Results of a joint auger/esca round robin sponsored by astm committee E-42 on surface analysis. Part II. Auger results

We report the results of a round robin involving kinetic-energy (KE) and relative-intensity measurements on high-purity samples of copper and gold by Auger-electron spectroscopy. These results were obtained using 28 different instruments or analyzers manufactured by four companies. We found that the spread in reported KE values ranged from 7 eV at a KE of 60 eV to 32 eV at a KE of ～2025 eV. The total spread in reported intensity ratios ranged from a factor of ～38 for the ～6O eV and ～92O eV peaks of Cu to a factor of $\text{~}\text{?}$120 for the ～70 eV and ～2025 eV peaks of Au. We have analyzed the observed trends in some detail. The systematic error of kinetic-energy measurements increases with kinetic energy for many instruments. Even though all instruments were adjusted with the use of 2 keV elastically scattered electrons, the spread in the reported positions of the ～2025 eV Au peak indicates that the instruments were not adequately calibrated. Examples of erratic response were found in the measurements of relative intensities; it was believed, though not proved, that the more extreme values of intensity ratios were associated with instrumental malfunctions or operator mistakes. As in the similar ESCA round robin (Part I), the spread in reported Auger kinetic energies and relative intensities demonstrates clearly the need for standards (e.g., calibration methods, operating procedures, and data analysis) to ensure that data of known accuracy can be obtained routinely. Until suitable standards are available, interested individuals may find it useful to compare measurements using their own Auger or ESCA instruments with the group results and the trends found in the round-robin results.We have conducted an extensive round robin consisting of AES measurements on high-purity samples of Cu and Au. Participants were asked to measure the kinetic energies and relative intensifies of designated Auger peaks under specified conditions. This round robin was conducted contemporaneously with a similar ESCA round robin, the results of which have already been published [1].The AES round robin had three principal objectives. First, it was intended to assess the overall accuracy of KE and relative-intensity measurements in a relatively straightforward AES measurement. An earlier round robin [2] with catalyst samples demonstrated substantial spreads of reported data, and it was believed that comparisons of data obtained for cleaned metallic samples should give a more accurate picture of the current state-of-the-art. With a larger number of participants in the present round robin than in the catalyst round robin, we in fact find a comparable spread in the raw data. The spread in the reported KE measurements is a function of kinetic energy, and ranges from 7 eV at a KE of 60 eV to 32 eV at a KE of 2 keV. The imprecision of the KE measurements is typically ～1–3 eV. The total spread in the reported intensity ratios ranges from a factor of ～38 for Cu (at an incident energy of 3 keV) to a factor of ～120 for Au (at the same incident energy). The imprecision of the intensity ratios is typically less than 10%.Second, it was desired to determine the variation of AES intensities as a Cu sample was displaced along the analyzer axis with respect to its optimum position. Many of the participants found maxima in the intensities of the Cu ～60 eV and ～920 eV peaks at or very close to that sample position found to be optimum for the 2 keV elastic peak. Other participants found intensity maxima at sample positions up to ～4 mm away from the 2 keV elastic-peak position; these participants found that when the sample was at the optimum position for the 2 keV elastic peak, the intensities of the ～60 eV and ～920 eV Cu peaks could be as low as 50% of the corresponding maximum peak intensities that were found when the sample was displaced.Third, it was intended to measure the Auger KE for the “adventitious” carbon that forms on initially clean samples in the ambient vacuum of each instrument. Few participants made this measurement, although it was observed that the spread of reported energies for the carbon Auger peak was comparable to that found for the low-energy (60–7OeV) peaks in Cu and Au.We have examined the KE and relative-intensity data in some detail. We found it useful to compute deviations of individual KE measurements from the median values of the reported measurements for the selected Auger peaks. These deviations were plotted as a function of kinetic energy and lines were drawn connecting data points obtained using the same instrument. Such plots can be regarded as “error functions” or “calibration curves” based on the use of the group median values