Oxidation of the (100) surface of a NiFe alloy

The initial stages of oxidation of the (100) surface of a single crystal alloy specimen of approximate atomic composition Ni 59, Fe 41 (at%) have been studied by Auger spectroscopy and electron diffraction techniques. The clean alloy surface shows only a slight iron enrichment over the temperature range of the oxidation studies (373–873 K). Oxidation studies were performed over the O₂ pressure range 5 × 10^?9 to 1 × 10^?6 Torr. Within these experimental conditions the rate of oxygen uptake was found to be linear in pressure and essentially independent of temperature. LEED studies showed that a chemisorbed c(2 × 2) structure preceded the formation of surface oxides. The interaction of oxygen with the surface induced a marked segregation of iron and this was particularly pronounced at elevated temperatures. Chemical shifts were observed in the low energy Ni and Fe Auger spectra during oxidation; these were similar to those previously observed in separate studies of the oxidation of pure Ni and of pure Fe. At the higher temperatures the initial oxide layer grew epitaxially apparently as a (111) cubic oxide on the (100) substrate. The Ni to Fe concentration ratio in oxides several layers thick was found to depend on the temperature of the reaction; at higher temperatures the oxide were more Fe-rich. The Fe to Ni ratio in oxides produced at lower temperatures could be increased by annealing. At large O₂ exposures (about 5000 L) a transition was observed in the structure of the oxide layer.     

ELS study on initial oxidation of Ni(100) surfaces

Electron energy loss spectra of clean and oxygen-covered Ni(100) surfaces were observed with concomitant measurements of LEED, work function change, and Auger peak height ratio O(KL_{2, 3}L_{2, 3})/Ni(L_{2, 3}VV). The observed electronic transitions are interpreted on the basis of primary election energy dependence, and of comparison with the loss spectrum for a UHV-cleaved NiO(100) surface and optical data of Ni. The observed loss peaks at 9.1, 14, and 19 eV in the clean surface spectrum are ascribed to the bulk plasmon of the 4s electrons, the surface plasmon, and the bulk plasmon of the coupled 3d + 4s electrons, respectively, and the weak but sharp peak at 33 eV is tentatively attributed to the localized many-body effect in the final state. Three oxygen-derived peaks at 6.0, 8.0, and 10.3 eV in the low oxygen exposure region (?4 L) are ascribed to the O 2p(e) → Ni 3d, O 2p(a₁) → Ni 3d, and O 2p → Ni 4s transitions, respectively. In the high oxygen exposure region (?50 L), the spectra become quite similar to that of the UHV-cleaved NiO(100) surface. The oxidation process consistent with LEED, Auger peak height ratio and work function change measurements is discussed.     

Growth and properties of oxide films on Zn(0001)

The initial stages of the formation of ZnO films on cleavage planes of Zn single crystals have been studied. The growth rate is found to be linear in pressure and independent of temperature over the range from 77 to 425 K. The growth law is consistent with the rate limiting step being the adsorption of oxygen rather than ion transport through the oxide layer. Both the atomic and electronic structure have been monitored during oxygen exposure. The LEED patterns and intensity-voltage data indicate that at low temperatures the oxide is either amrophous or a fine grained polycrystal while above room temperature it is single crystal ZnO epitaxially oriented on Zn(0001). The changes during oxygen exposure of the Zn M₂₃M₄₅M₄₅, M₂₃M₄₅V, M₂₃VV and corresponding M₁ Auger spectra have been studied in some detail. The details of the spectra are identified through comparison with calculated and measured band structures and with previous observations of LMM transitions. The relaxation energy associated with the two d-band holes in the final state (i.e., δE(M₄₅M₄₅)] is found to be ～9 eV in good agreement with a recent calculation. The corresponding relaxation energy for the oxide is ～13.5 eV. The development of the electron energy loss spectra with oxygen exposure has been followed for a range of primary energies. A growth model which is consistent with the LEED observations, Auger peak heights and lineshapes and energy loss spectra is that the oxide grows heterogeneously on the Zn surface with no distinguishable precursor adsorbed state. However, contrary to previous models, it is shown that the surface is completely covered by oxide at a mean thickness of 2–3 monolayers. LEED, Auger and energy loss data for the two polar cleavage faces of ZnO are presented for comparison with those from the oxide overlayer.     

