Angular Distribution Of Auger Electrons Research Articles

Angular distribution of Auger electrons following photoionization

An overview is given of the angular distribution of Auger electrons following photoionization in atoms. Emphasis is placed upon experiments observing Auger electrons alone and in coincidence with the primary ejected photoelectron, first within the two-step model, and finally for some cases which go beyond this model.

Alignment after inner-shell ionization by electron impact near and at threshold

The alignment A 20 of Ar + (2 p 3 2 −1) , Mg + (2 p 3 2 −1) and of Zn + (3 d 3 2 −1) ions after electron impact ionization have been measured via the angular distribution of Auger electrons and the linear polarization of fluorescence radiation for the range of impact energies E 0 from several times the threshold energy down to near or even to the threshold. The experimental results are compared to PWBA and DWBA values which are available for Ar and Mg 2 p 3 2 ionization. From the alignment values obtained near or at threshold it is concluded that two-electron partial waves with L = 0 and L > 0 (for Zn 3 d 3 2 ionization this can be specified up to L = 0,1,2) contribute to the Wannier threshold cross section.

Angular distributions of Auger electrons from surfaces: Conclusions from full multiple‐scattering simulations

We address some of the recent controversies regarding the interpretation of Auger electron diffraction patterns by simulating those from the 64 eV M2,3VV and the 914 eV L3VV lines from a Cu(001) surface. The lower‐energy diffraction pattern, in particular, is found to be strongly dependent on the angular momentum l of the emitted Auger electrons, in line with the observations of some other authors. However, we find that both peaks and dips are observed on the same diffraction pattern at the projections of different atomic rows, indicating that the angular momentum of the source wave cannot be the only determinant of these features. We find that the distance of the scatterer from the source, as well as overlap and interference effects from scatterers in other atomic rows to be an equally important determining factor. On comparing our simulations with experimental diffraction patterns, we find that, for an l=3 source wave, our simulations agree remarkably well with the data from both Auger lines.

Alignment effects in fast ion-atom collisions

Angular distribution of Auger electrons and X-rays, and X-ray polarization in high energy ion-atom collisions, measured in the last few years, are reviewed. The excitation mechanisms of interest are the resonant and nonresonant transfer and excitation (RTE and NTE respectively), capture, ionization, excitation and transfer ionization. An alignment of the excited states formed through these excitation mechanisms is often observed and seems to be strongly dependent on the excitation mechanism.

Auger electron angular distributions from Pt(111): The effect of adsorbed iodine monolayers

AbstractComplete Auger electron angular distributions have been measured from a Pt(111) substrate in the presence and absence of three distinct iodine adlattices in order to investigate the effect of adsorbate layer structure on substrate emission. The adlattices investigated were Pt(111)(□3 × □3) R30°‐I, Pt(111)(□7 × □7)R19.1°‐I and Pt(111)(3 × 3)‐I. Three Pt Auger transitions were employed (65, 258 and 1968 eV) to explore the effect of Auger electron kinetic energy. For each kinetic energy investigated, the qualitative features of the substrate angular distributions were the same in the presence and absence of the iodine overlayer, although small quantitative differences were observed. An energy‐dependent quantitative effect was also observed, in which the absolute intensities of the distributions were attenuated substantially at lower kinetic energies but were relatively unaffected at higher kinetic energies. These results show that lower kinetic energy Auger electron angular distributions (<300 eV) are relatively sensitive to the presence of the adlattice, although careful quantitative measurements and analyses are required to observe the differences in the distributions. The origin of attenuation features in the distributions is not yet fully understood, and further experimental and theoretical development is required to relate the observed attenuation features to the surface structure.

