Angular Distribution Of Auger Electrons Research Articles

State-resolved differential cross sections of single-electron capture in swift collisions ofC4+(1s2s3S) ions with gas targets

We report on double and single differential cross section measurements of the $1s2{s}^{2}\phantom{\rule{0.16em}{0ex}}^{2}S$, $1s2s2p\phantom{\rule{0.16em}{0ex}}^{2}P_{\ifmmode\pm\else\textpm\fi{}}$, and $1s2s2p\phantom{\rule{0.16em}{0ex}}^{4}P$ doubly excited states produced by single-electron capture in 6--15 MeV collisions of pre-excited ${\mathrm{C}}^{4+}(1s2s\phantom{\rule{0.16em}{0ex}}^{3}S)$ ions with ${\mathrm{H}}_{2}$, He, Ne, and Ar targets. Ab initio cross section calculations based on a three active electrons atomic orbital close-coupling approach are compared to the measurements for collisions with the lighter ${\mathrm{H}}_{2}$ and He targets, showing an overall good agreement. Theoretical details concerning Auger electron angular distributions, partial cross sections, impact parameter dependencies, and radiative cascade feeding effects are discussed. The populations of the $1s2s2p\phantom{\rule{0.16em}{0ex}}^{4}P$, $^{2}P_{\ensuremath{-}}$, $^{2}P_{+}$ multiplets concerning the spin symmetry are finally presented for the four collision systems and discussed following the comparisons between theory and experiment.

Effect of the Breit interaction on the angular distribution of Auger electrons following electron-impact excitation of highly charged Be-like ions

The electron-impact excitation from the ground state to the autoionizing level $1s2{s}^{2}2{p}_{1/2}\phantom{\rule{0.16em}{0ex}}J=1$ and the subsequent nonradiative Auger decay $1s2{s}^{2}2{p}_{1/2}\phantom{\rule{0.16em}{0ex}}J=1\ensuremath{\rightarrow}1{s}^{2}2s\phantom{\rule{0.16em}{0ex}}J=1/2$ of Be-like ions have been studied by using the multiconfigurational Dirac-Fock method and the relativistic distorted-wave theory. Special attention has been paid to the effect of the Breit interaction on the angular distribution of the Auger electrons emitted. To do so, the partial cross sections, alignment parameters of the autoionizing level, and intrinsic anisotropy parameters of the Auger decay are evaluated for Be-like ${\mathrm{Mg}}^{8+}, {\mathrm{Fe}}^{22+}, {\mathrm{Mo}}^{38+}, {\mathrm{Nd}}^{56+}, {\mathrm{Au}}^{75+}$, and ${\mathrm{U}}^{88+}$ ions, from which the angular distribution of the Auger electrons is further obtained. It is found that the Breit interaction hardly contributes to the angular distribution for low-$Z$ ions such as ${\mathrm{Mg}}^{8+}$, especially at low impact energies, while for medium- and high-$Z$ ions the situation becomes fairly different. To be specific, the Breit interaction contributes to lowering the anisotropy of the angular distribution, which first becomes quickly more prominent with increasing atomic number up to certain high-$Z$ ions and then behaves very slowly less prominently for higher-$Z$ ions. Moreover, for high-$Z$ ions the effect of the Breit interaction on the angular distribution is found to be nearly independent of the atomic number $Z$. Owing to opposite effects of the Breit interaction on the alignment parameters and the intrinsic anisotropy parameters, such an ``abnormal behavior'' is speculated to be caused very likely by a ``competition'' between the opposite effects of the Breit interaction on both of them.

Auger electron angular distributions following excitation or ionization from the Xe 3d and F 1s levels in xenon difluoride.

