Interatomic Auger Processes Research Articles

Early stage of the electron kinetics in swift heavy ion tracks in dielectrics

A Monte Carlo approach was applied for simulations of the early stage (first tens of femtosecond) of kinetics of the electronic subsystem of silica $({\text{SiO}}_{2})$ in tracks of swift heavy ions (SHIs) decelerated in the electronic stopping regime. At the first step multiple ionizations of target atoms by a projectile (${\text{Ca}}^{+19}$, $E=11.4\text{ }\text{MeV}/\text{amu}$) were described that gave the initial spatial distributions of free electrons having different momenta as well as distributions of holes in different atomic shells. Spatial propagation of fast electrons results in secondary ionizations of target atoms as well as in energy transfer to the lattice at times much shorter than the times of atomic oscillations (phonons). The well detected front of excitation in the electronic and ionic subsystems is formed due to this propagation which cannot be described by models based on diffusion mechanisms (e.g., parabolic equations of heat diffusion). At times $\ensuremath{\sim}10\text{ }\text{fs}$ after the projectile passage, about $\ensuremath{\sim}0.1\mathrm{%}$ of the energy is already transferred to the lattice. About 63% of the energy deposited by the ion is accumulated in holes at these times. Calculated distributions of these holes through the atomic shells are in excellent agreement with the spectroscopy experiments. Comparison with these experiments demonstrated also that relaxation of the electronic subsystem in SHI tracks in solids cannot be described adequately without taking into account intra-atomic and interatomic Auger (Knotek-Feibelman) processes.

Effect of inter-atomic Auger processes on relaxation of electronic vacancies at deep levels of highly ionized atoms in swift heavy ion tracks

We present the results of Monte-Carlo simulations of excitation and subsequent relaxation of the electronic subsystem of quartz in the close vicinity of the swift heavy ion (SHI) trajectory. Comparison between numerical results and experiments detecting X-ray K-shell emission of silicon atoms (KαL n transitions) from quartz aerogel irradiated with SHIs demonstrated that the adequate description of the relaxation kinetics of the excited electronic subsystem of a solid in a SHI track cannot be possible without taking into account fast inter-atomic Auger processes which are absent in gaseous targets.

Electronic effects in ion-stimulated desorption of positive halogen ions from semiconductor surfaces

Energy distributions of ions emitted during either 6–10 keV O 2 + or low-energy (<1 keV) electron bombardment have been studied for different ion species implanted into gallium arsenide. Some positive ions, such as F + and Cl +, have been shown to exhibit close similarity between ion-stimulated and electron-stimulated energy distributions indicating the dominance of electronic effects in ion-induced ejection of these ions. A tentative explanation based on an interatomic Auger process is given for the ion-stimulated ejection and results are correlated with measurements of the positive secondary ion yield dependence on ionization potential.

Electron emission in collisions of slow rare gas ions with partially cesiated W (110)

We report the spectra of electrons emitted after slow collisions (50 eV) of rare gas ions (He+, Ne+, Ar+, and Xe+) with partially cesiated W(110) surfaces. The electron spectra are discussed in terms of the two interatomic Auger processes, Auger Capture and Auger Deexcitation and the intra-atomic Auger process, Autodetachment of Rg−* (He−* (1s 2s22S) in the case of He). Model calculations including these processes can qualitatively reproduce the measured spectra as well as experimental values for the electron emission coefficients, γ. This is demonstrated for collisions of He+ ions and thermal metastable He atoms with clean and partially cesiated W(110).

