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.     

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 CO from Pd1/W(110)

The electron impact behavior of CO adsorbed on Pd₁/W(110) was investigated. The desorption products observed were neutral CO, CO⁺, and O⁺. After massive electron impact residual carbon, C/W = 0.15, but not oxygen was also found, suggesting that energetic neutral O, not detected in a mass analyzer must also have been formed. Formation of β-CO, i.e., dissociated CO with C and O on the surface was not seen. The total disappearance cross section varies only slightly with coverage, ranging from 9 × 10 ⁻¹⁸ cm² at low to 5 × 10⁻¹⁸ cm² at saturation (CO/W = 0.75). The cross section for CO⁺ formation varies from 4 × 10⁻²² cm² at satura to 2 × 10⁻²¹ cm² at low coverage. That for O⁺ formation is 1.4 × 10⁻²² cm² at saturation and 2 × 10⁻²¹ cm² Threshold energies are similar to those found previously [J.C. Lin and R. Gomer, Surf. Sci. 218 (1989) 406] for CO/W(110) and CO/Cu₁/W(110) which suggests similar mechanisms for product formation, with the exception of β-CO on clean W(110). It is argued that the absence or presence of β-CO in ESD hinges on its formation or absence in thermal desorption, since electron impact is likely to present the surface with vibrationally and rotationally activated CO in all cases; β-CO formation only occurs on surfaces which can dissociate such CO. It was also found that ESD of CO led to a work function increase of the remaining Pd₁/W(110) surface of 500 meV, which could be annealed out only at 900 K. This is attributed to surface roughness, caused by recoil momentum of energetic desorbing entities.     

Electron stimulated desorption of negative ions from gases adsorbed on molybdenum

The purpose of this letter is to present the results of an initial qualitative study of negative ions produced by electron stimulated desorption (ESD) of adsorbed gas layers.     

Electron stimulated desorption of condensed,branched alkanes

Electron stimulated desorption of H⁺, CH₃⁺, H₂⁺ and D⁺ has been measured as a function of excitation energy for condensed phase neopentane, tetramethylsilane, 2-methylpropane-1-d₁, and 2-methylpropane-2-d₁. The data show that the processes which result in both H⁺ and CH₃⁺ production are initiated by the same excitation. The excitations which produce H⁺, CH₃⁺ and H₂⁺ appear to be largely localized on the methyl groups. The results indicate that multi-electron (hole) final states are responsible for H⁺ and CH₃⁺ desorption.     

Electron stimulated desorption from CO chemisorbed on Pt(111): A dynamical study of positive ion and metastable CO emission

The desorption by electron impact of significant quantities of electronically excited neutral species, CO¹, from CO adsorbed on Pt(111) has been discovered. Comparison of the yield of this species as a function of electron energy, coverage, and temperature, with the yield of O⁺ and C0⁺ has led to the conclusion that the CO¹ species is mainly produced by direct excitation. Studies of the angular distribution of the three ESD-derived species have been made as a function of temperature, and high amplitude bending vibrational modes are observed. Isotope effects in the three desorption channels have been measured.     

Electron stimulated desorption of neutral species from (100) KCl surfaces

Composition changes of a (100) KCl surface bombarded by 1 keV electrons have been studied by Auger electron spectroscopy. Intensity ratios of characteristic alkali and halogen Auger lines were monitored as a function of target temperature and beam current density. In addition, for the first time angle-resolved energy distributions of electron desorbed K and Cl atoms were measured using mass-analyzed time of flight techniques. For temperatures higher than about 100°C, a near-stoichiometric surface composition was obtained and a significant non-thermal component was observed in the kinetic energy distributions of Cl atoms emitted normal to the (100) surface. These results can be interpreted in terms of new concepts regarding the excitonic mechanism of electron stimulated desorption (ESD).     

