Interatomic Auger Transitions Research Articles

Evidence for a covalent surface KCN species on Pd(100) from inter- atomic Auger transitions.

The He i and Ne ii uv spectra and ${\mathrm{He}}^{\mathrm{*}}$ Penning ionization electron spectra of coadsorbed K and CN annealed to above 800 K show extra features, not present in Ne i and ${\mathrm{Ne}}^{\mathrm{*}}$ spectra, that are observed at constant kinetic electron energies. Using the observed valence levels of ${\mathrm{KCN}}_{\mathrm{ads}}$ and the excitation energies, the extra features are shown to result from interatomic Auger transitions involving a K(3p) hole and mixed K-CN states. This strong interadsorbate interaction is attributed to the formation of molecular orbitals of a more covalently bonded KCN molecule.

Interatomic radiative transition rates for the sodium fluoride crystal.

Interatomic electric-dipole radiative transition rates for the NaF crystal are calculated for initial 1s, 2s, and 2p holes in the Na/sup +/ ion. The crystal is simulated by a (NaF/sub 6/)/sup 5 -/ cluster embedded in a large number of point ions, and the one-electron molecular orbitals are obtained from open-shell restricted-Hartree-Fock calculations of the initial state and final states. This allows the transition matrix element to be calculated in the self-consistent-field (..delta..SCF) formalism where both initial- and final-state determinantal wave functions are used, and the nonorthogonality of the two sets of molecular orbitals is taken into account. One interatomic radiative transition, the K..beta../sub 1/ satellite line, has been observed as a high-energy satellite of the Na K..cap alpha../sub 1/..cap alpha../sub 2/ line. The measured intensity ratio ..beta../sub 1/..cap alpha../sub 1/..cap alpha../sub 2/ is 1.2 x 10/sup -2/;our theoretical ratio is 1.13 x 10/sup -2/. This level of agreement between theory and experiment is similar to, but somewhat better than, that recently obtained by us for interatomic Auger transitions in NaF, using molecular orbitals from the same calculations. The other interatomic radiative transitions in NaF have considerably smaller transition energies and unobservably small transition rates. The sensitivity of the ratesmore » to the choice of Hartree-Fock Gaussian basis set is explored. Rates based on a single determinantal wave function are compared with the ..delta..SCF rates. The one-center approximation for the K..beta../sub 1/ rate was found to account for 82% of the rate« less

Auger characteristics of BaO layers on various substrates

The charge transfer interactions between a BaO monolayer adsorbate and various substrates were determined from interatomic Auger transitions (Ba 4d, Ba 5p, valence) in the 65 to 75 V energy range as well as from the changes in the O KLL lines in the 480 to 520 V region. The results showed that a greater charge transfer existed between O and oxide-forming metals such as W and Nb than on “noble” metals such as Ir and Rh. (For the purpose of this paper, these metals were designated here as “interactive” and “noninteractive” respectively.) For the interactive case, the majority of the contribution to the negative charge on the O seemed to come from the substrate rather than the Ba and these electrons also appeared to be bound more tightly. For the noninteractive substrates, there were fewer and more loosely bound electrons on the O and these were obtained mostly from the Ba rather than from the substrate. However, the smaller charge transfer noted between the O and these substrates was compensated here by a sizeable charge transfer interaction between the Ba and the substrate, i.e. the Ba giving 6s electrons to the substrate, thus becoming more positively charged. Variations were noted in the intensity of the interactions between the BaO adsorbate and different substrates of the same type (i.e. either interactive or noninteractive types), and these are discussed as are also the effects of BaO-BaO interactions.

Interaction of oxygen with sodium at 80 and 20 K.

We have studied the interaction of oxygen with evaporated Na films at 80 and 20 K, by means of core-level and valence photoemission spectroscopy and photoexcited Auger spectroscopy, as well as plasmon loss structures associated with core-level emission. The initial oxidation at 80 and 20 K is characterized by an oxygen 2p-italic resonance at about -3.4 eV with respect to the Fermi level, increasing intensity in the Na-O interatomic Auger transition, increasing width and intensity of the Na 2p-italic core level, and changes in the surface-plasmon loss region which might be interpreted as a decreasing ''free-electron'' density. Further oxidation leads to oxygen valence emission at -7.4 and -8.4 eV, which may be due to O-O interaction. Larger O/sub 2/ exposures yield a valence band with structure at -3.2, -5.3, -7.2, and -9.5 eV.