Electrons from ultra- and interatomic Auger processes in low-energy grazing collisions of H + and H +2 with W(110) partially covered by alkali atoms

Electron energy spectra from slow H + and H + 2 ions (below 100 eV, typically) colliding under grazing incidence with W(110) surfaces are reported. The surface work function was varied by the exposure to alkali (Li, Na, Cs) atoms. The electron emission spectra can be interpreted in terms of various inter- and intra-atomic Auger processes: for clean W(110) Auger capture involving two electrons of the surface dominates both for H + and H + 2 collisions. For sufficiently large coverages by alkali atoms resonant capture of one or two surface electrons leads to the formation of excited states of the neutralized projectile and to the formation of core excited states with two electrons occupying outer-shell orbitais (H −(2l2l′) and possibly H − 2 (1s2l2l′), respectively. These states manifest themselves by their decay via Auger deexcitation and, in particular, by autodetachment. For H + 2 projectiles the implications of these electronic processes for the molecular survival probability are discussed.

Electrons from intra- and interatomic Auger processes in low-energy grazing collisions of H+ and H2+ with W(110) partially covered by alkali atoms

Electrons from intra- and interatomic Auger processes in low-energy grazing collisions of H+ and H2+ with W(110) partially covered by alkali atoms

Autoionization and autodetachment in collisions of slow inert gas ions with partially K covered W(110) surfaces

We report the spectra of the electrons emitted after slow collisions (50 to 1000 eV) of inert gas ions (He +, Ar +, Xe +) with partially K covered W(110) surfaces. At collision energies below 250 eV autodetachment of the excited negative ion states X −∗ (He −∗(1s2s 2), Ar −∗(3p 54s 2), Xe −∗(5p 56s 2)), formed by resonant two-step capture of two surface electrons, is collisions the formation of the He −∗ resonance could be confirmed by model calculations which simulate the electron spectra. At higher energies in addition the autoionization of Ar ∗∗(3p 44s 2) and K ∗∗(3p 54s 2) is detected. These states are formed via promotion in inert gas ion collisions with the surface adsorbed K atoms. Besides these intra-atomic Auger processes also interatomic Auger processes involving one or two electrons of the surface (Auger de-excitation and Auger capture) are observed.

Intra- and inter-atomic auger processes in collisions of low energy He++, He+, He* (23,S, 21 S) and Li+ with Li covered W(110) surfaces

Electron spectra from He++, He+ and Li+ (10 to 1500 eV) ions colliding under grazing incidence with Li covered W (110) surfaces are reported. The results are compared with those obtained from thermal collisions of (23S; 21S) metastable He atoms. In these collisions 1s vacancies are either produced during the collision event (energetic He+ (Li+) collisions) or are brought into the collision (slow He++ (He+, He*) collisions). Population of the 2s orbitals by two electrons produces states which decay by intraatomic Auger processes: we observe autoionization of He** (2s2) and Li** (1s 2s2) as well as autodetachment of He−* (1s 2s2). Alternatively the 1s-holes in the projectile or target (Li) can be filled by Auger processes involving one or two surface electrons. The processes leading to electron emission are studied as a function of the Li coverage in the submonolayer region (0≦ΘLi≦1Ml) and as a function of the projectile energy. It is concluded that with one or two 1s vacancies present in the projectile the double capture of two surface electrons constitutes an important process responsible for electron emission of low work function surfaces.

Electrons from intra- and inter-atomic Auger processes in low-energy He 2+ collisions with partially alkalated W(110) surfaces

Electron energy spectra from He 2+ (100 eV) ions colliding under grazing incidence with partially alkali(Na, Cs)-covered W(110) surfaces are reported as a function of the alkali coverage θ (0 ⩽ θ ⩽ 1 ML). For clean W(110) the sequence of electronic transitions during the collision is similar as reported previously for other clean metals. For θ > 0.4 ML He 2+ is preferentially neutralized into He ∗∗ via He +∗(n = 3) states by two-step resonant capture of two electrons. Electrons are mainly emitted by Auger de-excitation of He +∗(n = 3 → 2) and autoionization of He ∗∗ to He + at this stage. In a second step the He + ions produced in the cours neutralization process again capture resonantly one or two electrons. Auger de-excitation of He ∗ and autodetachment of He −∗( 2S) then primarily give rise to electron emission whereby He neutrals in the ground state are formed. It is furthermore concluded that in a major portion of the collision events three electrons are emitted.