Linearly polarized synchrotron radiation has been used to record polarization dependent, non-resonant Auger electron spectra of XeF2, encompassing the bands due to the xenon M45N1N45, M45N23N45, M45N45N45 and M45N45V and fluorine KVV transitions. Resonantly excited Auger spectra have been measured at photon energies coinciding with the Xe 3d5/2 → σ* and the overlapped Xe 3d3/2/F 1s → σ* excitations in XeF2. The non-resonant and resonantly excited spectra have enabled the Auger electron angular distributions, as characterized by the βA parameter, to be determined for the M45N45N45 transitions. In the photon energy range over which the Auger electron angular distributions were measured, theoretical results indicate that transitions into the εf continuum channel dominate the Xe 3d photoionization in XeF2. In this limit, the theoretical value of the atomic alignment parameter (A20) characterizing the core ionized state becomes constant. This theoretical value has been used to obtain the Auger electron intrinsic anisotropy parameters (α2) from the βA parameters extracted from our non-resonant Auger spectra. For a particular Auger transition, the electron kinetic energy measured in the resonantly excited spectrum is higher than that in the directly ionized spectrum, due to the screening provided by the electron promoted into the σ* orbital. The interpretation of the F KVV Auger band in XeF2 has been discussed in relation to previously published one-site populations of the doubly charged ions (XeF22+). The experimental results show that the ionization energies of the doubly charged states predominantly populated in the decay of a vacancy in the F 1s orbital in XeF2 tend to be higher than those populated in the decay of a vacancy in the Xe 4d level in XeF2.

Effect of angular momentum transfer on the angular distribution of Auger electrons following atomic inner ns2 -shell photoionization

A mechanism of angular-momentum transfer between photoelectrons and Auger electrons is applied to Auger electron emission in process of photoionization of deep inner atomic shells followed by Auger decay. This mechanism proofs to be the leading contribution to distortion of angular distribution of the Auger electrons and highlights important features of postcollision interaction effect in the inner shells photoionization. The theory developed is applied to the photoionization of $n{s}^{2}$ shells of noble gases. Calculations show the noticeable influence of angular-momentum transfer on the Auger-electron angular distribution in a number of cases. Photoionization processes suitable for experimental observation of the proposed effect are discussed.

Angular distribution of Auger electron following photoionization in Xe atom

The angular distribution of Auger electron following photoionization in Xe atom is investigated using the multiconfiguration Dirac-Fock (MCDF) method and the density matrix theory. The alignment parameters of residual ions produced by the np3/2(n=2−5) and md3/2,5/2(m=3,4) sub-shell photoionization and the anisotropic angular parameters of Auger electron for different transition channels are calculated using our modified relativistic program, based on the RATIP package. The relationships of the angular distribution of Auger electron and the alignments of the core-ionized atomic states are given. The influences of the multipole radiation field, the Breit interaction, and the electron exchanged interaction on the photoionization and subsequent Auger decay processes are discussed. Our results show that the alignment parameters of the residual ions and the angular distribution of Auger electrons are strongly related to the atomic shell, yet they show a low sensitivity to the Breit interaction and the electron exchange effect. This work is of significance to the experimental study of photoionization induced Auger electrons.

Photoionization of the I 4d and valence orbitals of methyl iodide

The photoabsorption and photoionization dynamics of the I 4d and valence orbitals in methyl iodide have been studied both experimentally and theoretically. Synchrotron radiation has been employed to measure the total ion yield in the vicinity of the I 4d ionization thresholds. The observed structure, due to excitations into Rydberg or valence states, has been assigned using transition energies and relative intensities computed with time-dependent density functional theory within the Tamm–Dancoff approximation. Photoelectron spectra, recorded with plane polarized radiation in two polarization geometries, have allowed the effect of autoionization on the valence electron angular distributions to be investigated. The spectra obtained at photon energies of 50.62 and 52.34 eV, coinciding respectively with the I 4d5/2 → σ* and 4d3/2 → σ* transitions, reveal, in addition to valence shell photoelectron bands, features not associated with simple photoionization of the parent molecule. High resolution photoelectron spectra of the I 4d main-lines display structure resulting from spin–orbit coupling and molecular field splitting. The binding energies of the five states contributing to the (I 4d)−1 ionization have been determined. The iodine (in CH3I) N45VV Auger spectrum has been measured and the observed structure has been assigned using the core hole binding energies derived in the present work together with established ionization energies of the doubly charged ion. The experimentally determined Auger electron angular distributions have been discussed in relation to the theoretical angular distribution parameter characterizing the spatial alignment of molecular axes in the (I 4d)−1 state.

