Evidence for electron emission stimulated desorption from negative electron affinity GaAs surfaces

While desorption from surfaces caused by impinging electrons (electron stimulated desorption) is a well-established effect, electrons to be emitted also may give rise to desorption from an emitting surface (electron-emission stimulated desorption). Evidence for this effect is derived from data on the degradation of electron emission from negative electron affinity GaAs surfaces. The time dependence of the degradation is calculated, and agreement with the observed linear time dependence is found. Using the experimental degradation ata, the desorption cross section for the electron-emission stimulated desorption is obtained as 2 × 10^?25 cm².     

A radiotracer study of slow electron interaction with co on polycrystalline Mo

The interaction of slow (50–300 eV) electrons with CO chemisorbed on a polycrystalline Mo ribbon was investigated. A radiotracer was employed to monitor C concentration on the surface and desorbed ions were collected. From the time variation of the ion current, total electron induced desorption cross sections of 3.7 × 10^?17 to 1.6 × 10^?16 cm² were obtained. The desorption cross section for C containing species was found to be less than 1.4 × 10^?19 cm². In addition, the sticking coefficient for CO on Mo was found to be 0.13–0.16. These results are discussed in terms of the states of chemisorbed CO and the conclusions of other investigations of this systems.     

Electron stimulated desorption of CO from

The electron impact behavior of CO adsorbed on 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 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.     

Studies of clean and oxidized tellurium surfaces

Clean and oxidized surfaces of tellurium films have been studied using electron-excited Auger electron spectroscopy, X-ray photoelectron spectroscopy, energy loss spectroscopy, and electron-stimulated desorption. The results for clean tellurium are in general agreement with previous studies, but the oxidation studies have provided new information. Reaction between oxygen gas and tellurium was found to be an activated process requiring tellurium temperatures in excess of 60°C to produce detectable oxide for 30 min exposures to ~800 Torr of oxygen. Increasing the temperature to 200°C produced a 10.6 A layer of TeO₂. This layer was rapidly removed by electron irradiation with a cross-section of ~3 × 10^?18 cm² for electron-stimulated desorption of oxygen by 2 keV electrons; however, there was evidence for reduced cross sections for thinner TeO₂ layers.     

Electron beam interactions with CO on W {100} studied by Auger electron spectroscopy

The interaction of 2500 eV electrons with carbon monoxide chemisorbed on tungsten {100} was investigated by rapid-scan Auger electron spectroscopy. When no α state was present the O and C signals from the β state of CO were invariant during electron bombardment, giving an upper limit estimate for the electron stimulated desorption cross section, Q_β of 2 × 10^?21 cm². With the crystal at room temperature and saturated with CO, however, electron-beam induced accumulation of carbon was observed and characterised, the rate of the process being independent of CO pressure at pressures above 2 × 10^?8 Torr. At 450 K the rate was found to be pressure dependent up to at least 6 × 10^?7 Torr. A model is proposed for the accumulation process, which is based on electron beam dissociation of α₂-CO to form adsorbed carbon and gaseous O and the creation of new sites for further α₂-CO adsorption; it is in quantitative agreement with the results and yields a cross section for ESD of α₂-CO (Q_α2 = 1.55×10^?18cm²) in close agreement with direct measurements.     

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.     

Adsorption of CO on the (110) plane of tungsten electron impact,thermal desorption,and work function measurements

The adsorption of CO on the (110) plane of tungsten has been studied using electron impact desorption, thermal desorption, and work function measurements in a single apparatus combining these various techniques. It is concluded that a single molecular adsorption state exists at 20–250 K (virgin-CO). At 300–400 K, 60% of the low temperature layer desorbs, the remainder converting principally to a beta-1 state, which has very small electron impact cross section; in addition to beta-1 an O⁺ yielding state, which we call beta-precursor is formed. The beta-1 state is stable to 900 K, where some desorption and conversion of the remaineder to a beta-2 state occurs. The O⁺ yielding state decays with increasing T and is gone at 800 K. Readsorption on beta-1 leads to two types of adsorption states called alpha and gamma, which seem to be site specific. Electron impact desorption yields mostly CO⁺ and CO for virgin, O⁺ for beta-precursor, and CO⁺ and CO for the readsorption states. There is no isotopic mixing in virgin or in readsorbed CO, nor does readsorbed CO exchange with beta-1 or beta precursor. There is complete isotopic mixing in beta desorption. In addition, massive EID creates another state, characterized by a large dipole moment, also yielding O⁺ in EID. This state can be converted to beta-1 by heating to 400 K. The total disappearance cross sections for the various states are virgin-CO5 × 10^?17cm²; γ-CO 1.6 × 10^?16cm²; α-CO 5 × 10^?17cm²; β-precursor 6 × 10^?18cm²and 1.2 × 10^?19cm²; EID induced state 8 × 10^?18cm². In addition, cross sections for ion production are determined and found to be several orders of magnitude less than total disappearance cross sections. These results, and Leed and coverage data obtained in parallel investigations are used to formulate models of the various adsorption states. It is concluded that virgin and readsorbed CO are molecular and beta-precursor and beta dissociated, although strong interactions between C and O remain. The electron impact desorption of physisorbed CO was investigated and found to yield C⁺, O⁺, and neutral CO, but very little CO⁺. These results suggest primary dissociation of CO by electron impact, and desorption of neutral physisorbed CO by the energetic fragments. Physisorbed CO⁺, although undoubtedly created, lies on the attractive part of its potential curve relative to the surface, and thus does not desorb as CO⁺.     

