D.J. Fabian

X-ray photoelectron studies of the reaction of clean metals (Mg, Al, Cr, Mn) with oxygen and water vapour

Abstract Variation in the X-ray excited photoelectron spectrum during exposure of clean magnesium, aluminium, chromium and manganese to oxygen and water vapour, provide data on the kinetics of reaction with these gases. With exposures of up to 10 −2 torr sec the intensity of the oxygen 1s peak is approximately proportional to the quantity of oxygen adsorbed on clean surfaces of magnesium, aluminium and chromium at room temperature. The X-ray photoelectron spectra are shown to differentiate between oxides, hydroxides and adsorbed water. The metals investigated give principally oxides in their reaction with water vapour at room temperature but hydroxides are commonly formed at low temperatures. On reaction with oxygen the surfaces of magnesium, aluminium and chromium become protected by the first layers of oxide formed, which is not the case for manganese. Small shifts in the O 1s peak are observed during the reaction of Al and Mg with oxygen.

X-ray excited Auger and photoelectron spectra of magnesium, some alloys of magnesium and its oxide

KLL, and KLM Auger and X-ray photoelectron spectra for pure magnesium, for Mg2Cu, Mg3Au and Mg3Bi2, and for MgO are reported. Assignments are made for almost all of the observed features in the Auger spectra. For the pure metal, internal photoemission peaks and a plasmon gain peak are shown to be present in the X-ray excited spectra.

The reaction of oxygen and water with iron films studied by X-Ray photoelectron spectroscopy

Abstract Adsorption of oxygen on iron at ambient temperature and low pressure is shown by XPS to give a chemisorbed species and the oxide, Fe 2 O 3 . At low temperatures a further adsorbed species is detected, similar to the nickel-oxygen system. Correction of the intensity of the oxygen signal for depth results in an oxidation curve in agreement with reported work using other techniques, i.e. oxidation is fast until about four layers of oxide are formed, at an exposure of ca. 10 2 L, and then proceeds slowly to about ten layers. Adsorption of water vapour produces an overlayer less than one layer in depth at an exposure of 10 5 L. Comparison of the overlayer depths calculated from the decrease in unoxidised iron signal intensity and from the increase in oxygen intensity gives good agreement for the thick oxide film produced by oxygen adsorption, but not for the thin overlayer formed by exposure to water vapour. This suggests a difference in packing of the ions in the thin overlayer compared to the arrangement in the bulk oxide.

X ray photoelectron studies of thorium and uranium

The X-ray induced photoemission from both clean and deliberately oxidized thorium and uranium surfaces is described. The results are compared with previously reported XPS and X-ray emission measurements of electron binding energies for the heavy elements. While agreement between photoelectron and X-ray spectroscopic results is fair for most of the electron energy levels studied large discrepancies were observed in the energies of the P1, P2 and P3 levels. The authors discuss also the effect of oxidation on the spectra and on the valence bands for the metals.

The reactions of oxygen and water with the rare-earth metals terbium to lutetium studied by x-ray photoelectron spectroscopy

Abstract The X-ray excited oxygen 1s photoelectron spectra of water adsorbed on clean evaporated films of the heavy rare-earth metals, terbium to lutetium, is characterised by two peaks: (1) at 531.0 ± 0.5 eV binding energy, and (2) at 533.0 ± 0.5 eV binding energy, assigned respectively to oxide and hydroxide species. Variation of the relative intensities of these peaks with exposure to water leads to the postulate that the oxidation mechanism is island growth with a layer of hydroxide at the surface of the oxide island. At low temperatures, adsorption of water gives two additional peaks: (3) at 534.5 ± 0.5 eV, and (4) at 535.7 ± 0.2 eV, assigned respectively to chemisorbed and condensed water. On adsorption of small amounts of dry oxygen the O 1s spectra exhibit solely peak (1), whose intensity increases with further oxygen treatment to reach a steady value after ∼40 L exposure. The kinetics of reaction with oxygen follow a logarithmic relation, once correction is made for the effect of escape-depth on peak intensity. However, ytterbium, with a closed 4f shell in the metallic state, exhibits oxidation characteristics different from the other rare-earth metals; its oxygen 1s intensity increases linearly with exposure, and the steady plateau level is reached sharply rather than asymptotically. Island growth with a limited number of nucleation sites may explain this behaviour.