as reference data. These curves indicate that for many instruments the error of KE measurements increases approximately linearly with KE (unlike the behavior found in similar plots for binding-energy measurements in the ESCA round robin, in which the error was often nearly constant or slowly varying with binding energy). The large spread (32 eV) in the reported positions of the Au M₅N_6,7N_6,7 peak at a KE of ～2024 eV was considered particularly significant, since most instruments were adjusted and aligned using elastically scattered electrons at an energy of 2 keV. This observation clearly indicates that the working KE scales of the instruments were not adequately calibrated using the elastic-peak method; this problem is believed to be due to the insufficient accuracy of the 2 kV power supplies or the voltmeters used to display the KE scales rather than to any intrinsic deficiencies in the use of the elastic-peak method. There were several examples, however, in which plots of the intensities of the ～60 eV and ～920 eV Cu peaks as a function of sample position had maxima for both peaks at a position different from that found optimum for the 2 keV elastic peak. These observations indicate that sample alignment by the elastic-peak method was not done with sufficient accuracy in some laboratories. Finally, while the imprecision in the locations of the elastic peak and of Auger peaks in the round robin was typically 1–3 eV, the overall inaccuracy of the KE measurement was usually substantially larger.Most participants found that ratios of peak heights for the low-energy and high-energy transitions in Cu and Au decreased slowly as the incident electron energy was increased from 3 keV to 8 keV. Some participants, however, obtained qualitatively different dependences on incident energy; these results were attributed to mistakes, instrument malfunctions, or to inadequate alignment. Our experience in the ESCA round robin indicated that operator mistakes or instrumental problems were responsible for most of the outliers in comparisons of measured intensity ratios. We suspect (although we have not proved) that the more extreme values of peak-height ratios in the AES round robin have a similar origin. The AES intensity data were analyzed to search for mechanisms that could account for the large range of reported intensity ratios. We considered several possible origins for the more extreme data values. First, we examined the reported peak-height ratios for Cu and Au, to search for possible variations of the instrumental transmission functions from their “ideal” values. Second, we considered whether relatively large amounts of residual surface carbon could account for the observed intensity ratios. Third, we tested whether the instruments which exhibited “non-ideal” behavior (probably because of significant stray magnetic fields or inadequate sample alignment) when the samples were translated parallel to the analyzer axis were also the ones which gave the more extreme peak-height ratios. Fourth, we investigated whether probable variations in the amplitude of the modulation voltage applied to the analyzers would modify significantly the ratios of the observed intensities. Fifth, we considered the effects of the differing energy resolutions of the analyzers in the round robin. Finally, we considered effects due to variations in surface roughness caused by the different ion-sputtering conditions used for initial cleaning of the samples. None of these factors alone could account for the more extreme variations of the peak-height ratios, although it is possible that some of these factors could affect certain specific instruments while a different combination of the factors could be important in other cases. Although we were unable to demonstrate conclusively the nature of instrumental artifacts or possible operator mistakes in the various intensity measurements, we believe that the spread in the reported intensity ratios is associated with specific measurement problems in particular individual laboratories. A variety of factors have been identified here to account for the more modest but nevertheless distressing range of intensity ratios (a factor of ～2 for Cu and of ～5 for Au) for the majority of the participants.The spreads in the energies and relative intensities of Auger and photoelectron peaks in this AES and the previous ESCA round robin indicate clearly that improved calibration and operating procedures are required for both Auger and ESCA measurements. Published data, for example, are of little value unless credible statements of accuracy can be associated with the numerical results. We hope that the standards needed for improved measurements can be developed by the ASTM Committee E-42 on Surface Analysis together with other interested parties.     