Isotope effects in electron impact desorption of CO and O2 adsorbed on the (110) plane of tungsten

Results on the isotope effect for total and ionic desorption cross sections in the electron impact desorption of various binding states of CO on the (110) plane of tungsten, and of oxygen on this plane are presented and discussed. It is shown that the observations allow a dissection of cross sections into excitation cross sections and escape probabilities, and that the latter can be used to estimate lifetimes of excited or ionic states. It is found that excitation cross sections for total desorption are of the order of 10^?16–10^?17 cm², but seem to be significantly smaller in some cases for excitation to ionic states, suggesting that different excitations are involved. In all cases examined here the isotope effect for total desorption is much smaller than for ion production. This can be explained by the fact that ion lifetimes are somewhat shorter than those of excited neutrals. Lifetimes are estimated, in the cases examined, to be of the order of 10^?14s.     

The adsorption of Xe and CO on Ag(111)

The adsorption of Xe and CO on Ag(111) in the range 66 to 123 K and 10^?7 to 10^?1 Pa has been studied by surface potential, low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and electron energy loss spectroscopic (EELS) measurements. Isotherms derived from both surface potential and AES measurements for submonolayer Xe adsorption reveal successive stages and a two-dimensional phase change. Isosteric heats were 18 ± 1 kJ mol^?1. Temkin isotherms were observed for CO, the heat falling linearly with coverage from an initial value of 27 ± 1.5 kj mol^?1. No ordered CO overlayer structure could be detected. EEL spectra of clean Ag(111) agree with previous studies. Additional loss peaks were recorded for Xe and CO overlayers, and the assignment of the substrate loss features is discussed in relation to the effects of adsorption.     

The origin of the background radioactive isotope <Superscript>127</Superscript>Xe in the sample of Xe enriched in <Superscript>124</Superscript>Xe

The results of investigation of ¹²⁷Xe radioactive isotope production in the xenon sample enriched in ¹²⁴Xe, ¹²⁶Xe, ¹²⁸Xe are presented. The isotope is supposed to be the source of the background events in the low-background experiment on search for 2K-capture of ¹²⁴Xe. In this work we consider two channels of ¹²⁷Xe production: the neutron knock-out from ¹²⁸Xe nucleus by cosmogenic muons and the neutron capture by ¹²⁶Xe nucleus. For the first channel the upper limit of the cross section of ¹²⁷Xe production was found to be σ ≤ 0.007 × 10^?24 cm² at 95% C.L. For the second channel the value obtained for the cross section was found to be equal to σ = (2.74 ± 0.4) × 10^?24 cm², which coincides well, within the statistical error, with reference value.     

ESD study of CO adsorbed on platinum

The technique of electron stimulated desorption is used to study the adsorption of CO on a recrystallized Pt ribbon. Desorption of both CO⁺ and O⁺ is observed with total cross sections of 2.8×10^?18 cm² and 4.3×10^?18 cm² respectively. Evidence is presented which indicates that CO⁺ desorbs from a single state while the O⁺ species may originate from multiple states.     

The adsorption and reaction of H2O on clean and oxygen covered Ag(110)

The adsorption and reaction of water on clean and oxygen covered Ag(110) surfaces has been studied with high resolution electron energy loss (EELS), temperature programmed desorption (TPD), and X-ray photoelectron (XPS) spectroscopy. Non-dissociative adsorption of water was observed on both surfaces at 100 K. The vibrational spectra of these adsorbates at 100 K compared favorably to infrared absorption spectra of ice Ih. Both surfaces exhibited a desorption state at 170 K representative of multilayer H₂O desorption. Desorption states due to hydrogen-bonded and non-hydrogen-bonded water molecules at 200 and 240 K, respectively, were observed from the surface predosed with oxygen. EEL spectra of the 240 K state showed features at 550 and 840 cm^?1 which were assigned to restricted rotations of the adsorbed molecule. The reaction of adsorbed H₂O with pre-adsorbed oxygen to produce adsorbed hydroxyl groups was observed by EELS in the temperature range 205 to 255 K. The adsorbed hydroxyl groups recombined at 320 K to yield both a TPD water peak at 320 K and adsorbed atomic oxygen. XPS results indicated that water reacted completely with adsorbed oxygen to form OH with no residual atomic oxygen. Solvation between hydrogen-bonded H₂O molecules and hydroxyl groups is proposed to account for the results of this work and earlier work showing complete isotopic exchange between H₂¹⁶O_(a) and ¹⁸O_(a).     

