Autoionization Emission Research Articles

Observation of electron-excited autoionization emission from nickel in the intermetallic compound NiTi

Autoionization electron emission from nickel in NiTi, initiated by electron-beam excitation is reported. It is the first observation of such electron-excited Ni emission, and unlike the case of elemental nickel, the feature is distinctly resolved from the Ni M 2,3VV Auger line. The measurements were done at temperatures corresponding to the CsCl-type phase of NiTi, and the emission intensity was found to depend critically on the annealing treatment of the sputter-cleaned sample. The appearance of Ni autoionization emission in Niti is attributed to changes in the distribution of the empty and filled d-states at the nickel sites on alloying with titanium. Emission energy shifts are estimated on the basis of reported theoretical band-structure calculations for NiTi.

Autoionization and Auger features of clean and oxygen-exposed surfaces of Fe, Co and Ni.

The ${M}_{2}$,3VV Auger and 3p core loss spectra for clean and oxygen-exposed surfaces of the transition metals Fe, Co, and Ni have been studied. All the peaks observed in the Auger spectra are accounted for in terms of both atomic and bandlike ${M}_{2}$,3${\mathrm{M}}_{4}$,5${\mathrm{M}}_{4}$,5 transitions and an autoionization emission involving the decay of a localized excited state. For the clean surfaces the localized excitation and subsequent deexcitation by autoionization emission is considered, within a quasiatomic model, to be 3${p}^{6}$3${d}^{n}$\ensuremath{\rightarrow}3${p}^{5}$3${d}^{n}$${d}^{\mathrm{*}}$\ensuremath{\rightarrow}3p $^{6}$3${d}^{n\mathrm{\ensuremath{-}}1}$+ef. However, upon oxidation of these surfaces, the splitting observed in the 3p core loss spectra and the energy of the observed autoionization peaks indicate changes in the nature of the excitation due to the chemical environment. This is contrary to the accepted notion for such quasiatomic processes, which are considered essentially unaffected by the chemical environment. Further, for these oxidized surfaces, the observed autoionization features are shown to be associated with the excitation of 3p electrons to a localized state at the bottom of the conduction band as opposed to the empty d states at the top of the valence band.

Autoionization emission by electron impact in the3dtransition-metal seriesSc21toCu29

Autoionization emission associated with direct recombination of the resonant $3p\ensuremath{\rightarrow}3d$ transitions have been examined for all the $3d$ transition-metal elements from $_{21}\mathrm{Sc}$ to $_{29}\mathrm{Cu}$. Two characteristic trends with atomic number $Z$ are observed which correlate with those previously reported in the $3p$-electron energy-loss spectra. Firstly, the intensity of this emission decreases with increasing $Z$. Secondly, the separation of the emission peak from the $3p$ threshold is large (\ensuremath{\sim} 10 eV) toward the beginning of the series ($_{21}\mathrm{Sc}$ through $_{24}\mathrm{Cr}$) and abruptly drops to a few electron volts near the middle of the series ($_{25}\mathrm{Mn}$, $_{26}\mathrm{Fe}$). This latter reduction in peak separation causes the autoionization and (${M}_{2,3}\mathrm{VV}$) Auger emission to merge into a single spectral feature. We show for $_{25}\mathrm{Mn}$ (and $_{22}\mathrm{Ti}$) that the two contributions can be separately identified by means of oxygen-dosing experiments. Oxidation dramatically shifts the Auger emission to lower energy leaving the autoionization emission as a separate peak pinned above the $3p$ threshold. These measurements shed light on the spectral details in the $3p$ line shapes of the autoionization and the electron-energy-loss spectra. We report angle-resolved measurements for Cr which show different angular dependences of the emission and absorption processes.