Auger electron angular distributions following excitation or ionization of the I 3d level in methyl iodide.

Auger electron spectra following excitation or ionization of the I 3d level in CH3I have been recorded with horizontally or vertically plane polarized synchrotron radiation. These spectra have enabled the Auger electron angular distributions, as characterized by the β parameter, to be determined. The I 3d photoionization partial cross section of CH3I has been calculated with the continuum multiple scattering approach, and the results show that in the photon energy range over which Auger spectra were measured, the I 3d cross section exhibits an atomic-like behavior and is dominated by transitions into the εf continuum channel. In this limit, the theoretical value of the alignment parameter (A20) characterizing the core ionized state in an atom becomes constant, independent of photon energy. This theoretical value has been used to obtain the Auger electron intrinsic anisotropy parameters (α2) from the β parameters extracted from our normal (non-resonant) molecular Auger spectra. The resulting anisotropy parameters for the M45N45N45 transitions in CH3I have been compared to those calculated for the corresponding transitions in xenon, and the experimental and theoretical results are in good agreement. Anisotropy parameters have also been measured for the M45N1N45, M45N23N45, and M45N45O23 transitions. For the M45N1N45 and M45N23N45 Auger decays in CH3I, the experimentally derived angular distributions do not exhibit the strong dependence on the final ionic state that is predicted for these transitions in xenon. Resonantly excited Auger spectra have been recorded at 620.4 and 632.0 eV, coinciding with the I 3d5/2 → σ* and 3d3/2 → σ* transitions, respectively. The resulting Auger electron angular distributions for the M4N45N45 and M5N45N45 decays were found to exhibit a higher anisotropy than those for the normal process. This is due to the larger photo-induced alignment in the neutral core excited state. For a particular Auger transition, the Auger electron kinetic energy measured in the resonantly excited spectrum is higher than that in the normal spectrum. This shift, due to the screening provided by the electron excited into the σ* orbital, has been rationalized by calculating orbital ionization energies of I 3d excited and I 3d ionized states in CH3I.

Separation of Surface- and Bulk-specific Ti L-edge XANES Spectra of Rutile (110) Surface

The electronic structure of the rutile TiO2(110) surface passivated with pyrrolidine dithiocarbamate (PDTC) was investigated by depth-resolved Ti L-edge X-ray absorption spectroscopy. We measured a series of Ti LMM Auger electron angular distributions as a function of photon energy. X-ray absorption spectra measured at the grazing and normal emission directions were deconvoluted into surface- and bulk-region-specific spectra. The spectra at an emission polar angle of approximately 47.5° were identical to the emission-angle integrated spectrum. This suggests that the TiO2 surface electronic structures have two main components: those of the surface layers, with a thickness of approximately 0.47 nm, and those of the bulk regions. The separated surface spectrum suggested that the Ti 3d band is partially occupied, but no additional peak related to the defect state was found. These results showed that the surface treatment was effective in removing the electron recombination defect sites at the photovoltaic cel...

Imaging the Temporal Evolution of Molecular Orbitals during Ultrafast Dissociation

We investigate the temporal evolution of molecular frame angular distributions of Auger electrons emitted during ultrafast dissociation of HCl following a resonant single-photon excitation. The electron emission pattern changes its shape from that of a molecular σ orbital to that of an atomic p state as the system evolves from a molecule into two separated atoms.

Auger decay and subsequent fragmentation pathways of ethylene followingK-shell ionization

The fragmentation pathways and dynamics of ethylene molecules after core ionization are explored using coincident measurements of the Auger electron and fragment ions by employing the cold target recoil-ion momentum spectroscopy method. The influence of several factors on the dynamics and kinematics of the dissociation is studied. These include propensity rules, ionization mechanisms, symmetry of the orbitals from which the Auger electrons originate, multiple scattering, conical intersections, interference, and possible core-hole localization for the double ionization of this polyatomic molecule. Energy correlation maps allow probing the multidimensional potential energy surfaces and, in combination with our multiconfiguration self-consistent field calculations, identifying the populated electronic states of the dissociating dication. The measured angular distributions of the Auger electrons in the molecular frame further support and augment these assignments. The deprotonation and molecular hydrogen ion elimination channels show a nearly isotropic Auger electron angular distribution with a small elongation along the direction perpendicular to the molecular axis. For the symmetric breakup the angular distributions show a clear influence of multiple scattering on the outgoing electrons. The lowest kinetic energy release feature of the symmetric breakup channel displays a fingerprint of entangled Auger and photoelectron motion in the angular emission pattern identifying this transition as an excellent candidate to probe core-hole localization at a conical intersection of a polyatomic molecule.