Auger electron angular distributions from Al(100): Resolution of an apparent contradiction

Complete Auger electron angular distributions have been measured from an Al(100) surface for the two principal Auger transitions, 64 eV(LMM) and 1388 eV(KLL). The results are consistent with angular distributions measured for other crystalline samples: intensity minima are observed along the internuclear directions at low kinetic energies (<100 eV), while intensity maxima are observed along those same directions at higher kinetic energies (≳300 eV). The earliest reported polar profiles for the Al(100) 64 eV Auger peak appeared to contradict this trend, exhibiting intensity maxima along the surface normal. However, the present results do not confirm that aspect of the earliest measurements. Furthermore, the present results are found to be in good agreement with more recent measurements reported by other laboratories, suggesting that the apparent discrepancy is attributable to experimental factors in the earliest measurements. That is, the Auger emission from Al(100) follows the trends observed for other elements.

Effects of channel interaction, exchange, and relaxation on the angular distribution and spin polarization of Auger electrons from noble-gas atoms.

The angular anisotropy ${\mathrm{\ensuremath{\alpha}}}_{2}$ and spin polarization ${\ensuremath{\xi}}_{2}$ parameters have been calculated for Ar ${\mathit{L}}_{3}$MM, Kr ${\mathit{M}}_{4,5}$NN, Xe ${\mathit{N}}_{4,5}$OO, and Xe ${\mathit{M}}_{4,5}$NN Auger transitions using the multichannel multiconfiguration Dirac-Fock method (MMCDF). The results of our MMCDF calculations are in good general agreement with experiment. We have also determined the separate contributions of the exchange, relaxation, and channel interaction to the angular anisotropy of the Auger process. We have shown that especially for Auger lines having at least two strong partial amplitudes corresponding to different values of the orbital angular momentum both the exchange and channel interaction can change the value of the ${\mathrm{\ensuremath{\alpha}}}_{2}$ parameter significantly and sometimes even change its sign. Because of the additional eigenchannel phase shifts the ${\mathrm{\ensuremath{\alpha}}}_{2}$ parameters are more sensitive to channel interaction than to the exchange. In contrast the relaxation has a small effect on the angular distribution of Auger electrons. A comparative study showed that the ${\mathrm{\ensuremath{\alpha}}}_{2}$ parameters are usually less sensitive to correlation than the branching ratios or even the total Auger rates. This was traced back to universal scaling of the modulus of transition amplitudes and to the existence of one dominating ionization channel in most transitions. The ${\ensuremath{\xi}}_{2}$ parameters were small for all strong Auger lines and exceedingly sensitive to all correlation effects.

Subshell-dependent relaxation in the Auger effect.

The effect of relaxation on KLL, LMM, and MNN Auger-electron spectra of Ne, Mg, Ar, Ca, Zn, and Kr has been studied by the Dirac-Fock method. The relaxation is shown to depend universally on the Auger-electron energy and on the number of peel electrons (${\mathit{N}}_{\mathit{r}}$) in subshells above the final-state holes. In the sudden-approximation limit (large ${\mathit{N}}_{\mathit{r}}$), the relaxation saturates to 14% reduction of the Auger rates in analogy to the 20% reduction of deep inner-shell Dirac-Fock photoionization cross sections well above threshold. The isotropy of the relaxation is emphasized by its small influence on the angular distribution of Auger electrons.

Angularly resolved Auger rates of LiF and HF.

Angularly resolved Auger rates are investigated. The angular distributions of Auger electrons display a strong dependence on the final dicationic state which is created by the Auger process. The angular distributions show interference structures which can be related to the geometry of the molecule. Although there are presently no experimental data on angularly resolved Auger rates, we suppose that the angular distributions are measurable, e.g., by coincidence techniques

Auger electron and photoelectron angular distributions from surfaces: Importance of the electron source wave.

Angular distributions of Auger electrons and of photoelectrons emitted at high (&gt;500 eV) and low (100 eV) kinetic energies are compared. The high-energy patterns can be interpreted as forward-projected images of real space crystal geometry. At low energies, the angular distributions are dominated by diffraction effects, and the structural information is no longer obvious. On the other hand, the sensitivity to the geometry and the atomic composition of the surface is here dramatically enhanced. Moreover, the angular momentum character of the source wave strongly influences these patterns.

Structural determination of a palladium single crystal using photoelectron and Auger-electron angular distributions.