Electron stimulated desorption: Principles and recent developments

A survey is given of the basic experimental findings in electron stimulated desorption and of the accepted theoretical model for their explanation. As an example for the test of the theory, the isotope effect in ESD of hydrogen on tungsten surfaces is described, which shows excellent agreement with the predictions of the model. The use of ESD as a surface probe is exemplified with recent work on hydrogen adsorption states on tungsten; the need for generalization of the one-dimensional model to three dimensions becomes clear in this case. Recent developments in theory (a quantum-mechanical treatment of the process) and experiment (promising measurements of the angular distribution of ionic ESD) are shortly discussed.     

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.     

Associative electron stimulated desorption of neutral CO molecules

The Electron Stimulated Desorption (ESD) of neutral CO molecules was studied on polycrystalline Ni surface. Combined ESD and temperature programmed desorption (TPD) measurements were used to observe the variations of neutral ESD yield in dependence on CO coverage, containing direct information about the cross section for ESD of neutral CO molecules in relation to various CO adsorption states. The results show evidence of ESD of neutral CO molecules from dissociative adsorption states. The cross section for this associative ESD process is relatively high in comparison with the CO ESD cross section for molecular CO adsorption. This work was supported by the Czech Grant Agency under the Grant No. 202/99/1714 and by the Czech Ministry of Education, Youth and Sports under the Project No. VS97116.     

Electron stimulated desorption of O2 from gadolinia-doped ceria surfaces

The interactions of gas phase oxygen with gadolinia-doped ceria (GDC) surfaces are investigated by electron stimulated desorption (ESD). The primary desorbed cationic species related to molecular oxygen adsorption is O₂⁺. The threshold energy for ESD of O₂⁺ is 13–14 eV, indicating electron impact ionization of molecular oxygen bound at oxygen vacancies. Dependence of O₂⁺ velocities upon incident electron energy and substrate temperature reveals the dominant influence of the effective charge of the adsorption complex. The O₂⁺ velocity distribution is bimodal, and the onset of the faster components at room temperature is related to the balance between fluxes of incident electrons and secondary electron emission, causing effective hole production and neutralization of trapped electrons at surface states.     

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 induced by the scanning tunneling microscope

The use of the scanning tunneling microscope (STM) as an excitation source and a probe of electron stimulated desorption on the atomic scale is reviewed. The case of H desorption from H-terminated Si(001) is examined in detail. Experimental results on excitation thresholds, desorption cross-sections, isotope effects and site-selectivities are presented. Evidence for mechanisms involving direct electronic and hot ground-state desorption, as well as a novel multiple-vibrational excitation mechanism is discussed. Using the latter mechanism, the ultimate resolution limit of selective single atom desorption is achieved. New results on desorption from Si dihydride, including a proposed mechanism for the STM-induced H/Si(001)-3 × 1 to 2 × 1 conversion, are presented. Possible applications of STM-induced desorption in nanofabrication are considered.     

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.     

Electron stimulated desorption of CO molecules from chemisorbed CO layers on Ru(001): a comparison of the distributions of translational and internal energies

The translational and internal energies of CO desorbed by electron impact from well-ordered × layers on Ru(001) are investigated with time-of-flight techniques and resonance enhanced multiphoton ionization (REMPI) spectroscopy. We discriminate four desorption channels with different translational and internal energy distributions. Translational and vibrational energies are positively, crrelated; rotational and vibrational–translational energies are negatively correlated. The microscopic origins of these correlations are discussed.     

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.     

Electronically stimulated desorption

In the valence region, desorption of neutrals predominates; nevertheless most research has been on ions up to recently. New methods have made the investigation of neutrals easier, and angle as well as energy distributions have been reported; on this basis the salient mechanisms can be deduced both for chemisorbed and physisorbed layers. Desorption of ions and, in particular, of fragment ions, is strongly enhanced following higher excitations. These can most easily be reached via Auger decay of adsorbate core holes. Considerable detail of understanding is obtainable through polarization-dependent measurements with synchrotron radiation. The importance of many-body interactions at surfaces (increased localization by hole-hole interaction; increased delocalization by screening) becomes obvious. The field of stimulated desorption has profitted strongly from the improved understanding derived from the mature state of surface spectroscopies.     

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.