Distribution Of Auger Electrons Research Articles

Angular distributions of selected N2 Auger transitions.

The angular distribution of Auger electrons produced in the collision of 1634-eV electrons with ${\mathrm{N}}_{2}$ is reported. The angular distributions are measured relative to the internuclear axis of the molecule from 0\ifmmode^\circ\else\textdegree\fi{} to 90\ifmmode^\circ\else\textdegree\fi{} in 6\ifmmode^\circ\else\textdegree\fi{} steps. When an Auger transition occurs to an unstable doubly ionized state the molecular ion dissociates into two ${\mathrm{N}}^{+}$ fragments. Because the time required for dissociation of the ${\mathrm{N}}_{2}^{2+}$ ion is much less than that for rotation of the ion, the axial recoil approximation holds to first order, and detecting one of the ${\mathrm{N}}^{+}$ ions determines the orientation of the target molecule at the time of the projectile-molecule collision. Therefore, a coincidence experiment between the Auger electrons of appropriate energy and the ${\mathrm{N}}^{+}$ fragments is devised. The rotation of the ${\mathrm{N}}_{2}^{2+}$ ions during the course of the dissociation process has been taken into account as a second-order approximation. As a result of this rotational effect, the predicted angular distribution function is smeared out by an amount depending on the different ${\mathrm{N}}_{2}^{2+}$ final states. The data are analyzed in terms of a two-center model in which prolate spheroidal coordinates are used.

Influence of multiple elastic and inelastic scattering on photoelectron line shape.

The influence of multiple elastic and inelastic scattering on photoelectron spectra from semi-infinite random solids has been investigated. A theoretical expression describing the energy and angular yield has been derived which adequately accounts for the anisotropic source emission of photoelectrons. Comparison of theory with results from an efficient Monte Carlo code yields very satisfactory agreement. The resulting energy and angular distribution for four subshell transitions in gold are presented. A pronounced dependence of the particle transport on the line shape is seen in the results for different geometries. As an application of the presented theory, a procedure to correct the observable experimental spectrum in the quasielastic regime for multiple elastic and inelastic collisions is developed and tested. In all studied cases the proposed approach yields background corrected spectra agreeing within 3% with the original line shape, while methods not accounting for the anisotropic source emission generally display discrepancies of up to 30% in the background intensity. The main advantage of the proposed approach lies in its generality. It not only accurately describes the influence of particle transport on the energy and angular distribution of Auger electrons and photoelectrons, but can also be applied in a straightforward manner to many other spectroscopic techniques using electrons, ions, or other probing particles.

The angular distribution of Auger transitions in N 2

The angular distributions of Auger electrons emitted by nitrogen molecules are reported. The distributions are measured relative to the orientation of the internuclear axis of the molecule. The initial inner-shell vacancies are produced by bombardment with 1634 eV electrons. The ensuing Auger transitions are followed promptly by the dissociation of the doubly charged molecular ions. The experiment measures the counting rate for the coincident detection of an N + fragment and an Auger electron of the appropriate energy. Since rotation times are long compared to dissociation times, the axial recoil approximation holds, and detection of the N + fragment serves to determine the orientation of the target molecule. The data will be discussed in terms of a two-center model in which prolate spheroidal functions are employed.

Circular dichroism in photoinduced Auger decay

It is demonstrated that the angular distribution of Auger electrons produced in photoionization of non-polarized atoms by circularly polarized light is different for left and right circular polarization, provided the Auger electron is detected in coincidence with the photoelectron ejected in a certain direction. A general formulation of the theory of circular dichroism in angular-resolved double photoionization is presented based on the statistical tensor formalism. Extreme cases of direct and two-step double photoionization are considered and the inherent similarities and differences between the two cases as applied to circular dichroism are discussed. As an example, photoionization in the 3p32/ subshell of Ca and the following M3-N1N1 Auger transition is considered. The connection of the dichroism in the angular distribution of Auger electrons to the photoelectron spin polarization is discussed as well as their dependence on the dynamical properties of photoionization amplitudes.

Implementation of a position sensitive detection system in a scattered electron-Auger electron coincidence experiment

A coincidence data acquisition system, incorporating a position sensitive detector and a custom-built data acquisition card installed in an IBM-PC 486 computer, has been implemented in a coincidence experiment in which scattered electrons and Auger electrons are detected in coincidence. Three analog-to-digital converters (ADCs) are used to digitize all the timing and energy information, and custom-designed and written control software is used to accumulate the data, display it in real time, and store it in expanded memory. The new system has ben used to measure the energy and angular distribution of Auger electrons ejected from argon after electron impact ionization.

Relativistic effects on angular distribution of Auger electrons emitted from Be-like ions following electron-impact excitation.

We have calculated the angular distribution functions for Auger electrons emitted from Be-like Mg, Fe, and Mo following 1[ital s]-2[ital p] excitation by electron impact. The calculations were carried out in intermediate coupling with configuration interaction using the relativistic distorted-wave and multiconfiguration Dirac-Fock methods. We found strong angular asymmetry for many transitions. We also found that relativistic effects can completely change the characteristics of the angular distribution for transitions which have many contributing partial waves, even for ions as light as Fe.

Coherence and correlation in the anisotropy of Ne KL-LLL satellite Auger decay.

The energies, intensities, and angular anisotropies of the Ne KL-LLL satellite Auger lines have been studied by the multiconfiguration Dirac-Fock method. In addition to the initial- and final-state correlation effects we have studied the influence of the quantum beat effect on this Auger spectrum. Since the energy splitting of the Ne 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$2${\mathit{p}}^{\mathrm{\ensuremath{-}}1}$ $^{3}$P multiplet is much smaller than the lifetime broadening, the coherent excitation of these initial states by the in time and space localized electromagnetic pulse of the projectile has a drastic effect on the angular distribution of Auger electrons. To analyze this coherence effect we have generalized the theory of the angular distribution of Auger electrons to the case of coherent excitation of partially overlapping initial states. The results of our calculations are in good overall agreement with experiment. However, for a quantitative study of the influence of the coherence and the initial spin state on the anisotropy of these Auger lines new measurements with lower error limits are necessary.

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.

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 (>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.

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.

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 <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.

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.