E´tude par la diffraction dese´lectrons lents et la spectroscopie auger de l'adsorption du soufre sur l'or

A study has been made of the adsorption of sulphur on the (111), (100) and (110) faces of gold using LEED and AUGER spectroscopy combined with a radioactive tracer technique employing ³⁵S. The content of the surface unit mesh can thus be determined. A preliminary adsorption state with localization of sulphur atoms at sites of maximum coordination of the metal is observed. This state is transformed by reconstruction into a surface compound, or adsorbed two-dimensional sulphide. The results are compared with previous results from HEED and experiments of reversible chemisorption. On the (110) face the equilibrium between different adsorbed phases of sulphur is studied under conditions for which sulphur dissolution or evaporation is negligible. A temperature- coverage diagram is determined which shows the regions where sulphur is present at equilibrium in one or two phases.     

Étude par la diffraction des électrons lents et la spectroscopie auger de l'adsorption du soufre sur le platine

The adsorption of sulphur on the (111), (100) and (110) faces of platinum has been studied by LEED and Auger spectroscopy. By the combined use of radioactive S³⁵ the contents of the surface unit meshes were determined “in situ” for each state of adsorption. On the (111) face one observes a preliminary adsorption with localization of sulphur atoms at sites of maximum coordination of the metal. At higher pressures and temperatures, S-S distance contracts and a compact hexagonal arrangement, with sulphur atoms outside sites, is obtained. The thermal stability of the various structures was studied as a function of sulphur coverage. During adsorption on the (100) and (110) faces, two steps may be distinguished: the first one seems to correspond to an adsorption in sites, the second corresponds to the formation of a two-dimensional compound whose square mesh is slightly different on the two faces.     

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.     

The local adsorption site for sulphur on Ni {111} in the low coverage lattice gas

Angle-resolved sulphur L_2.3VV Auger electron spectra have been taken from sulphur adsorbed on Ni {111} at a range of coverages both below and above those corresponding to the ordered (2 × 2) structure. These data indicate that the local adsorption site in the low coverage lattice gas is the same single three-fold hollow site adopted in the ordered overlayer. This contrasts with the low coverage occupation of both three-fold hollow sites for the system I/Ag<{111}.     

Quantitative Auger electron spectroscopy; A comparison of techniques for adsorbed tin on iron

Three independent approaches are discussed for the quantitative calibration of tin adsorbed on the surface of iron monitored by Auger electron spectroscopy. Two of the approaches involved measurement of the substrate and condensate Auger electron intensities at stages during the condensation of tin or stannic oxide on pure iron. The condensate quantity was directly monitored by a quartz crystal oscillator and the growth form by measurements of the elastic scattering. Allowances were made for changes in the tin Auger electron peak structure due to its combination with the oxygen in the oxide condensate. These two calibrations compare very closely with a calibration obtained from the cleavage face of a single crystal of α-iron containing 7.9 wt % of tin in solid solution. A calibration in terms of relative peak intensities is obtained for tin on iron and it is concluded that the quantifica- tion procedures are applicable, even for strongly bonded compounds.     

Surface structure from angular dependence of auger electron emission

Auger electrons emitted from atoms in the vicinity of a solid surface can show strong angular dependence in their emission due to elastic scattering from the surrounding ion cores of the solid. Model calculations for the emission of Auger electrons from sulphur adsorbed on a Ni (100) surface show that these angular features are very sensitive to the adsorption site of the emitting atoms, and that a study of these effects should produce a method of surface structure analysis for the location of preferred surface adsorption sites. This approach would appear to be an improvement over LEED because of its high sensitivity to structural changes and because it does not require long range order in the adsorbate layer.     

Segregation of sulphur on a molybdenum surface studied by auger electron spectroscopy

The kinetic behaviour of sulphur segregation was studied during heat treatment of a molybdenum ribbon between 750°C and 1350°C by using an Auger electron spectroscopic technique. When the specimen is heated to high temperatures, the sulphur present as an impurity in the polycrystalline molybdenum diffuses on to the surface along grain boundaries. The intensity changes of Auger electron signals of sulphur and molybdenum showed that the rate of the diffusion remained constant until the surface was covered with a first sulphide layer in the temperature range studied. The activation energy in the initial stage of the diffusion was 26kcal/mol. High resolution Auger spectra of the sulphur were measured and explained from the density of states of MoS₂.     