Charge state of low energy reflected particles

Low energetic noble gas particles, scattered from a metal surface, have only a small probability to leave this surface in the charged state. Even particles scattered from target atoms in the outermost surface layer are nearly all neutralised. Consequently analysis of the charged fraction of the scattered beam guarantees that information is obtained of only this layer. To quantify these low energy ion scattering (LEIS) measurements data on the ion fractions must be known, especially because these fractions have such small values. Besides this practical aspect there is the fundamental question: how does it work, this neutralization mechanism, does the transition rate for electron capture of the scattered particle depend upon its distance from the surface, or are it distant binary collisions with individual metal atoms along its trajectory which give rise to interatomic Auger transitions. Already for many years Hagstrum's theory has been applied to correct for the neutralization effect. This theory is developed to study the ejection of Auger electrons as a result of the interaction of low energetic noble gas ions with a metal surface. Due to the low ion energies used in those experiments, namely the tens eV-region, Hagstrum assumed a structureless metal surface. This means that the normal velocity of the scattered particle, v 1, takes part in determining the ion fraction and not v, the velocity of the scattered particle in its trajectory. It is a question, however, whether this theory may be applied for keV-ions. The distance between scattered particle and metal surface is for these energies namely much smaller than for tens eV's and the surface can no longer be regarded as a plane; this may result in dominating interatomic Auger transitions. But then the trajectory of the scattered particle with its velocity v plays a part in determining the occurrence of neutralization. Another mechanism that may determine the charge state of the scattered particle occurs in the violent collision; neutralization as well as (re)ionization of this particle may take place and influence its charge state. These models are treated in this review and discussed with the help of recent measurements and results of computer simulations of reflected particles.

An electron spectroscopic study of the surface oxidation of glassy and crystalline Cu-Zr alloys

Surface oxidation of three metglasses in the Cu-Zr system has been investigated by employing X-ray photoelectron spectroscopy and Auger electron spectroscopy with a view to comparing their oxidation behaviour with that of the corresponding crystalline states of the alloys. Surface oxidation of pure Zr metal has also been examined in detail using these techniques. Sub-oxides of Zr are formed during the initial stages of oxidation of Zr (at oxygen exposures <10L), while at higher exposures, ZrO2 is formed together with the highest possible sub-oxide which the authors designate as 'ZrO'. The relative proportion of 'ZrO' goes through a maximum in the range 25-50 L. Both the glassy and the crystalline states of the Cu-Zr alloys exhibit preferential oxidation of Zr. The glassy alloys exhibit a higher rate of oxidation at intermediate exposures compared with the crystalline states of the alloys; the extent of oxidation at higher oxygen exposures is, however, higher for crystalline alloys. Interatomic Auger transitions have been found in the Zr+O2 system as well as in Cu-Zr alloys.

Observations of electron stimulated desorption during SIMS profiling of insulators

There are currently two theories to describe electron stimulated desorption from the surface of oxides. Model 1 describes the process as a Franck-Condon type excitation of an adsorbed atom leading to desorption of an ion. This model predicts an isotope effect. The other (model 2) describes the process as taking place via inter-atomic Auger transitions leading to the formation and subsequent desorption, of an O + ion. This process should only occur in solids where the cation attains maximal valency and should not occur in oxides such as NiO. No isotope effect is predicted for this model. We have measured the ESD signal from oxide surfaces enriched in the stable isotope 18 O. We report 1. (1) the presence of an isotope effect from all the samples measured is consistent with model 1; 2. (2) ESD signals from samples of NiO are again consistent with model 1 but not model 2. There is good evidence to suggest that both models are, as yet, inadequate to fully describe all the processes occurring in ESD from oxide surfaces.