Relaxation energies of oxygen auger transitions in some oxides

Relaxation effects are of major importance in the so-called chemical shifts observed in XPS and AES. These chemical shifts mainly arise from changes in the electrostatic environment due to the field of the neighbouring atoms in the initial neutral state and from electron redistribution in the surrounding electron cloud in order to screen the final state holes of the excited atom. Using a three-step model for the Auger process, we succeeded in deriving cross-relaxation energies from the KLL spectrum of oxygen in oxides, provided a careful calibration of experimental binding and Auger kinetic energies is achieved. It has been shown that the extra-atomic relaxation energy increases with ionicity for oxides of non-transition metals. In the case of transition metal oxides, it has been found that the cross-relaxation energies are larger than for the other oxides; it is believed that this is due to a more efficient screening effect of the d-electrons of the neighbouring metal atoms.     

ESCA studies of Cu,Cu2O and CuO

Cu 2p, Cu 3d and O 1s electron spectra and Cu L₃M_4,5M_4,5 Auger electron spectra from Cu, Cu₂O and CuO have been studied at 25°C and at 400°C. The height of the Cu 2p satellite peaks from copper oxides was lowered when the temperature was raised. The intensity of the satellites also decreased if the sample stayed in vacuum for prolonged periods.Two commercial cuprous oxides were different with respect to the behaviour of the satellite peaks. One produced very weak satellites, while the other produced strong ones as previously reported in the literature for cuprous oxide. The colour of the oxides was slightly different, indicating that the stoichiometry was not the same.The change in satellite intensity is accompanied by changes in oxygen spectra, Cu L₃M_4,5 M_4,5 Auger spectra and valence band spectra.It is useful to study Auger electrons in addition to the direct electron spectrum, since Auger signals can be more sensitive to surface conditions than direct electron spectra.     

The adsorption of hydrogen and the reaction of hydrogen with oxygen on Pt (100)

The adsorption of hydrogen on Pt (100) was investigated by utilizing LEED, Auger electron spectroscopy and flash desorption mass spectrometry. No new LEED structures were found during the adsorption of hydrogen. One desorption peak was detected by flash desorption with a desorption maximum at 160 °C. Quantitative evaluation of the flash desorption spectra yields a saturation coverage of 4.6 × 10¹⁴ atoms/cm² at room temperature with an initial sticking probability of 0.17. Second order desorption kinetics was observed and a desorption energy of 15–16 kcal/mole has been deduced. The shapes of the flash desorption spectra are discussed in terms of lateral interactions in the adsorbate and of the existence of two substates at the surface. The reaction between hydrogen and oxygen on Pt (100) has been investigated by monitoring the reaction product H₂O in a mass spectrometer. The temperature dependence of the reaction proved to be complex and different reaction mechanisms might be dominant at different temperatures. Oxygen excess in the gas phase inhibits the reaction by blocking reactive surface sites. At least two adsorption states of H₂O have to be considered on Pt (100). Desorption from the prevailing low energy state occurs below room temperature. Flash desorption spectra of strongly bound H₂O coadsorbed with hydrogen and oxygen have been obtained with desorption maxima at 190 °C and 340 °C.     

Adsorption of oxygen on W(100): Adsorption kinetics and structure

The adsorption of oxygen on W(100) single crystal surfaces is studied by Auger electron spectroscopy (AES), flash desorption, low-energy electron diffraction (LEED) and retarding field work function measurements with the aim of obtaining a better understanding of the adsorption kinetics and of the structures of the adsorbed layer. The AES results reveal step-wise changes of the sticking coefficients in the coverage range 0 to 1, and activated adsorption at higher coverages. Upon room temperature adsorption a series of complex LEED patterns is observed. In layers adsorbed at 1050 K and cooled to room temperature, the well-known p(2 × 1) structure is the first ordered structure observed. This structure shows a reversible order-disorder transition between 700 K and 1000 K and is characterized by a work function which is lower than that of the clean surface. Heating room temperature adsorbates changes their structure irreversibly. At temperatures below 750 K some new structures are observed. Combining the results obtained in this study with other published work leads to a considerable revision of the previously accepted model of the adsorption of oxygen on W(100).     