X-ray photoelectron core-level studies of the heavy rare-earth metals and their oxides

Systematic measurements of the core-electron binding energies of the rare-earth metals terbium to lutetium, and their surface and bulk oxides, have been made with a VG-ESCA3 X-ray photoelectron spectrometer coupled with a PDP/8E computer. Comparison of the measured binding energies for the core levels with accepted tabulations reveals discrepancies that cast doubt on normally accepted values for the heavier elements. Multiplet splitting of the 4f, 4d, 4p, 4s and 5s levels is observed for the metals terbium to thulium, which have open 4f-shells. The 4f and 4d spectra of pure ytterbium and lutetium metals, with closed 4f-shells, exhibit spin-orbit doublets; whereas multiplet splitting is also detected for oxidized ytterbium, while for oxidized lutetium the simple 4f and 4d doublets are retained. Satellite structure is observed in the 4p spectra; while the 3d spectra of dysprosium and terbium - accessible to excitation by AlKα radiation - have complex structures. Spin-orbit splitting is also observed in 5p spectra for the whole of the series terbium to lutetium. The present results agree well with available high-resolution x.p.s. data, and are also in good agreement with many-electron predictions. The existence of additional structure predicted by the correlated hole theory is confirmed, and occurs at ca. 26 eV to the high binding-energy side of the most intense peak in the 4d-spectra for terbium to thulium (with incomplete 4f-shells). Measurements are also reported for bulk oxide samples, and the oxygen Is binding energies are compared for the surface oxidized metal and bulk oxides.

Electron energy loss processes in X-ray photoelectron spectroscopy

Abstract A technique is established in X-ray photoelectron spectroscopy (XPS), using spectra emitted from successively evaporated metallic films, to distinguish between electron energy loss mechanisms identified as, respectively, extrinsic and intrinsic to the photoelectron excitation process. It is demonstrated that tailing on the high kinetic energy side of many XPS peaks is due to intrinsic processes, while the background emission at energies generally some 30 eV below the peaks arises from extrinsic processes. Plasmon energy-loss peaks are believed to contain contributions from both intrinsic and extrinsic processes.

X-ray photoelectron studies of scandium and its hydride and oxide

Binding energies for the core levels of scandium metal and its oxide and hydride are recorded. Plasmon energy-loss peaks are observed for the metal and show a large shift on hydriding, approximately 3.7 eV, compared with a shift of approximately 0.2 eV for the 2p core levels. The large plasmon shift is consistent with the view that scandium forms a metallic hydride in which the H 1s electrons contribute to the Sc valence band.

On the comparison of the soft X-ray, X-ray photoemission and Auger electron spectra of the valence bands of Mg, Mg2Cu and Mg3Au

Interpretation of the data obtained using the experimental techniques soft X-ray spectrometry, X-ray photoelectron spectrometry, and Auger electron spectrometry, are discussed in relation to studies of magnesium, Mg2Cu and Mg3Au. Comparison of SXS and XPS leads to improved understanding of the band structure of these materials, but in the case of the Auger technique it is concluded that the origin of the spectra must be better understood for this to be useful in band structure studies.

Oxidation of scandium by oxygen and water studied by XPS

Abstract Reaction of scandium with water at 293 K gives initially the oxide and, above ∼50 L, also the hydroxide. The ratio of hydroxide to oxide is approximately constant in the later stages of reaction. The reaction of water with scandium is slower and less extensive then with the lanthanides. At low exposures of water at 80 K only the hydroxide and chemisorbed water are produced, again in the contrast to the lanthanides. Oxidation with oxygen is complex. For small adsorptions of oxygen, the O 1s BE does not agree with the value for the bulk oxide, whereas for a large single step adsorption, they are in good agreement. The overlayer depths calculated from the metal and from the oxygen signals also differ for stepwise additions of oxygen. The results are tentatively interpreted as dissolution of the oxygen in the scandium, with the oxygen concentration below the value required for nucleation of oxide.