Oxidation of Mg adsorbed on Ru(001): A photoemission study

We studied the interaction of oxygen with Mg overlayers adsorbed on Ru(001). Soft x-ray synchrotron radiation was used to explore photoemission from the valence band, the O 2p, O 2s, and Mg 2p levels, as well as intra- and interatomic Auger processes. The photoemission results are complemented by thermal desorption data. The O/Mg/Ru system shows photoemission features characteristic of molecular orbitals of a dioxygen species.

Lifetime and screening of the C 1s photoemission in graphite.

High-resolution soft-x-ray photoemission spectra of the C 1s line of graphite have been analyzed to determine the various contributions to the linewidth and line shape. The natural lifetime width \ensuremath{\Gamma} is 210 meV, much larger than those of small molecules (for which \ensuremath{\Gamma}\ensuremath{\simeq}110 meV), or that calculated for the isolated carbon atom (for which \ensuremath{\Gamma}=56 meV). The phonon width is no larger than 50 meV. The general trend toward an enhanced Auger decay rate from the isolated atom to the small molecule and solid is indicative of significant contributions from interatomic Auger processes. However, the large increase in width from molecules to solids must be due to another phenomenon, perhaps to the formation of the excitonic final state in the solids. Our line-shape analysis confirms that the line has an asymmetry like that associated with metallic screening, which had been earlier related to the excitonic final state in graphite, but the singularity index is significantly smaller than that obtained in the previous analysis.

On the appearance threshold and the reverse motion of ions in electron-stimulated desorption

We report here the first observation of the ESD appearance threshold energy exceeding the energy of the inner shell ionization, which is interpreted as a consequence of a shake-off process. For a tightly bound chemisorbed system we discovered the possibility of reverse ion motion due to an interatomic Auger process. The observed expansion of the ion retardation curves and the weakening of the isotope effect in the case of reverse ion motion is discussed in terms of an improved ESD model considering local surface field relaxation.

Electron bombardment induced desorption of oxygen from LiNbO 3

Low energy (1 keV) electron bombardment of LiNbO 3 single crystals causes the emission of O + ions with a kinetic energy of 20 eV. The emission process can be identified as a Knotek-Feibelman mechanism, whereby the creation of Nb 4p core-holes is followed by an interatomic Auger process which leaves oxygen in a highly repulsive state. This oxygen subsequently desorbs.

A secondary ion mass spectometry study of the desorption mechanisms of F + from a LiF surface bombarded with Ar + and Ar 2+

Desorption yields of F + from solid LiF were measured as a function of the energy (1–5 keV) of bombarding Ar + and A 2+. The production of F + induced by the two argon ion species was separated into contributions from a mechanism which generates F + by inelastic energy transfer between an argon ion and a surface fluorine atom and a second mechanism, producing a F + intensity which is relatively constant over the 1–5 keV range of ion energy, which involves an interatomic Auger process between an Ar 2+ ion and a surface fluorine atom. A tentative explanation for the interatomic Auger process is given based on physical and energetic considerations.

Model calculations for the anomalous M4,5VV Auger spectrum for dilute palladium in silver.