Reaction products and subsequent thermal decomposition of Ba films exposed to CO, CO 2, H 2O and O 2

The reactions involving thin Ba films (monolayer as well as 10-layer films on Ir) and CO, CO 2, H 2O and O 2 have been studied using Auger Electron Spectroscopy (AES) and thermal desorption. Common to these reactions is the formation of BaO which is evident from the growth of the interatomic 68 eV (Ba 4d, 5p, O 2p) Auger line and decreases in 73 eV (Ba 4d, 5p, 6s) line. From CO and CO 2 exposures, BaC 2 has also been observed to form as indicated by the Auger peak shape in the carbon Auger lines. A quantitative analysis has been used to determine the chemical equation governing each reaction. The presence of BaC 2 in BaO has been observed to increase the work function to 2.0 eV (compared to 1.4 eV for an active BaO film). Although this poisoning is moderate compared to that resulting from BaCO 3 or Ba(OH) 2 formation, it requires higher temperature for reactivation. In addition to BaC 2, BaS has also been analyzed in terms of interatomic Auger transitions. The results showed that BaS like BaO exhibited such a transition indicating a charge transfer from Ba to S in the molecular bond. BaC 2 on the other hand showed no such transition.

Interatomic Auger transitions in maximal valent V and Cr compounds

The Auger and x-ray photoelectron spectra of V and Cr compounds, where V and Cr exist in their maximal valence state with nominal 3d 0 electron configuration, have been studied. In these compounds, the mere presence of Auger peaks involving the 3d electrons unequivocally identifies their interatomic character. Furthermore, using ion bombardment to induce valence state reductions of transition metal ions, we are able to observe dynamically, and in situ, the interatomic to intra-atomic transitions of the Auger spectra. In particular, the intensity of the cation L3M23M45 transitions has been observed to increase dramatically with ion bombardment. The correlation between the interatomic Auger transition and covalency as well as the movements of the 3d electrons are also discussed.

Auger electron spectroscopy of TiO 2: Inter-and intra-atomic transitions connected with the valence band

Auger electron spectroscopy was employed as a probe of the valence band density-of-states. Cleaved TiO 2 (110) surfaces were annealed, bombarded with Ar ions, and exposed to oxygen. The Ti-LMV Auger spectral shape was changed with various surface treatments. The annealed surface was nearly stoichiometric and exhibited the inter-atomic Auger transition, connected with the valence band originating from the oxygen 2p level. i.e., L(Ti)M(Ti)V(O). The ion bombarded surface, however, showed both inter- and intra-atomic Auger processes, the latter of which, L(Ti)M(Ti)V(Ti), occurred with the titanium 3d level occupied with electrons by oxygen defect formation.

Study of electron states of solids by techniques of electron spectroscopy

Studies of valence bands and core levels of solids by photoelectron spectroscopy are described at length. Satellite phenomena in the core level spectra have been discussed in some detail and it has been pointed out that the intensity of satellites appearing next to metal and ligand core levels critically depends on the metal-ligand overlap. Use of photoelectron spectroscopy in investigating metal-insulator transitions and spin-state transitions in solids is examined. It is shown that relative intensities of metal Auger lines in transition metal oxides and other systems provide valuable information on the valence bands. Occurrence of interatomic Auger transitions in competition with intraatomic transitions is discussed. Applications of electron energy loss spectroscopy and other techniques of electron spectroscopy in the study of gas-solid interactions are briefly presented.

Interatomic Auger transitions in transition-metal oxides

Interatomic ${L}_{3}(M){M}_{23}(M)V(O)$ and ${L}_{3}(M)V(O)V(O)$ Auger transitions of some transition-metal oxides are reported for the first time. The interatomic mode of decay becomes progressively more dominant (relative to the intra-atomic mode) as the metal $d$ level gets depleted or as the oxidation state of the metal increases. The usefulness of interatomic Auger transitions in studying oxidation of metals has been examined.

Auger studies of transition metal oxides

Intra-atomic Auger transitions involving metal energy levels are found to be useful in studying the surface oxidation state as well as the oxidation of metals. Transition Metal oxides also exhibit interatomic Auger transitions, the intensities of which depend on the occupation of the metal d level. The probability of the interatomic transition is therefore highest in oxides where the metal has the d ° configuration. The competition between intra-atomic and interatomic Auger transitions in oxides will be discussed as also the use of the interatomic transitions in the study of metal oxidation.

Surface oxidation of cadmium, indium, tin and antimony by photoelectron and Auger spectroscopy

Surface oxidation of Cd, In, Sn and Sb has been investigated by employing valence bands, metal 4d levels and plasmon bands in X-ray photoelectron spectra. O(KLL), metal M 4N 45N 45, and plasmon transitions in electron-induced Auger spectra as well as Auger transitions due to the metal (metal oxide) and plasmons in X-ray-induced Auger spectra. The surface oxides are In 2O 4, CdO and a mixture of SnO and SnO 2 in the case of In. Cd and Sn respectively. The facility of surface oxidation is found to vary asIn>Cd>Sn>Sb. Inter-atomic Auger transitions involving oxygen valence bands have been identified on oxidized surfaces of Cd and In.