Spectres auger et sections efficaces d'ionisation de la couche L23 du magnésium et de l'aluminium sous impact de protons et d'ions lourds (10–100 keV)

A comparison of Auger structures observed on the energy distributions of secondary electrons emitted from Mg and A1 solid targets bombarded by either light particles (H⁺ and He⁺) or heavy ions (Ne⁺, Ar⁺, …) is presented. With incident protons, it essentially appears a broad peak corresponding to a L₂₃VV transition and a weak shoulder due to the surface and bulk plasmon excitation. The Auger structures obtained with heavy ions are richer and the peaks which compose it are sharper. Such atomic-like structures correspond to Auger transitions from excited (with one or two L₂₃ holes) moving recoiling atoms. The experimental L₂₃ Mg and A1 ionization cross sections were determined from Auger spectra. In H⁺?Mg (or A1) collisions our results are in good agreement with the theoretical values calculated in a PWBA model. In the case of heavy ion-target interactions, we compared the experimental measurements with ionization cross section calculations obtained in a Landau-Zener model.     

Étude par spectrométrie auger et par spectrométrie de masse de la chimisorption de l'oxyde de carbone sur le molybdène polycristallin

A combination of Auger spectrometry and mass spectrometry was employed to study CO chemisorption on polycrystalline Mo surfaces at room temperature. We observed five adsorption states and calculated the binding parameters (E,n₀,τ₀) for the three important states. The results obtained by the two methods are in accord but we pointed out the occurence of electronic desorption in Auger experiments. Contamination effects by C atoms in such studies were investigated by repeated cycles of adsorption-desorption and a characteristic evolution of flash desorption peaks was observed. The results are discussed in this point of view enhancing the importance of a control of the adsorption surface cleanness by a method of great sensibility like Auger spectrometry.     

Etude par spectroscopie de photoelectrons et d'electrons auger des etapes initiales de l'oxydation du chrome polycristallin

The joint application of various techniques of surface analysis (XPS, UPS, AES and work function measurements) allowed us to pinpoint the initial stages in the oxidation of polycrystalline chromium in the temperature range 180–720 K. At weak exposures ( < 10 L) the oxygen was dissociatively adsorbed without providing any information on the amount of oxygen involved or the sites of adsorption (chemisorption vs. physical interaction due to the roughness of the surface). At stronger exposures the oxide Cr₂ O₃ is observed to form, and this is covered by a layer of chemisorbed oxygen at ordinary temperatures and below. The maximum thickness of the oxide layer depends on the temperature, as would be expected from the logarithmic oxidation kinetics. The change in shape of the Auger peaks of chromium during oxidation has been interpreted in terms of interatomic transitions made via the 2p levels of oxygen.     

Spectres Auger sous bombardement d'ions Ar+ de 60 keV de quelques éléments purs et de composés binaires

Auger spectra of different elements, alloys and compounds excited by 60 keV argon ions have been obtained. These spectra show a detailed structure with, in particular, a sharp Auger peak. This peak is interpreted as a superposition of two kinds of Auger transitions: the first one occurs with two electrons of the electronic band, the second, takes place from electrons situated on “quasi-atomic” outer shells. On the low energy side of Auger line, two equidistant peaks attributed to Auger electrons having suffered energy losses by ionization of electronic outer shell of target-atoms were observed. From Mg, Al, Si, P and S Auger transitions due to initial double L_ii,iii level ionization were detected. The interpretation of the creation of a vacancy in electronic inner shell at the origin of Auger effect by ionic bombardment is given from the electronic promotion model of Fano and Lichten.     

Surface analysis by ion-electron spectroscopy; Silicon target

The energy spectra of secondary elections emitted from a Si(111) surface due to bombardment by 6 keV He⁺ and O⁺₂ ions have been examined. The fine structure in the spectra is explained on the basis of a novel mechanism of creation of Auger electrons at the surface. There are two stages of interaction between incoming ions and the substrate via adsorbed atoms. In the first stage, due to a level promotion mechanism, vacancies in the adsorbed atoms are created. In the second stage, Auger neutralization processes accompanied by the emission of electrons from a solid with characteristic energies take place. These electrons provide a good indication of the degree of coverage of the silicon surface with contaminant atoms. The energy losses of escaping electrons are also discussed.     

Sulphur overlayers on the Au(1 1 0) surface: LEED and TPD study

The adsorption and desorption of sulphur on the clean reconstructed Au(1 1 0)-(1 × 2) surface has been studied by low energy electron diffraction, Auger electron spectroscopy and temperature programmed desorption. The results obtained show a complex behaviour of the S/Au(1 1 0) system during sulphur desorption at different temperatures. Two structures of the stable ordered sulphur overlayer on the Au(1 1 0) surface, p(4 × 2) and c(4 × 4), were found after annealing the S/Au(1 1 0) system at 630 K and 463 K, respectively. The corresponding sulphur coverage for these overlayers was estimated by AES signal intensity analysis of the Au NOO and S LMM Auger lines to be equal to 0.13 ML and 0.2 ML, respectively. Both sulphur structures appear after removing an excess of sulphur, which mainly desorbs at 358 K as determined from TPD spectra. Furthermore, it was not possible to produce the lower coverage p(4 × 2) sulphur structure by annealing the c(4 × 4) surface. In the case of the p(4 × 2) S overlayer on the Au(1 1 0)-(1 × 2) surface it is proposed that the sulphur is attached to “missing row” sites only. The c(4 × 4) S overlayer arises via desorption of S₂ molecules that are formed on the surface due to mobility of sulphur atoms after a prolonged anneal.     