A model Hamiltonian is introduced for an impurity in a filled band host, and where the intra-atomic electron interaction U on the impurity site is included. The Auger spectrum ending in two holes in the valence band (VV) is solved exactly for the impurity. For a certain range of values of U there is besides the normal Cini-Sawatzky quasiatomic peak a new, discrete two-hole state at much lower binding energy. For a one-dimensional system the two-hole problem is treated exactly and the solution is used to explain the nature of the new two-hole bound state. Also, a self-consistent mean-field type of treatment is applied to elucidate the origin of the two bound states. Extensions of the model to include the intra-atomic Coulomb interaction on the host atoms show also that interatomic quasiatomic Auger processes contribute to the decay of the initial impurity core hole. In connection with this, a detailed comparison between the Cini and Sawatzky expressions for the spectral distribution function is performed. The solution for the model Hamiltonian is used to explain the strong anomaly of the Pd ${M}_{4}$,5VV Auger spectrum for dilute palladium alloys with silver. By a schematization of the palladium local density of states, a direct comparison between the calculated and the experimental spectrum is made and a good agreement is found. By variations of the local density of states and the intra-atomic Coulomb repulsion, the conditions for the appearance of the anomalous peak are investigated.

Making and breaking of chemisorption bonds

A brief review of recent contributions from surface physics to the understanding of chemisorption and desorption is given. The first question that is addressed is: What happens to a molecule when itcomes close to a metal surface? On the basis of self-consistent calculations for atoms [1,2] and molecules [3] on simple metal surfaces general trends and concepts are described and a scenario for H 2 on Mg(0001) is presented [4]. Except for the physisorption occuring at distances ∼ 3 A ̊ or larger outside the surface the molecule-surface interaction is characterized by substantial redistributions of the electrons. Molecular electron energy levels are shifted and broadened. In particular, the antibonding affinity level of H 2 is shifted to lower energies. The spatial redistribution of the electrons is beneficial for an increased electron-proton interaction, and thereby for the chemisorption energy of H, i.e. for the constituents of H 2. The intramolecular bond between the constituents, on the other hand, is weakened on the surface by at least one order of magnitude and at some instances even becomes repulsive. The reason for this breaking of the molecular bond is the downward shift at the antibonding molecular-orbital resonance allowing its partial filling with conduction electrons from the metal [5]. Also for the dynamic aspects of the chemisorption, the variation of electron structure upon the molecular approach to the surface is of key importance. For instance, there can be electronic mechanisms for the dissipation of the molecular translational energy, required for the trapping of the molecule [6]. For the emission of ions, occuring in, e.g., sputtering, the variation of the electron structure of the emitted particle is also very important [7]. The second question concerns the desorption of adsorbed species upon electron or photon impact. A brief review is given of proposed mechanisms for electron- and photon-stimulated desorption, ESD and PSD, respectively [8]. The Redhead-Gomer-Menzel mechanism for ESD [9] assumes the incoming electron to excite a valence electron contributing to the adsorbate-substrate bond into an antibonding orbital. This changes the attractive potential energy curve of the adsorbate into a repulsive one, and the adparticle finds it energetically favourable to leave the surface, i.e., to desorb. The mechanism of Knotek and Feibelman [10] originated from studies of O + desorption from TiO 2, where the O + ion yield was found to increase considerably at incident electron energies that coincide with ionization potentials of core electrons of the metal ion. Their mechanism is an Auger decay model, where after the creation of a core hole there is a deexcitation by an interatomic Auger process, where due to the suddenness of the process and the Coulomb repulsion between electrons the Auger electron is likely to be followed by a second emitted electron. The resulting O + ion ends up on a repulsive potential-energy curve and desorbs. The mechanism has later been extended to intraatomic Auger processes and PSD and successfully applied to selective breaking of bonds [8].