Interatomic Auger transition spectroscopy as a probe for the study of O 2 and CO adsorption on Al

Auger electron spectroscopy (AES) is applied as a probe to study adsorption mechanisms of oxygen and carbon monoxide on clean aluminium. Low-energy AES spectra reflect chemical effects due to adsorbates in the form of chemical shifts of Al(LVV) resulting from oxygen or carbon monoxide adsorption and interatomic Auger transitions A1(L) O(L) O(L), A1(L) C(L) C(L) showing the formation of AlO, AlC bonds. From the analysis of exposure dependences of these signals, it is demonstrated that Auger electron spectroscopy is a powerful tool for the study of adsorption phenomena.

Study of adsorption phenomena on aluminium by interatomic Auger transition spectroscopy: II. CO adsorption onto Al

Adsorption of carbon monoxide on aluminium is studied by the combination of in situ evaporation of Al and the Auger electron spectroscopy. New Auger signals which are attributed to interatomic Auger transitions in Al 4C 3 type AlC bonds are found. It is revealed that two adsorption stages exist in the adsorption process, i.e., that after the formation of the AlC bond takes place, the adsorbed layer consisting of AlC and AlO bonds is formed.

Study of adsorption phenomena on aluminium by interatomic Auger transition spectroscopy: I. Initial oxidation of Al

The initial oxidation process of aluminium is studied by using the combined molecular beam evaporation and Auger electron spectroscopy under ultra-high vacuum. At the initial stage of oxygen exposure to the clean aluminium at room temperature, the chemisorption of the oxygen atoms on the surface of aluminium is turned out to be dominant from the behaviour of the energy shifted Auger signal of aluminium. Then, the formation of the alumina (Al 2O 3) type bonding is followed, which is concluded from the interatomic Auger signals.

Synchrotron radiation photoemission studies of the adsorption of oxygen on magnesium and aluminium

The sorption of oxygen on polycrystalline films of Mg and Al has been studied by photoemission using synchrotron radiation in the photon energy range 70–150 eV. The 2p levels, situated 50.0 eV (Mg) and 72.8 eV (Al) below the Fermi level in the clean metals, shift to higher binding energy by 1.3 (Mg) and 2.6 eV (Al) on heavy oxidation. There is some evidence of a smaller shift, corresponding to a chemisorbed state, coexistent with the oxide peak. For oxygen on magnesium, an interatomic Auger transition, as previously reported from electron-stimulated Auger electron measurements, was observed for a wide range of exposures. An O 2p valence-band resonance is seen 6.1 (Mg) and 7.1 eV (Al) below the Fermi level, with accompanying shoulders 2.6 eV deeper in both systems. Oxidation also produces a marked increase in the total 2p emission from the metallic species, which we attribute to a longer electron mean free path in the oxide than in the metal. The ratio of the mean free path in oxide and metal is typically a factor of 2 for both Mg and Al, and this ratio is energy dependent.

Study of the initial oxidation of single-crystal aluminum by inter-atomic Auger yield spectroscopy

Adsorbate-substrate interactions in the initial oxidation of the (100), (110), and (111) single-crystal faces of aluminum have been studied by partial photoyield spectroscopy with the final-state energy tuned to coincide with an inter-atomic Auger transition [A1 (2p) O (2s) V.B.]. This higher energy final state is shown to provide significantly better surface sensitivity than that obtained with typical constant final-state spectroscopy using low-energy, secondary electrons. Models for the initial oxidation are discussed.

Interatomic Auger processes in NaCl

The decay rate of a Na $2s$ hole in NaCl by interatomic Auger transitions associated with a valence electron located on the nearest-neighbor chlorines is calculated. The Cl orbitals of the octahedral cluster surrounding the Na $2s$ vacancy are combined into molecular orbitals and the Auger transitions to spherical waves having $l\ensuremath{\le}4$ are determined. The total calculated rate is nearly twice that calculated for the $2s$ vacancy in a free Na atom and supports the earlier suggestion by Citrin that the effective valence charge on the ${\mathrm{Na}}^{+}$ ion due to the ligand orbitals is even larger in NaCl than it is in the free atom.