The angular distributions of Pd 3${\mathit{d}}_{5/2}$ (${\mathit{E}}_{\mathit{k}}$=1151.5 eV), Pd 3${\mathit{d}}_{3/2}$ (${\mathit{E}}_{\mathit{k}}$=1146.3 eV), and Pd 3${\mathit{p}}_{3/2}$ (${\mathit{E}}_{\mathit{k}}$=954.5 eV) photoelectrons and Pd ${\mathit{M}}_{5}$VV Auger electrons (${\mathit{E}}_{\mathit{k}}$=327.5 eV) were investigated for a single-crystal Pd(100) surface. We have observed pronounced anisotropies in both photoelectron and Auger-electron intensities as a function of the polar emission angle \ensuremath{\theta} along the principal azimuthal directions 〈001〉 and 〈011〉. We find that the observed angular distributions can be explained by the forward scattering of the outgoing electron waves (regardless of whether they are photoelectrons or Auger electrons) by the atoms overlying the emitter. Direct structural information was obtained by using the kinetic-energy dependence of the angular distributions of photoelectrons and Auger electrons in that zeroth-order forward-focused peaks are independent of energy, whereas the first-order interference intensities shift with energy. Forward-scattering spectroscopy is a very powerful technique in determining the relaxation of surface atoms despite a lack of translational symmetry normal to the surface. We have shown that the Pd(100) surface does not relax within 1.0\ifmmode^\circ\else\textdegree\fi{}. The experimentally observed angular distributions are consistent with a single-scattering model in which forward focusing dominates the photoelectron and Auger-electron angular distributions.

Angular distribution of Auger-electron emission following double-electron capture in C6++He collisions.

Angular distributions of Auger electrons ejected in 60-keV C{sup 6+}+He collisions were measured. This study extends previous work where cross sections for the production of the configuration 2{ital lnl}{prime} ({ital n}{ge}3) have been determined from measurements at the observation angle of 0{degree} only. For {ital n}=3 and 4, the present angular distributions were measured to be anisotropic, whereas for {ital n}=5 and 6 the electron emissions were found to be isotropic within the experimental uncertainties. The integration over the observation angle yields total production cross sections {sigma}{sub 2{ital l}{ital n}{ital l}} that were found to follow an {ital n}{sup {minus}3} law, in agreement with previous calculations. The observed angular distributions are discussed in terms of different capture mechanisms.

Effect of intermediate coupling on angular distribution of Auger electrons.

The Auger decay anisotropy parameter ${\mathrm{\ensuremath{\alpha}}}_{2}$ of angular distribution has been calculated for the ${\mathit{M}}_{45}$-NN transitions in Kr and ${\mathit{M}}_{45\mathrm{\ensuremath{-}}}$NN and ${\mathit{N}}_{45\mathrm{\ensuremath{-}}}$OO transitions in Xe atoms using the multiconfiguration Dirac-Fock model in intermediate coupling with configuration interaction. For transitions involving many contributing partial waves and leading to the strongly mixed final ionic states in intermediate coupling, the effects of relativity and spin-orbit interaction are found to be quite important for the calculations of ${\mathrm{\ensuremath{\alpha}}}_{2}$ values. The existing discrepancies between theory and experiment are largely removed by the present relativistic intermediate coupling calculations.

Angular distribution in resonant Auger decay

The angular distribution of Auger electrons in the decay of resonantly excited atomic states is studied theoretically. The two-step model is used for a description of the Auger process with the excited electron as a spectator. Expressions for the angular anisotropy parameters in the jK and jj coupling schemes are obtained for excitation of a closed-shell system. A close relation between the angular anisotropies of the resonant Auger and normal Auger processes is found within the framework of the spectator model. Numerical calculations for resonant Auger transitions in Ar, Kr and Xe are presented and compared with recent experiments.