X-ray photoemission study of Ce-pnictides

All the Ce pnictides crys allize in the NaCl structure with lattice constant increasing systematically from CeN to CeBi. Since Ce has the largest $4f$-orbital radius in the entire lanthanide series, it is quite interesting to follow the evolution of his level as the distance between the Ce atoms is varied. The core levels and the valence-band region of CeN, CeP, CeAs, and CeSb have been studied by high-resolution x-ray photoemission spectroscopy. The $4d$ and $3d$ core-level spectra of Ce show clearly that the $4f$ state remains strictly localized in all compounds. In CeN a superposition of lines corresponding to two valences is found which reveals the intermediate valence character of this compound. In the valence-band spectrum, the $4f$ level is found to be pinned at the Fermi level and superimposed on the $5d$ states. The extended valence states originating from the anion $p$ states are located at higher binding energies. In the compounds of heavier rare-earth elements the $4f$ state moves away from the Fermi level and gradually overlaps in energy with $p$ states, so that the probability for interatomic Auger processes in the photoemission final state increases rapidly. The natural width of the $4f$ peak is already 0.8 eV in CeP, and becomes so large in CeAs and CeSb that this line can no longer be unambiguously identified in the other valence-band structure. Finally, it can be concluded from the similarity of the $4d$ spectrum of metallic Ce and CeN that the $4f$ electron remains localized in the $\ensuremath{\alpha}$ phase of this metal.

KVV Auger spectrum of oxidized lithium

The effect of oxygen adsorption on the KVV Auger signal from lithium has been investigated. After using deconvolution techniques to extract electron‐loss‐independent structure from the measured data, a double‐peaked N (E) lithium spectrum is obtained. The two peaks lie 11 and 16 eV below the position of the single N (E) peak observed for the clean lithium surface, and have a height ratio of roughly two to one. The corresponding KVV oxygen N (E) spectrum is similar in width and shape, being dominated by a peak at 505 eV. This dominant peak has a peak height, however, that is more than three times that of the other structure in the oxygen spectrum. These results are discussed in terms of band structure, relaxation effects, and interatomic Auger processes occurring in ionic‐compound surfaces.

Electronic Structure and4f-Hole Lifetime in Rare-Earth Borides

X-ray photoelectron spectra of rare-earth borides exhibit systematic variations of the $4f$ lifetime width with the rare-earth number across the series, with the $4f$ binding energy, and with the number of boron neighbors. Valence band spectra and theoretical band structures make it possible to understand these observations in terms of an interatomic Auger process, here a $5d\ensuremath{\rightarrow}4f$ recombination, with simultaneous excitation of an electron-hole pair from boron $2sp$ states.

Interatomic Auger processes and the density of states

The correlation between the line shape of Auger peaks and the density of states near the surface has been the subject of recent controversy. In certain cases, it has been possible to obtain the density of states by numerical deconvolution of a KVV peak (Amelio, 1970) or directly using a KLV peak (Cardona et al., 1973). However, the extension of this technique to transition metals (Cu, Zn) has encountered serious difficulties, related to the perturbation created by the presence of localized charges either in the initial or in the final state, although it is not yet clear why this perturbation is strong only in certain cases. The purpose of the present communication is to show a series of results that can throw some light on the abovementioned problem. The main point is that Auger processes of interatomic type, as those occurring in the INS technique of Hagstrum, are free of these perturbations. Recently, the authors have studied the line shape of the Auger peaks of O, C, N and S adsorbed on Cu, Ni and Fe. These results show that only that part of the Auger structure originated by interatomic transitions between substrate and adsorbate atoms can be related to the local density of states (LDOS). The rest of the structure, due to normal intraatomic processes, is dominated by the spectral terms in the final configuration of the ion. This new interpretation allows a separation of perturbation effects and clarifies the contribution of the LDOS to the peak line shape. In this communication, we present the line shape analysis of the L 2,3 VV and KVV Auger peaks of Mg and O in MgO. Due to the strong ionic character of this compound, the L 2,3 VV peak of Mg ++ is mainly due to interatomic processes between Mg ++ and O = ions, whereas the KVV peak of O is mainly due to interatomic processes. This analysis shows that good agreement exists between the L 2,3VV Mg ++ Auger peak and the self-convolution of MgO density of states, whereas the KVV Auger peak of O = is dominated by the spectral terms of the final configuration. Only a small peak in the high energy side of the latter peak can be related to the density of states and could be interpreted as an interatomic transition between two neighboring oxygen ions, in agreement with the interpretation given by others.