Room temperature adsorption and growth of Ga and In on cleaved Si(111)

Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS) measurements have been performed on a set of ultrahigh vacuum cleaved Si(111) surfaces with different bulk dopings as a function of Ga or In coverage θ. The metal layers are obtained by evaporation on the unheated substrate and θ varies from zero to several monolayers (ML). First, the 2×1 reconstruction of the clean substrate is replaced by a $\text{3}\text{×}\text{3}$ R30° structure at $\text{1}\text{3}$ ML, meanwhile the dangling bond peak at 0.6 eV below the valence band edge E_vs is replaced by a peak at 0.1 eV for Ga or 0.3 eV for In, below E_vs. At the same time, the ionization energy decreases by 0.4 eV (Ga) or 0.6 eV (In), while the Fermi level pinning position gets closer to the valence band edge by about 0.1eV. Upon increasing θ, new LEED structures develop and the electronic properties keep on changing slightly before metallic islands start to grow beyond θ ～1 ML.     

Design of an Auger Electron Mössbauer Spectrometer (AEMS) using a modified cylindrical mirror analyzer

We have designed and built an Auger Electron Mössbauer Spectrometer (AEMS) for the detection of resonant ⁵⁷Fe Auger electrons using a modified commercial cylindrical mirror analyzer (CMA). The CMA final aperture was modified intentionally in order to increase electron transmission at the expense of reducing its energy resolution, from an original value of 0.5 % to a value of 11 % after the modification. The Channeltron detector electronics and the pre-amplifier were also modified in order to increase the counting efficiency. The electron energy analyzer is selective in energy in the 30 eV–3000 eV range, so the spectrometer can be used to detect MNN (45 eV) and LMM (600–700 eV) Fe Auger signals, what gives it a high surface sensitivity for Fe containing samples. We have used it to acquire the Fe LMM Auger signals generated from the de-excitation process after γ-Ray resonant nuclear absorption. The spectrometer can be used to study samples non-enriched in ⁵⁷Fe, with acquisition times from 5 to 7 days, what is a big advantage. From electron trajectory Monte Carlo simulations in metallic iron, the mean-escape-depth of the detected Auger signals has been estimated in approximately 1 nm. Fe K conversion electrons and KLL Auger electrons with mean escape depths of 129 nm and 78 nm respectively also contribute to the detected signal although in a lesser proportion.     

Quantitative AES of binary alloys: A comparison between handbook and pseudo-first principles corrections

Quantitative Auger electron spectroscopy of a number of binary alloys was studied. A pseudo-first principles correction scheme was applied to series of chromium/iron, chromium/nickel, and copper/gold alloys. Peak-to-peak height ratios of differentiated spectra were corrected for elemental differences in ionization cross section, Auger transition probability, atomic density, electron escape depth, backscattering factor, and sputter yield. These corrections changed the original ratio by factors ranging from 1.12 (for the chromium/iron series with a 10 kV primary electron beam) to 4.63 (for the copper/gold series with a 5 kV primary beam). The calculated surface concentrations were compared to wet chemistry bulk concentrations and in all cases, this first principles correction was an improvement over using uncorrected peak height ratios for concentration determination. For a $\text{50}\text{50}$ at% alloy in each series, the corrected data error for the chromium/iron alloy was 15%, for the chromium/nickel alloy was 2%, and for the copper/gold alloy was 36%. In all series except the chromium/iron system, concentrations obtained using this method had a smaller error than concentrations obtained using sensitivity factors from the Handbook of Auger Electron Spectroscopy.     

Low energy electron spectroscopy of Pt (100) and Pt (100)/CO

Fine structure in the n_vi, _VIIVV spectrum of clean Pt (100) has been observed, and interpreted as “band like” in origin rather than quasi-atomic. Differences in the dependence of the Auger yield on primary beam energy are observed between the N_VI, _VIIVV and O_IIIVV peaks, and are associated with anomalies in the dependence of the inner shell ionization crossection of the 4f level. Low energy electron loss spectra on the clean surface have been investigated at primary energies in the range 71–774 eV and at angles of incidence of the beam 0–60°. The results are related to high energy loss and optical data, and assignments are given for inter-band and plasmon losses. With approximately $\text{3}\text{4}$ of a monolayer of CO on the surface there is a prominent additional loss at around 13.5 eV, which is interpreted as a one electron transition from a σ state below the d band to available states several electron volts above the Fermi level.     

Delayed onset of 4d photoemission relative to the giant 4d photoabsorption of La

For the giant 4d photoabsorption of La, both the total photoabsorption spectrum and the N_4.5-derived Auger emission intensity spectrum increase significantly above hν ? 112 eV, with spectral peaks at hν = 118 and 119 eV, respectively. However, the predominant 4d photoemission partial cross section shows a delayed onset of ～ 4 eV, with a peak at hν = 121 eV, while the 5s, 5p, and 5d partial cross sections all show a strong resonant enhancement at lower energies, with spectral peaks at hν = 116.6 eV. These results are compared with a recent many-body calculation for Ce. The photon energy dependence of the La 4d_{$\text{5}\text{2}$}/4d_{$\text{3}\text{2}$} photo-emission branching ratio is consistent with a “final-state model.”     

High energy xps using a monochromated Ag Lα source: resolution,sensitivity and photoelectric cross sections

Resolution, sensitivity and calibration data are presented for a novel high energy XPS source, monochromated Ag Lα radiation (hv = 2984.3 eV). Adequate resolution is attainable for good signal/noise spectra, whilst values for experimental sensitivity factors agree well with theoretical cross section values calculated by Nefedov. This allows an evaluation of ESCA 3 Mk. II transmission function up to 3000 eV, which appears to obey an approximate E^{$\text{?1}\text{2}$} dependence. Monochromated Ag Lα (linewidth 1.3 eV) overcomes the problem of broad natural linewidths for high energy sources, such that chemical state information can be gained. Various new core level and Auger peaks are developed, a notable feature being the 1s core level and KLL Auger transition capability from Al through to Cl. Improved sensitivity is experienced for elements whose major peaks occur in the 1500–3000 eV BE range, whilst there is no serious reduction of sensitivity in the conventional XPS energy range.     

