁ and α₂-CO states.     

Electron stimulated desorption of H3O from 316L stainless steel

Surface ions generated by electron stimulated desorption from mass spectrometer ion source grids are frequently observed, but often misidentified. For example, in the case of mass 19, the source is often assumed to be surface fluorine, but since the metal oxide on grid surfaces has been shown to form water and hydroxides, a more compelling case can be made for the formation of hydronium. Further, fluorine is strongly electronegative, so it is rarely generated as a positive ion. A commonly used metal for ion source grids is 316L stainless steel. Thermal vacuum processing by bakeout or radiation heating from the filament typically alters the surface composition to predominantly Cr₂O₃. X-ray photoelectron spectral shoulders on the O 1s and Cr 2p_3/2 peaks can be attributed to adsorbed water and hydroxides, the intensity of which can be substantially increased by hydrogen dosing. On the other hand, the sub-peak intensities are substantially reduced by heating and/or by electron bombardment. Electron bombardment diode measurements show an initial work function increase corresponding to predominant hydrogen desorption (H₂) and a subsequent work function decrease corresponding to predominant oxygen desorption (CO). The fraction of hydroxide concentration on the surface was determined from X-ray photoelectron spectroscopy and from the deconvolution of temperature desorption spectra. Electron stimulated desorption yields from the surface show unambiguous H₃O⁺ peaks that can be significantly increased by hydrogen dosing. Time of flight secondary ion mass spectrometry sputter yields show small signals of H₃O⁺, as well as its constituents (H⁺, O⁺ and OH⁺) and a small amount of fluorine as F⁻, but no F⁺ or F⁺ complexes (HF⁺, etc.). An electron stimulated desorption cross-section of σ⁺ ∼ 1.4 × 10⁻²⁰ cm² was determined for H₃O⁺ from 316L stainless steel for hydrogen residing in surface chromium hydroxide.     

ESD on metal oxides and oxygen adsorption layers: Evidence for different mechanisms in electron stimulated desorption

Electron stimulated desorption (ESD) of several metal oxides (W, Mo, Ta, Fe, Ni, and Cu oxide) and oxygen adsorption layers on Ni and Fe has been investigated. ESD of O⁺ from oxides can be easily understood in terms of the Auger decay model for ESD proposed recently by Knotek and Feibelman (KF). The measured O⁺ desorption from oxygen adsorption layers on Fe and Ni however, can hardly be explained in the frame of the KF model. There is strong evidence that at least two different ESD mechanisms are operative for ionically and covalently bound oxygen, respectively.     

X-ray initiated molecular photochemistry of Cl-containing adsorbates on a Si(1 0 0) surface using synchrotron radiation

X-ray initiated molecular photochemistry for SiCl₄ and CCl₄ adsorbed on Si(1 0 0) at ∼90 K following Cl 2p core-level excitation is investigated by photon stimulated ion desorption and ion kinetic energy distribution measurements. The Cl excitation of solid SiCl₄ induces the significant enhancement (∼900%) of the Cl⁺ yield, while the Cl excitation of condensed CCl₄ leads to a moderate enhancement (∼500%) of the Cl⁺ yield. The enhancement of Cl⁺ yield at the specific core-excited states is strongly correlated to the ion escaped energy. Upon X-ray exposure for CCl₄ adsorbed on Si(1 0 0) (20-L exposure), the Cl⁺ yields at resonances decrease and new structures at higher photon energies are observed. Cl⁺ yields at these new resonances are significantly enhanced compared to those at other resonances. These changes are the results of desorption and surface reaction of the CCl₄-Si surface complex due to X-ray irradiation. We have demonstrated that state-specific enhancement of ion desorption can be successfully applied to characterize the reaction dynamics of adsorbates adsorbed on surfaces by X-ray irradiation.     