Oxygen KLL Auger spectra from O2 and CO adsorbed on Ni

High resolution oxygen KLL Auger spectra from O₂ and CO adsorbed on clean Ni (110) have been measured and compared. These Auger spectra are quite different from one another and are compared with spectra of the free molecules. Similarities between the oxygen spectra from adsorbed and gaseous CO are noted and interpreted as due to the weak bonding of CO to Ni with the O atom furthest from the surface. These Auger studies also characterize the change in bonding of adsorbed CO due to electron beam interaction.     

Influence de l'absorption d'atomes de plomb sur la résistance électrique de films d'or

Precise electrical resistance and thickness measurements are associated to study lead adsorption on gold films in ultra-high vacuum. Fuchs' theory is used to interpret the large size effects observed when the lead deposit is less than one monolayer. At low coverages the ratio of electrical resistivity variation to lead concentration is independent of temperature and equal to 1.8 ± 0.1 microOhms cm/at%. With temperatures ranging from ? 150 to + 150°C, the evolution of size effects is related to structural changes in the lead overlayer. A saturation in the diffuse scattering of conduction electrons by adsorbed adatoms is shown to occur when the lead deposit is about half a monolayer; this is used for comparing surface roughness of annealed and unannealed gold films. For lead coverage greater than a monolayer, an alloy identified by X-ray diffraction as AuPb₂ is formed. The electrical resistance variation is consistent with the thinning of the gold film by removing atoms which diffuse towards the free surface through the alloy layer.     

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.     

L'adsorption du bismuth sur des films minces d'or etudiée par variation de résistance electrique

Adsorption of bismuth on gold thin films is studied by electrical resistance variations. At low coverage the increase of gold resistivity due to Bi adatoms is 2.3 ± 0.1 μΩ cm/at% and is independent of temperature and thickness of the Au layers. At 20° C and ?150°C, the shape of the resistance variation curve for increasing Bi coverage indicates that the adatoms form a first monolayer having a higher density at lower temperature. At 85°C two Bi monolayers are made successively: the first is adsorbed on the free surface of the gold film; the second grows on the other face, between the gold and the glass substrate, after migration of the Bi atoms through the grain boundaries or other defects. These results are verified by Auger electron spectroscopy. They are used to show that the initial reflectivities for conduction electrons on the two surfaces are similar in recrystallized gold films; the specularity coefficients P and Q have been evaluated using the Fuchs theory. For clean surface, the electron reflection is large specular (P ? Q ? 0.75 ± 0.05) and becomes entirely diffuse when a monolayer of bismuth is adsorbed on each face of the gold film.     

Détermination par spectroscopie auger,des sections efficaces d'ionisation de la couche L23 du magnésium et de l'aluminium sous impact d'ions He+

The L₂₃ Mg and L₂₃ Al Auger spectra excited by helium-ion bombardment have been studied. These spectra essentially show an intensive peak interpreted in terms of L₂₃ MM atomic-like and L₂₃ VV transitions. By integration of this peak, the Auger emission cross sections were determined and their variations as a function of the incident particle energy were studied from a few keV up to 100 keV. These results have been compared to the values of the ionization cross sections calculated in a classical binary encounter approximation (BEA). One observed a good agreement for the aluminium target. For the magnesium, the discrepancy in the low energy range shows that, in those case, the electronic promotion in a molecular orbital model added at the electronic excitation due to the direct Coulomb interaction.     

Diffuse scattering in RHEED induced by linear disorders of sulphur segregated on nickel (111) surface

RHEED diffuse scattering intensity is related to the growth of one-dimensional sulphur chains segregated on a nickel (111) surface. These linear disorders lead to wall domains in the reciprocal lattice which give the well-known “arching” phenomenon in RHEED patterns. The diffuse scattering process disappears when the (5√3 × 2)R30° superstructure is obtained. Knowing the absolute coverage of sulphur by calibrated Auger spectroscopy, it is possible to give a new high symmetry model for this superstructure.     

Utilisation de l'heterodynage optique pour des mesures locales de la vitesse de phase d'ondes elastiques

A new optical method for measuring the acoustical wavelengths with a great accuracy has been investigated. An acousto-optical interaction is performed and the acoustical frequency range is such that the diffraction take place in “Bragg regime”. The phase modulation of the scattered laser beam is visualized by means of an optical heterodyning. When the location of the acousto-optical interaction is linearly displaced, the detected beating signal supplies a local measurement (10 μ) of the acoustical wavelength.