Oxide thickness measurements up to 120 Å on silicon and aluminum using the chemically shifted auger spectra

Thicknesses of oxides on Si or Al can be determined up to about 120 Å using Auger electron spectroscopy, without ion-mill depth profiling, by using the ratio of the chemically shifted and unshifted peaks from the oxide and substrate, respectively. Measurement standard deviations of ± 1 Å at oxide thickness of 30 Å and spatial resolution < 10 μm are readily attainable. The absolute accuracy of the present calibration is about ± 30% at 30 Å for SiO₂. A comparison of the measured thickness d with ellipsometry revealed a disagreement which was largest at $\text{d(}\text{ellips.}\text{) = 50}\text{A}\text{?}$, where d(Auger) was 33 Å. We propose that most of this disagreement is a consequence of the finite extent of the oxide/Si interface, and the measurement of different physical parameters in the two techniques. It is demonstrated that the milling rate of SiO₂ (within about 100 Å from the SiO₂/Si interface) can be determined from ion-mill depth profiles alone and the position of the interface in the depth profile can be located within serveral Å. The electron mean free path in SiO₂ at 1615 eV was determined to be 31 ± 9 Å.     

A study of the charging and dissociation of SiO2 Surfaces by AES

AES is used to determine the initial spectrum of a vacuum-broken SiO₂ surface and to follow its dissociation under the electron beam probe. Both Auger peaks heights and energies are affected by the irradiation. The change in stoichiometry is accompanied by a decrease of the surface charge by 5–8 V. The relation between stoichiometry and charge is explained by the influence of radiation-induced defects on secondary electron emission. The reduction of SiO₂ is characterized in terms of irradiation dose, dissociation cross-section and electron impact efficiency. Resistance to radiation damage is increased by surface carbon contamination. The chemical contribution to the Auger peak energy can be distinguished from the charging effect leading to a shift between element and compound of 12 eV for the silicon peak.     

Characteristic energy losses on carbon contaminated layers correlated with secondary electron,auger electron emission and contrasts in scanning microscope

The secondary electron (SE) spectrum (0 < E < 50 eV) has been analysed by means of a CMA. Samples were clean aluminum, aluminum becoming carbon contaminated, sintered graphite powder, electro chemically deposited polymer on platinum and monocrystals of silicon carbon contaminated. When the clean Al surface is becoming carbon contaminated a quick decrease of surface plasmon and bulk plasmon losses is observed whereas a main characteristic energy loss peak (ELS) at 20 eV and a secondary electron peak at 20 eV appear simultaneously. Both peaks are very sensitive general features of carbon contaminated surfaces. The main loss peak is attributed to the excitation of the carbon-carbon bounds (σ → σ¹) as already proposed in the transmission ELS. The few eV change of the loss peak energy of various carbon compounds may correspond to slightly different carbon-carbon distances. The 20 eV secondary electrons could be produced by the relaxation of the excited state (σ¹ → σ transition) via an Auger process. The cross section for molecular electronic excitation is higher than that of atomic ionization for inner level. The loss peak is as intense as the SE peak and higher by more than two orders of magnitude than the C KLL Auger peak. The modification of secondary emission under carbon contamination has been observed on a silicon sample by Scanning Electron Microscopy (SEM) in the Secondary Electron Image (SEI) mode.     

A study of the (00) LEED beam intensity at normal incidence from CdS(0001), Cu(001), Cu(111), and Ni(111)