An Auger and X-ray photoelectron spectroscopic study of sodium metal and sodium oxide

Photoelectron and Auger electron measurements have been made on polycrystalline films of sodium metal evaporated in ultra high vacuum, and on Na₂O produced by in-situ oxidation by dry oxygen. Most of the spectra were recorded using Mg Kα (1254 eV) radiation but excitation by 5 keV electrons or monochromatized Al Kα (1487 eV) X-rays was used for specific purposes. Core and valence electron binding energies, photoionization cross-sections relative to Na 1s, KLL and KLV Auger energies and transition probabilities are reported. Energy losses in the metal and oxide are discussed and the relative intensities of surface and bulk plasmon losses have been used to calculate mean electron escape depths in the metal. When corrections were made for experimental geometry, escape depths of 10 Å at 180 eV and 31 Å at 1200 eV were obtained. An escape depth of 23 Å at 980 eV was obtained by Na 1s-Na K-Auger intensity correlation and this is consistent with the plasmon data. Data on Auger satellite lines are presented and, in particular, evidence has been obtained which indicates that a high energy satellite should not be attributed to a plasmon gain mechanism. Valence band influences on the KLV Auger spectra are discussed with reference to the XPS spectrum and other sources of valence band information. Unexpected structure was found in the KLV spectra of the metal which, pending thorough interpretation, offsets the sensitivity and resolution advantages which these spectra otherwise offer for valence band studies.     

The adsorption of oxygen on gold

The oxidation of gold has been studied under UHV conditions by AES, XPS, and TDS. The previously reported adsorbed oxygen state, which formed by heating the sample above 600 K in 10^?5 Torr of oxygen and which remained after subsequent heating to 1100 K in vacuo, has been shown to result from the reaction of oxygen with silicon diffusing from the bulk. No oxygen adsorption was detected on a clean sample for oxygen pressures up to 10^?4 Torr and sample temperatures between 300–600 K. Chemisorption of oxygen atoms could be induced by placing a hot platinum filament close to the sample during exposure to oxygen. The activation energy for desorption of this oxygen state was estimated from the thermal desorption spectra to be about 163 kJ mol^?1. The chemisorbed oxygen atoms and the oxygen associated with silicon were distinguished by different O(1s) binding energies (529.2 and 532.3 eV respectively) and by different O(KVV) Auger fine structure.     

X-ray excited auger spectra (XAES) from chemisorbed species

Using X-ray excitation, well-resolved oxygen KLL Auger spectra have been obtained from chemisorbed layers of oxygen and carbon monoxide on Ru(001). These spectra are markedly different, while the spectrum of CO on Ru(001) is quite similar to the Auger spectrum from gaseous CO. The shift between the spectra of gaseous and adsorbed CO is much larger than for the corresponding XPS spectra, a fact understandable in terms of image charge screening. The use of XAES as a fingerprinting method for adsorbed species is recommended.     

The clean thermally induced W {001} (1 × 1) → (√2 × √2) R 45° surface structure transition and its crystallography

A (√2 × √2)R45° surface structure on W {001} produced only by cooling below ~370 K, first reported by Yonehara and Schmidt, has been investigated by LEED, AES, work function change, characteristic loss and low energy Auger fine structure measurements. No significant changes at any energy up to 520 eV occur in the standard Auger spectrum upon cooling to 220 K for as long as 30 min after a flash to >2 500 K. The work function of the (√2 × √2) R45° at 210 K is 20 ± 10 mV below that of the (1 × 1) surface, and a sensitive feature in the fine structure of the N₇VV AES transition shows approximately 60% attenuation. Unlike for H₂ adsorption, the “surface plasmon” loss peak exhibits little if any measurable attenuation and no measurable shift in energy as the crystal cools to form the (√2 × √2)R45°. The rate of intensity buildup in the $\text{1}\text{2}$-order LEED beams is strictly temperature dependent, and significant differences exist between the $\text{1}\text{2}$-order LEED spectra produced by cooling and those produced by H₂ adsorption. Only 2-fold symmetry was observed in the LEED beam intensities at exactly normal incidence, rather than 4-fold as expected for statistically equal numbers of rotationally equivalent domains. The LEED I-V spectra for 24 fractional order beams and 12 integral order beams, taken over large energy ranges at normal incidence, clearly establish that the beam intensities display 2 mm point group symmetry, and hence a preference of one domain orientation over the other. No beam broadening or splitting effects were apparent, implying only incoherent scattering from the various domains. The half-order beam spectra (±h/2, ±h/2) are identical in relative intensity to the (±h/2, ±h/2) spectra but different in absolute intensity by a constant factor, which can be explained only by domains with p2mg space group symmetry rather than just p2mm. Adsorption of H₂ onto the cooled (√2 × √2)R45° structure restores the 4-fold symmetry in the LEED beam intensities at normal incidence, giving a c(2 × 2) hydrogen structure, the same as when adsorbing H₂ onto the above room temperature (1 × 1) crystal. This strongly supports the observed p2mg symmetry as being a true property of the cooled (√2 × √2)R45° surface structure. These results show that the (1 × 1) → (√2 × √2) R45° transition produced by cooling is a transition involving displacement of surface W atoms, and that it apparently can be characterized as an order-order, second degree, homogeneous nucleation process, which is strongly prohibited by the presence of impurities or defects.     