Adsorption and coadsorption of carbon monoxide and hydrogen on Pd(111)

The adsorption and coadsorption of CO and H₂ have been studied by means of thermal desorption (TD) and electron stimulated desorption (ESD) at temperatures ranging from 250 to 400 K. Three CO TD states, labelled as β₂, β₁, and β₀ were detected after adsorption at 250 K. The population of β₂ and β₁ states which are the only ones observed upon adsorption at temperatures higher than 300 K was found to depend on adsorption temperature. The correlation between the binding states in the TD spectra and the ESD O⁺ and CO⁺ ions observed was discussed. Hydrogen is dissociatively adsorbed on Pd(111) and no ESD H⁺ signal was recorded following H₂ adsorption on a clean Pd surface. The presence of CO was found to cause an appearance of a H⁺ ESD signal, a decrease of hydrogen surface population and an arisement of a broad H₂ TD peak at about 450 K. An apparent influence of hydrogen on CO adsorption was detected at high hydrogen precoverages alone, leading to a decrease in the CO sticking coefficient and the relative population of CO β₂ state. The coadsorption results were interpreted assuming mutual interaction between CO and H at low and medium CO coverages, the “cooperative” species being responsible for the H⁺ ESD signal. Besides, the presence of CO was proved to favour hydrogen penetration into the bulk even at high CO coverage when H atoms were completely displaced from the surface.     

Coadsorption of oxygen and carbon monoxide on tungsten: Desorption spectra,electron stimulated desorption and field emission microscopy

CO/W desorption spectra are characterized by an α state and multiple β states; using electron stimulated desorption (ESD) the α state was shown to comprise two sub-states, α₁ and α₂. In this paper the consecutive interactions of O₂ and CO on W are investigated using ESD, flash desorption and field emission microscopy (FEM).Desorption spectra show that the α-CO state is displaced by O₂, in two stages. The ESD probe provides an identification of the first stage with the removal of the α₁-CO state, and energy analysis of ESD ions reveals a large energy shift (～ ? 1.5 eV) during O₂ coadsorption which can be attributed to an incresae in the α₁-CO WC bond length of ～ 0.15 Å. During this O₂-induced displacement, the two β peaks converge into a single peak at the β₁ position; this is ascribed to adatom interactions in the mixed O and C adlayer. Isotope exchange experiments with ²⁸CO and ³⁶O₂ reveal (i) no exchange in the α-CO states, and (ii) complete exchange in the β-CO states, which is consistent with dissociative adsorption in the latter. The amount of coadsorbed O₂ is estimated from these results, and from FEM data: a full monolayer of O adatoms can be coadsorbed on CO-saturated W, but CO pre-adsorption inhibits the formation of W oxides. The β₁-O₂ (ESD active) state also forms on the CO-covered surface: this state is identical in population, ESD cross section and ion energy distribution to β₁-O₂ on clean W, and retains its identity in the mixed layer (it does not undergo isotopic exchange). CO₂ desorption spectra from the mixed layer were also characterised, complete isotopic scrambling being observed.Pre-exposure of tungsten to O₂ inhibits CO adsorption: a monolayer of O₂ is sufficient to prevent CO adsorption, and at low O₂ coverages, every O₂ molecule preadsorbed prevents one CO molecule from adsorbing. Isotopic exchange is again complete in the β states, and a lateral interaction model for desorption kinetics, based on dissociative adsorption in the β-CO state, quantitatively describes the CO desorption spectra.     

Electron stimulated desorption of oxygen from nickel

Electron Stimulated Desorption (ESD) of O⁺ ions from oxygen-covered Ni(100) has been investigated at 390 K and 500 eV primary energy. The ion energy distribution is found to peak at 7.5 eV and to extend to 11 eV, over our whole exposure range (0–1000 L). The 7.5 eV peak height as a function of exposure shows that desorption takes place both in the chemisorption and the oxidation region. Emission of O⁺ occurs preferentially along the surface normal, with a base width of ≈ 60°. No azimuthal structure is observed. Additional electron energy dependent measurements clearly show a threshold near the oxygen 2s level.     

Photon stimulated ion desorption of H+ ions from GaAs (110)/H2O

The photon stimulated ion desorption yield of H⁺ ions from a H₂O dosed GaAs (110) surface has been measured in the range 18eV ? hυ ? 30eV. There is a direct correspondence between the PSID H⁺ yield, reflectance, and the secondary electron yield spectrum of GaAs (110). The data provides evidence that the initial stages of PSID involve core level (Ga(3d), O(2s)) → conduction band excitation followed by Auger decay.