Classification of the 5p5n1l1n2l2 LSJ autoionizing states of Cs excited by 30 eV electron-impact

SynopsisClassification of the 5p5n1l1 (L'S')n2l2 LSJ autoionizing states of Cs excited by 30 eV electron-impact was performed by using more accurate calculations of energy levels, excitation cross sections and autoionization probabilities obtained in the Dirac-Fock-Slater approximation. Calculated parameters were used for simulation of the ejected Auger electron intensity spectrum and identification of the experimental ejected-electron spectrum of Cs excited by 30 eV electrons taking into account the asymmetry of the angular distribution of Auger electrons.

Theoretical analysis of Young-type electron interference inHe2++H2collisions using a semiclassical model

A four-body semiclassical model is developed to describe interferences observed in the angular distribution of Auger electrons emitted after double capture in 30-keV $\mathrm{H}{\mathrm{e}}^{2+}+{\mathrm{H}}_{2}$ collisions. The present model is based on both the corpuscular and wave behaviors of the emitted electron. The corpuscular aspect is used to determine the trajectories of the collision partners, while the wave behavior occurs only in the determination of the phase shift. The results of the calculation are found to reproduce the experiment remarkably well. Series of maxima and minima are found in the angular distribution, with periods that are close to the experimental values. In addition, at a fixed angle, oscillations in the energy distribution are clearly evidenced in both the experiment and calculation.

Strong asymmetry of the angular distribution of Auger electrons from electron-impact excited autoionizing states of Rb

The asymmetry parameters of the angular distribution of Auger electrons for a number of electron-impact excited states were calculated in single configuration intermediate coupling approximation. Strong dependence on the energy of exciting electrons was obtained for the angular distribution of the emitted Auger electrons from the states with J > 1/2.

Strong impact of the giant resonance on the radiationless decay of the 4d-vacancy in Xe: II. N4, 5OO Auger effect

The radiationless decay of the Xe N4, 5 levels is investigated theoretically. The strong influence of the giant resonance in the 5s5p5(1P)εf channel on the partial and total Auger widths via the interchannel mixing is revealed. The computed widths of the N4 and N5 levels, equal to 105.9 and 109.6 meV, respectively, are in excellent agreement with the measured values 104(3) and 111(3) meV (Jurvansuu et al 2001 Phys. Rev. A 64 012502). The shape of the computed N4, 5OO Auger spectrum is in good agreement with the measured one. Interchannel interaction reduces the probability of the 5p5p − 4dεg Auger decay by more than an order of magnitude. This conclusion is supported by the good agreement between computed and measured Auger electron angular distribution parameters.

Theoretical study of the 4p5nln′l′ autoionizing states of Rb excited by electron impact

Large scale configuration interaction calculations of energy levels, autoionization probabilities and electron-impact excitation cross sections of the autoionizing states 4p5nln′l′ (nl = 5s,4d,5p,4f; n′l′ = 4d,5s,5p,5d,5f,5g,6s,6p,6d,7s,7p) of Rb up to 19.6 eV were performed for the first time. The relativistic effects were taken into account in the Dirac–Fock–Slater approximation. The asymmetry parameters of the angular distribution of Auger electrons for a number of electron-impact excited states were calculated in a single configuration intermediate coupling approximation. Strong dependence on the energy of exciting electrons was found for the angular distribution of the emitted Auger electrons from the states with J > 3/2 at the polar ‘magic angle’ of 54.73°. The calculated data were used for the novel classification of lines in an experimental ejected-electron spectrum.