Initial oxidation of the Mg(0001) surface,an electron spectroscopy study

The initial oxidation of Mg(0001) has been studied using AES (Auger electron spectroscopy), LEED (low energy electron diffraction), and EELS (electron energy loss spectroscopy). The oxidation proceeds through different stages; first oxygen atoms are incorporated to chemisorption sites below the top layer magnesium. This chemisorption phase is followed by the formation of an oxide layer. The oxide layer covers the Mg surface after an oxygen exposure of ～ 10 L O₂. After this exposure the bulk-like MgO formation slowly increases the oxide thickness. The oxide layer formed for exposures up to ≤ 10 L O₂ gives rise to a diffuse LEED pattern of the same symmetry as the original “clean” LEED pattern; the possibility of an epitaxial oxide formation at this stage is discussed.     

Low energy Auger and loss electron spectra from magnesium and its oxide

Data have been obtained from Auger and energy loss processes in clean metallic Mg, Mg during stages of oxidation, and UHV cleaved MgO(100) surfaces. Particular attention has been paid to twenty features below 200 eV in the Auger spectra from these surfaces. A comparison of spectra from the metal, oxidised metal surface, and single crystal MgO has enabled estimates to be made of surface charging effects, and the MgO steady state surface potential is found to be near + 10 V above ground. All the Auger features are given assignments, two of which are interfacial processes involving ionic initial states and metallic final states. Several features in the low energy Auger spectrum are attributed to diffraction of true secondary electrons.     

Relationship between vibrational states of O-Ni systems and their superficial structure on (100) face of nickel single crystal

High resolution energy loss spectra of 4 eV electrons reflected in the specular direction from Ni(100) surface clean or covered by the ordered structures obtained in the different stages of the metal oxidation, are analysed with reference to LEED patterns. At room temperature, the successive p(2 × 2) and c(2 × 2) structures associated with the chemisorption of oxygen have been observed without modification of the energy loss spectra, in respect of the clean nickel surface. Surface phonons are known to occur in the case of the c(2 × 2)S ordered layer and their absence in the case of Ni-O corresponding system is discussed. After short exposures to oxygen between 200 to 500° C, the surface exhibits a so called “intermediate oxide”. It is identified by its hexagonal unit mesh (～5 Å) with two equivalent orientations along the [100] and [110] directions of the substrate and its vibrational spectra characterized by a loss peak at ? 112.5 meV (± 2.5 meV). Subsequent exposures to oxygen lead to the formation of the (100) face of NiO (in epitaxy on the Ni(100) face) accurately identified by its LEED pattern. The obtained typical multiple loss spectra with spacing 67.5 meV (± 15 meV) reveal a scattering of low energy electrons by long wavelength optical phonons associated to the oxide. The characteristic energy loss (67.5 meV) is in relative good agreement with the energy of the Fuchs-Kliewer surface phonon calculated from the optical constants of the nickel oxide.     

Oxidation of the hydrogenated diamond (100) surface

The surface composition and structure of natural diamond (100) surfaces subsequently oxidized with activated oxygen at T_sub≤35°C were investigated with high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy, electron loss spectroscopy (ELS) and low-energy electron diffraction (LEED). Complete surface oxidation (oxygen coverage θ=1 ML) required doses of hundreds of kilolangmuirs of O₂. HREELS vibrational spectra permitted identification of the specific surface oxygen species, and also provided information about the diamond surface states. Most surface sites lost their hydrogen at least once before becoming oxidized. The oxygen coverage θ increased quickly at first, and then more slowly as saturation was approached; different mechanisms or sites may have accounted for the decreased rate. The relative distribution of oxygen species varied with the oxidation conditions. Ether, carbonyl and hydroxyl groups appeared during the initial stages of oxidation, but the hydroxyl groups disappeared at higher coverages. Bridge-bonded ether groups dominated at saturation coverage, although smaller amounts of carbonyl and hydroxyl were still observed. The carbonyl and C---H stretch frequencies increased with oxygen dose due to formation of higher oxidation states and/or hydrogen bonding between adjacent groups. ELS revealed only a low concentration of C=C dimers on the oxidized surfaces, and no evidence of graphitization.