Summary Abstract: Autoionization emission from transition metals: An electron stimulated analog of resonant photoemission

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Electron Spin Polarization in Inner-Shell Excitations in a Solid

Using a ferromagnetic solid as an array of spin-polarized atoms allows one to measure directly the spin dependence of electron scattering in core excitations. Strong spin polarization of the $3p\ensuremath{\rightarrow}3d$ energy-loss electrons scattered from ferromagnetic ${\mathrm{Fe}}_{83}$${\mathrm{B}}_{17}$ has been observed. The large spin polarization in the autoionization emission satellite above the $\mathrm{MMM}$ Auger line is discussed.

Line-shape analyses ofXVVAuger spectra ofp(1×1)-V3Si(100): Evidence for autoionization emission

In the light of recent advances in understanding the Auger process, the local electronic structural origins of selected core-valence-valence ($\mathrm{XVV}$) Auger line shapes are analyzed for a transition-metal silicide prototype, ${\mathrm{V}}_{3}$Si. We report for clean $p(1\ifmmode\times\else\texttimes\fi{}1)$-${\mathrm{V}}_{3}$Si(100) Auger spectra that include the region of the vanadium ${M}_{2,3}\mathrm{VV}$ and ${M}_{1}\mathrm{VV}$ and the $\mathrm{Si}{L}_{2,3}\mathrm{VV}$ transitions. We compare the measured line shapes to spectra we generated based on the muffin-tin local density of states (DOS) calculated self-consistently by Klein et al. Good agreement in both the $\mathrm{Si}pp({L}_{2,3}{M}_{2,3}{M}_{2,3})$ and the $\mathrm{V}dd({M}_{2,3}{M}_{4,5}{M}_{4,5})$ peak positions between experiment and calculation verified that the final-state hole-hole repulsion for $\mathrm{Si} ({U}_{\mathrm{pp}})$ and $\mathrm{V} ({U}_{\mathrm{dd}})$ are both \ensuremath{\sim}0 eV. Also, the $\mathrm{Si}{L}_{2,3}\mathrm{VV}$ spectrum resembles that of elemental Si in that the line shape is predominantly a self-fold of the $\mathrm{Si}3p$ DOS. However, an unexpected result is that the V spectral region above the ${M}_{2,3}\mathrm{VV}$ threshold possesses a broad (\ensuremath{\sim}30-eV-wide) intense feature that is not amenable to conventional interpretation in terms of the ${M}_{1}\mathrm{VV}$ transition or ${M}_{2,3}\mathrm{VV}$ double-ionization or plasmon-gain satellites. We attribute this observation to the presence of Fano autoionization emission associated with deexcitation of the resonant $3p\ensuremath{\rightarrow}3d$ transition. Supporting evidence comes from a comparison of our x-ray- and electron-stimulated Auger spectra, and to the line shape of the $3p$ loss spectrum. In addition, oxygen-dosing Auger and x-ray photoelectron spectroscopy experiments (0-20 L) (1 langmuir = 1 L = ${10}^{\ensuremath{-}6}$ Torr sec) indicate dramatic $\mathrm{Si}{L}_{2,3}\mathrm{VV}$ line-shape changes associated with oxidation, similar to that observed previously for ${\mathrm{Pd}}_{4}$Si. The initial oxidation rate is \ensuremath{\sim}${10}^{2}$ faster than that for elemental Si. We hypothesize that the dissociation of ${\mathrm{O}}_{2}$ is a rate-determining step in the oxidation of elemental Si, but is rapid at transition-metal sites in the silicides. Atomic oxygen then rapidly spills over to the neighboring silicon sites where oxidation subsequently occurs.

Autoionization Emission from Transition Metals by Electron Impact

Intense gain satellites associated with the ${M}_{2,3}\mathrm{VV}$ Auger transition have been observed for representative $3d$ transition metals. The satellites have line shapes and intensities that resemble those of the ${M}_{2,3}$ electron-energy-loss and resonant photon-excited spectra of these materials and are identified as being due to autoionization electron emission. This identification satisfies the expectation that the Fano effect should be manifest in both absorption and emission experiments, whether photon or electron excited.