A Faraday cage apparatus is used for the measurement of the (00) LEED beam intensity, I₍₀₀₎, and the total secondary emission coefficient, δ(E_k), for angles of incidence from 0° ± 2° to 8° ± 2°, with an energy resolution of ± 0.037 of the incident beam energy, in the energy range 1 to 200 eV. The data are normalized and expressed as a fraction of the incident beam intensity. The basic principle of operation is the separation of the incident and specularly diffracted beams in a uniform magnetic field. Monolayer, or in-plane, resonances associated with the emergence of nonspecular beams, as well as beam threshold minima, are observed in I₍₀₀₎ at normal incidence from clean CdS(0001), Cu(111), and Ni(111). Some major differences are observed in the I₍₀₀₎ profiles for the clean (111) surfaces of nickel and copper. All secondary Bragg peaks, except the 2$\text{2}\text{3}$ order, have greater intensities for Ni(111) in the energy range 50–150 eV, thus indicating that the atomic scattering cross-section for electrons in this energy range is larger for nickel than for copper. For the (111) surface of nickel, the (11) resonance is missing, but the (10) resonance and all $\text{1}\text{3}$ order secondary Bragg peaks between the second and fifth orders are observed. For Cu(111) both the (10) and (11) resonances are observed, but the $\text{1}\text{3}$, $\text{2}\text{3}$, 1$\text{2}\text{3}$, and 3$\text{1}\text{3}$ order secondary Bragg peaks are missing in this energy range. These data indicate that multiple scattering with evanescent intermediate waves, or “shadowing”, is predominate on the (111) surfaces on nickel and copper for energies above 30 eV, and that below 30 eV multiple scattering with propagating intermediate waves is predominate on Cu(111). Correlation of the (00) beam intensity profiles from clean Ni(111) at 0°, 2°, and 6° with the intensity profiles of the (10). (1&#x0304;0), and (11) non-specular beams is nearly one-to-one from 30 eV to 100 eV, thus supporting the dynamical theories of LEED in which peaks in the (00) beam are expected to occur at nearly the same energies as peaks in the non-specular beams.     

Electron spectroscopic studies of clean and oxidized iron

X-ray photoelectron spectroscopy (XPS) and soft X-ray appearance potential spectroscopy (APS) together with Auger electron spectroscopy (AES) were used to study the electronic properties of clean and oxidized (Fe₃O₄) iron surfaces. The features arising from excitations of electrons from Fe 2p core levels are discussed consistently within the common one-electron picture (i.e. neglecting final state effects). For pure Fe the shape of the APS L₃ peak is evaluated taking into consideration the theoretical density of states above the Fermi level and is found to agree well with that observed. As a consequence it is shown that in this case the appearance potential is about 1 eV larger than the threshold energy for the excitation of a core electron to the Fermi level. Thus for $\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ electrons this quantity results to be 704.8 eV from both XPS and APS techniques. Successive oxidation at 500°C leads to an increase of the appearance potentials of the Fe 2p levels by only 0.5 eV, whereas the positions of the corresponding XPS peaks are shifted by as much as 3.5 eV. However this apparent disagreement can be eliminated by taking into account the above mentioned effect concerning the appearance potentials from pure Fe and the fact that the threshold energies (which determine the appearance potentials) of the XPS signals are shifted only by 1.7 eV. This example demonstrates that considerable care has to be taken in discussing “binding energies” or “chemical shifts” as derived from different electron spectroscopic techniques. The observed splitting of the MVV Auger transition of Fe at 47 eV upon oxidation is interpreted in terms of the qualitative features of the valence band structure of Fe₃O₄ and ascribed to the participation of a cross-transition between O 2p and Fe 3p states.     

Internal friction of deformed zinc single crystals

Zinc single crystals with the side in a (0001) basal plane were elongated and the internal friction measurements were carried out as a function of temperature in the mode of a flexural vibration. Two relaxation peaks were observed to appear: one at around 500°K and the other at about 570°K. Each activation energy was obtained 0.70±0.08eV and 1.01±0.06eV, respectively, by the peak shift method. The experimental results were discussed in terms of dislocations in pyramidal slip system {$\text{11}$22} 〈11$\text{2}$3〉 and twinning dislocations in the planes {$\text{1}$012} 〈10$\text{1}$1〉, respectively.     

Temperature dependence of specular and non-specular leed beams from Cu3 Au

The temperature dependence of intensities of the prominent diffraction peaks in the specular (00) and one non-specular ($\text{11}$) beam from the (100) surface of Cu₃Au have been measured in the range of 300 to 673 K. The effective Debye temperature associated with the specular beam appears to increase continuously with energy below 50 eV but varies discontinuously at high energies. For the ($\text{11}$) beam, which was available only in the higher energy range of 65 to 136 eV, the effective Debye temperature varies discontinuously with energy. Parallel and normal components of the Debye temperature were deduced from the two sets of data from which it appears that the two mean square displacements are approximately equal, compared to the harmonic approximation which indicates a difference of 30 percent. The log of peak intensity versus temperature (Debye plot) deviates from the straight line at 60° below the disordering temperature for all beams and all energies.     