Surfaces generated by oxygen addition and then desorption were more reactive than surfaces generated by hydrogen desorption. Oxidized surfaces that were heated in vacuum and then rehydrogenated did not recover the sharp LEED patterns and HREELS spectra of the original plasma-smoothed surface. This effect was presumably due to surface roughening caused by oxygen desorption as CO and CO₂, and creation of reactive high-energy sites that quickly bonded to available background gases and prevented large areas of organized surface reconstruction.     

The structure and stability of surface platinum oxide and of oxides of other noble metals

The formation and stability of surface layers of platinum oxides in platinum single crystals has been studied in ultrahigh vacuum. Low energy electron diffraction (LEED) was used to identify the ordered structures that formed on the surface of Pt(111), Pt(332), and Pt(110). It appears that these structures can be related to hexagonal planes of PtO₂. The cleanliness of the surface was monitored by Auger electron spectroscopy (AES). The presence of impurities like Ca and Si must be avoided as they oxidize preferentially to the Pt. It is shown that the Pt oxide layers are stabilized by the very slow kinetics of oxygen diffusion to the surface which is responsible for the observed long life of the oxide layers under most catalytic reactions that are carried out at temperatures below 500°C. The stability of other oxides of noble metals that have been observed in UHV studies is also reviewed.     

The diamond surface: II. Secondary electron emission

The total secondary electron emission spectrum from diamond has been examined, and details of the Auger spectra, characteristic loss spectra and K-level ionisation loss spectra have been presented. These features from the clean diamond surface were contrasted to those from graphite and amorphous-carbon. The fine structure in the carbon Auger spectra were compared with the line shape calculated using the band structure model, and the chemical sensitivity of the Auger spectra was demonstrated. A high energy Auger satellite peak in the diamond spectrum was ascribed to the Auger transition occurring from a doubly ionised carbon atom. This result was substantiated by the observation of loss peaks associated with the singly ionised level. The characteristic energy losses were assigned to interband transitions and plasma losses, and have been compared to optical data where possible.     

Some aspects of auger electron spectra of 3d transition metal oxides

Auger electron spectra of the transition metals Cr, Mn, Fe, Co and Ni as well as their oxides have been investigated in the energy range between 0–100 eV. In each case of the clean metal surface the observed spectrum consists essentially of one Auger line identified asM _2,3 VV transition. After oxidation a line doublet is observed revealing two transitions instead of one. Additional new Auger peaks appear in the low energy range between 0–30 eV. The “splitting” of the Auger line can be explained as resulting from aM _2,3 V _dV_d and aM _2,3 V _pV_p transition. The latter is characteristic for the compound and can in a simple way be interpreted as a cross transition.     

Obtaining density of states information from self-deconvolution of Auger band-type spectra

A systematic revision of the techniques to isolate Auger peaks in experimental spectra obtained with LEED/AES analyzers has been carried out: Background subtraction, deconvolution with the elastic peak and inversion of self-convolution. New techniques have been devised and applied to the Auger spectra of graphite, aluminum, magnesium, magnesium oxide and silicon. Self-deconvolution of these spectra leads to a transition density function that has been compared with the density of states of the valence band near the surface, as given by other competitive techniques (XPS, UPS, XES) and by theory. This comparison shows that in those meterials, Auger electron spectroscopy (AES) is a sensitive probe of the electron environment of surface atoms.     

Surface characterisation of indium phosphide

LEED, characteristic loss spectroscopy, Auger electron spectroscopy, photoemission and light modulated contact potential methods have been employed to study indium phosphide surfaces. Cleaved (110) faces have a surface unit mesh identical to that of the bulk and surface states lead to band bending in the surface region. Reaction with oxygen leads to loss of phosphorus and the formation of an oxide layer near the surface. Features in the characteristic loss spectrum are discussed in terms of interband transitions and plasmon losses and the Auger spectra of oxidized surfaces are thought to contain components due to cross transitions. The surface composition, band bending and photothresholds of etched (100) surfaces of epitaxial InP layers have been established. Surfaces cannot be cleaned by heat treatment alone since decomposition occurs at elevated temperature. The influence of evaporated silver on the sign of the surface photovoltage has also been investigated.