Theory of angular distributions in resonant transfer and excitation

It is shown that angular distributions of Auger electrons and X-rays from deexcitation of doubly excited states produced in the resonant transfer and excitation of projectiles in collisions with light targets are, in general, not isotropic. The X-ray emission is nonisotropic with respect to the projectile beam direction because the doubly excited states produced are collisionally aligned. The Auger electron emission to the ground state shows nonisotropic behavior due to the alignment, and the interference with the elastic scattering channel. The detailed theoretical results for the 2p 21D resonance in F 8+ + H 2 are presented at a projectile energy of 20 MeV. The angular distribution of the Auger electron is strongly peaked in the forward direction in the laboratory, and the Auger electrons exhibit significant deviations from the Lorentzian profile, in particular for large angles in the laboratory frame.

Direct imaging of monolayer and surface atomic structure by angular distribution Auger microscopy

Presented here are complete angular distributions of Auger electrons emitted from Pt(111), Ag(111), Cu(100), and an adsorbed monolayer of iodine on Pt(111). These samples were chosen because there is agreement as to their surface structure, providing a foundation on which to compare the nature of the observed distributions to the structural properties of the surface. In particular, the dependence of the angular distribution on the Auger electron kinetic energy was investigated by obtaining data at more than one energy for each metal. In each case, angular distributions from crystalline samples contain intensity minima (‘‘silhouettes’’) along the internuclear directions for Auger kinetic energies &amp;lt;100 eV. Above 100 eV, the intensity minima are gradually replaced with maxima along directions parallel to the clearest channels through the crystal. Near grazing angles of detection, the angular distributions of 507 and 518 eV iodine Auger electrons emitted from a monolayer of iodine on Pt(111) contains intensity minima along directions corresponding to the I–I internuclear directions. These results indicate that the presence of intensity maxima or minima along the internuclear directions is not merely a function of Auger electron kinetic energy, but also depends upon the nature of the sample.

Auger Electron Angular Distributions from Surfaces: Direct Comparison with Isoenergetic Photoelectrons

Angular distribution patterns of Auger electrons and of photoelectrons from a Cu (001) surface were measured at the same electron kinetic energy. These measurements reveal that the low kinetic energy angular distributions for Cu Auger electrons and Cu 3p(3/2) photoelectrons differ substantially. This direct comparison between the photoelectron and Auger electron angular distributions demonstrates that, in some circumstances, the Auger process produces a complicated source wave whose nature must be explored before Auger angular distributions can be used for surface structure analysis.

Connection between the angular distribution of Auger electrons and spectator autoionization electrons following 4d ionization/excitation in xenon

For xenon, the angular distribution of photoinduced N4,5-O2,3O2,3 Auger electrons and of the spectator autoionization lines following 4d5/2 to 6p3/2 and 4d5/2 to 7p3/2 excitation have been measured and analysed on the basis of a two-step formulation for the photoprocess and the subsequent radiationless transition. A close relationship between the angular distribution of Auger electrons and spectator autoionization electrons is established theoretically and found to be in general agreement with the experimental observation. It yields deeper insight into the radiationless decay mechanisms above and below the 4d photoionization threshold.

Angular distribution of Auger electrons emitted through the resonant transfer and excitation process following O5++He collisions.

This Letter reports the first measurements of the angular distribution of Auger electrons emitted from the decay of the (1s2s2${\mathit{p}}^{2}$${)}^{3}$D ${\mathrm{O}}^{4+\mathrm{*}\mathrm{*}}$ doubly excited state formed predominantly through resonant transfer and excitation (RTE) in collisions of 13-MeV ${\mathrm{O}}^{5+}$ projectiles with He. The (1s2s2${\mathit{p}}^{2}$${)}^{3}$D angular distribution is strongly peaked along the beam direction, in agreement with recent calculations of the RTE angle-dependent impulse approximation. Furthermore, interference effects between the RTE and the elastic target direct-ionization channels are observed.

Effect of intermediate coupling in angular distribution of Auger electrons

A general expression for angular distribution of Auger electrons in the intermediate coupling scheme is obtained. It is shown that the intermediate coupling can lead to an anisotropy of the Auger line which is isotropic in the pure LS-coupling