An aes study of a copper-iron sulphide mineral

Synthetic bornite, Cu₅FeS⁴ has been studied by Auger electron spectroscopy. Sputtercleaned bornite shows a sulphur spectrum with three peaks at 138, 147 and 149 eV. These Auger transitions are different from those observed when sulphur is adsorbed on metal surfaces, where the peaks are at 139, 149 and 154 eV. The adsorption of oxygen on the surface of bornite at room temperature results in the formation of a layer of iron oxide and, in addition, the sulphur spectrum loses its fine structure and shows only a single peak at 148 eV. Under the influence of both the ion sputter beam and the electron beam, the surface composition of bornite shows large and rapid changes which are due mainly to movement of mobile Cu⁺ ions through the lattice, this movement being caused by surface charging effects.     

Angular distributions of auger electron emission: Clean and gas-adsorbed polycrystalline Ni surfaces

The excitation angle (β) and emission angle (θ) dependences of the Ni M₂,₃VV (61 eV) and Ni L₃VV (850 eV) Auger emissions from clean polycrystalline Ni surfaces, and the S L₂, ₃ M₂, ₃M₂, ₃ (150 eV) Auger emission from S-adsorbed poly-Ni surfaces have been investigated. In the case of Ni (61 eV) and S Auger emissions, the β-dependence shows the $\text{1}\text{cos β}$ distribution, while a significant deviation from $\text{1}\text{cos β}$ is observed for Ni (850 eV) Auger emission. The cosθ distribution and the intermediate between isotropic and cosθ distributions are observed for Ni (61 eV), and for Ni (850 eV) and S Auger emissions, respectively. Those results have been found to be in fairly good agreement with the calculations based on the simple continuum model without consideration of the diffraction effect and the inherent anisotropic emission.     

Plasmon energy gain and double ionization satellites in the Be Auger spectrum

Two relatively weak, higher energy satellites are observed at 18 and 38 eV above the Be KVV Auger spectrum. The lower energy satellite is assigned to coupling of energy from bulk plasmon de-excitations $\text{(}\text{h}\text{?}\text{ω ～ 18 eV)}$ with Auger electrons and the higher energy event to Auger electrons ejected from Be atoms with doubly ionized K levels.     

A study of the adsorption of oxygen on Ni(111) using auger electron spectroscopy: Chemical shifts and valence spectra

Auger electron spectra have been recorded when oxygen is adsorbed on a Ni(111) single crystal surface. For the coverage range θ < 1, an analysis of the plot of the peak to peak height (H) of the oxygen KVV (516 eV) transition versus the total number of molecules cm^2? impinging on the surface (molecular beam dosing) shows agreement with the kinetic mechanism proposed by Morgan and King [Surface Sci. 23 (1970) 259] for the adsorption of oxygen on polycrystalline nickel films. In this coverage range, no energy shifts of the nickel or oxygen Auger peaks were recorded.At coverages θ > 1 (standard dosing procedure) shifts in the valence spectra M_{2, 3}VV (61 eV) and L₃M_{2, 3}V (782 eV) of ?2.3 eV and ?1.8eV respectively are recorded at 1.4 × 10^?2 torr-sec. Up to these coverages no shift of the L₃VV transition (849 eV) is observed. A chemical shift of ?2.1 eV is recorded in the L₃M_{2, 3}M_{2, 3} Auger transition (716 eV) at 1.4 × 10^?2 torr-sec.In the coverage range θ > 1, shifts in the energy of the oxygen Auger peaks are observed. At 5.8 × 10^?3 torr-sec. the KVV (516 eV) and KL₁V (495.2 ± 0.3 eV) transitions show shifts of ?1.5 eV and ?(1.0 ±0.3) eV respectively. No shift up to this coverage is recorded in the KL₁L₁ (480.6 ± 0.3 eV) transition.     

The chemisorption of sulphur on W(100)

The interaction of sulphur vapour with a W(100) surface is studied in detail with Auger Electron Spectroscopy (AES), LEED, work function difference (Δ?) measurements and thermal desorption spectroscopy (TDS). The dissociative adsorption of S occurs on the W surface without reconstruction. Several LEED structures are observed which indicate repulsive adatom interactions. TDS shows that the desorption energy of atomic S decreases from about 8 eV at θ = 0.1 ML to about 3 eV near saturation in close vicinity of 1 ML. Above $\text{θ =}\text{3}\text{4}$ ML, S₂ desorbs in addition to S in a high temperature peak which saturates at about 1 ML. Sulphur in excess of about 1 ML is desorbed in two low temperature peaks of which the lower one consists not only of S and S₂ but also of S₃ and S₄.