binding states of CO and H2 on clean and oxidized (111)Pt

The binding states and sticking coefficients of CO and H₂ on clean and oxide covered (111)Pt are examined using flash desorption mass spectrometry and Auger electron spectroscopy (AES). On the clean surface at 78 K there is one major binding state of CO with a desorption activation energy which decreases with coverage plus a second smaller state, while H₂ exhibits three binding states with peak temperatures of 140, 230 and 310 K and saturation density ratios of 0.5 : 1 : 1. Desorption kinetics of CO are consistent with a first order state with a normal pre-exponential factor of 10^{13 ± 1} sec^?1, while all three peaks of H₂ are broader than expected. Interpretations in terms of anomalous pre-exponential factors, coverage dependent desorption activation energies, and desorption orders are considered. On the oxidized surface saturation densities of both gases are nearly identical to those on the clean surface, but desorption temperatures are increased significantly and the initial sticking coefficient on the oxide decreases slightly for CO and increases slightly for H₂.     

Electron-stimulated processes in boron nitride-based ceramics

The low-energy electron radiation resistance of boron nitride-based BN + Si₃N₄ and BN + SiO₂ ceramics proposed as a construction material for ion space engines was studied. It was shown that a reduced silicon phase is formed on the BN + Si₃N₄ ceramics surface in the high-temperature region (～900 K), which results from material thermal destruction. The BN + SiO₂ ceramics exhibits high thermal stability and is not prone to destruction due to electron-stimulated oxygen desorption (the cross section of this process does not exceed 10^?20 cm²). It is preferable to use such ceramics as a construction material. Based on the results obtained, some models were proposed that explain variations in the erosion rate of ceramic units of ion engines under electron and ion irradiation.     

An AES study of electron-beam stimulated desorption of oxygen from a semiconductor surface

Electron impact desorption of oxygen from a Ge(111) surface obeyed a first order rate-equation with a desorption cross section of 6 × 10^-20cm².     

Identification of surface vibrations on clean and oxygen covered Pt(111) surfaces with high resolution electron energy loss spectroscopy (EELS)

Clean and oxygen covered {111} recrystallized Pt surfaces were studied by EELS after surface preparation at 150≤T≤165OK. The clean surface shows Stokes as well as anti-Stokes lines of surface phonons at ±195^?1. Adsorption of small amounts of (<10^?2 monolayers) of O₂ or H₂ leads to substrate-derived phonon losses at ±380cm^?1. Oxygen exposure at different pressures, times and temperatures leads to atomic and/or molecular adsorption as well as oxide-related features which have been identified by EELS.     

Vibrational characterization of carbon monoxide adsorption on sulfur modified Ni(100) surfaces

Carbon monoxide adsorption has been studied on a series of presulfided Ni(100) surfaces using vibrational spectroscopy. The sulfided Ni(100) surfaces were characterized using Auger electron spectroscopy and low energy electron diffraction, binding states were isolated by heating CO-dosed surfaces to prescribed temperatures, corresponding to the desorption temperatures of the CO. Adsorption of CO on Ni(100) with a p(2 × 2) array of sulfur lead to CO stretching frequencies of 1740 and 1930 cm^?1 corresponding to desorption temperatures of 370 and 290 K, respectively. Adsorption of CO into the c(2 × 2)S structure resulted in a CO stretching frequency of 2115 cm^?1 and a desorption peak near 140 K. The binding sites on the p(2 × 2)S structure were interpreted as metal four-fold hollows and bridging sites. The high frequency state was interpreted as weak bonding into the four-fold hollow with back donation into the π¹ orbital on CO restricted by stearic hindrance due to adsorbed sulfur. Both the thermal desorption and vibrational results indicated that local CO-sulfur interactions are dominant on the presulfided Ni(100) surface in the coverage range studied.     

Interaction of cesium and oxygen on W(110): I. Cesium adsorption on oxygenated and oxidized W(110)

Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10¹⁵ cm^?2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (～1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10¹⁵ oxygen atoms per cm². Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10¹⁵ cm^?2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.     

Characterization of the adsorption and decomposition of methanol on Ni(110)

The chemisorption, condensation, desorption, and decomposition of methanol, both CH₃OH and CH₃OD, on a clean Ni(110) surface have been characterized using high resolution electron energy loss spectroscopy, temperature programmed reaction spectroscopy, and low energy electron diffraction. The vibrational spectrum of the saturated chemisorbed layer, 7.4 × 10¹⁴ molecules cm^?2, is almost identical to the infrared spectrum of liquid or solid methanol. Condensation of multilayers of methanol is facile at 80 K. The only quasi-stable intermediate isolated during the decomposition is a methoxy species, CH₃O, which decomposes thermally to CO and H. The evolution of both CO and H₂ occurs in desorption limited processes.     

Search for long-lived superheavy elements in the reaction of136Xe with238U

A search with radiochemical methods for long-lived superheavy elements in 238U targets bombarded with intense beams of¹³⁶Xe ions produced negative results. A formation cross section of ≤1×10^?35 cm² is deduced at 95% confidence level for nuclides with half-lives between 1 and 200 d.