Angular correlation between B K-VV Auger electrons of BF3 molecules and coincident fragment ions: manifestation of the difference between the angular correlation and molecular frame Auger electron angular distribution.

We have measured the angular correlation between the B K-VV Auger electrons of BF(3) molecules and the coincident fragment-ion pairs of BF(2)(+)-F(+). Then, we have found that the measured angular correlation patterns depending on the mutual angle between the light polarization direction and molecular orientation are affected by the anisotropic axis distribution of the molecular ensemble of BF(3)(+) reflecting the anisotropic nature of photon-molecule interaction. In this context, we have pointed out generally that for coincidence experiments, so-called molecular frame Auger electron angular distributions are realized only if the axis distribution of the molecular ion ensemble is isotropic.

Auger decay of1σgand1σuhole states of theN2 molecule. II. Young-type interference of Auger electrons and its dependence on internuclear distance

Theoretical two-center interference patterns produced (i) by the $K$-shell photoionization process of the ${\mathrm{N}}_{2}$ molecule and (ii) by the Auger decay process of the $K$-shell hole state of the ${\mathrm{N}}_{2}$ molecule are compared for the case of equal photo- and Auger-electron energies of about 360 eV. The comparison shows that both the angular distribution of the photoelectrons and the angular distribution of the Auger electrons of equal energy in the molecular frame are primarily defined by the Young interference. The experimental data for the angular resolved $K$-shell Auger electrons as a function of the kinetic-energy release (KER) obtained earlier [Phys. Rev. A 81, 043426 (2010)] have been renormalized in order to visualize the angular variation in the regions of low Auger-electron intensities. That renormalized data are compared with the corresponding theoretical results. From the known behavior of the potential energy curves, the connection between the KER and the internuclear distance can be established. Since the Young interference pattern is sensitive to the internuclear distance in the molecule, from the measured KER dependence of the Young interference pattern one can trace the behavior of the Auger-electron angular distribution for different molecular terms as a function of internuclear distance. The results of that analysis are in a good agreement with the corresponding theoretical predictions.

Strong interference effects in the angularly resolved Auger decay and fluorescence emission spectra of the core-excited NO molecule

Angular distributions of Auger electrons and subsequent fluorescence photons are studied theoretically and experimentally in the vicinity of the core excitations of NO. In the calculations, lifetime vibrational interference and electronic state interference were taken into account ab initio. The interference between excitation and deexcitation amplitudes for transitions via symmetry-different intermediate resonances 1s−12π2(2Δ, 2Σ±), which is forbidden in the solid-angle-averaged or magic-angle-recorded decay spectra, plays a crucial role in the formation of the angularly resolved decay spectra. Experimentally, angular distribution parameters for the NO+(A1Π → X1Σ+) fluorescence induced by linearly polarized synchrotron radiation are determined in the vicinity of the N*O resonance in the Raman regime for core excitation. Theoretical results are in good agreement with the present experimental fluorescence spectra and with the available vibrationally and angularly resolved resonant Auger electron spectra.

Auger decay of1σgand1σuhole states of theN2molecule: Disentangling decay routes from coincidence measurements

Results of the most sophisticated measurements in coincidence of the angular resolved K-shell photo- and Auger-electrons, and of two atomic ions produced by dissociation of N2 molecule, are analyzed. Detection of photoelectrons at certain angles allows separating the Auger decay processes of the 1{\sigma}g and 1{\sigma}u core hole states. The Auger electron angular distributions for each of these hole states are measured as a function of the kinetic energy release of two atomic ions and are compared with the corresponding theoretical angular distributions. From that comparison one can disentangle the contributions of different repulsive doubly charged molecular ion states to the Auger decay. Different kinetic energy release values are directly related to the different internuclear distances. In this way one can trace experimentally the behavior of the potential energy curves of dicationic final states inside the Frank-Condon region. Presentation of the Auger electron angular distributions as a function of kinetic energy release of two atomic ions opens a new dimension in the study of Auger decay.

Separation of Auger transitions into different repulsive states afterK-shell photoionization ofN2molecules

The Auger transitions to different repulsive doubly charged molecular ion states are separated by measuring the angular resolved photoelectrons and Auger electrons in coincidence in the molecular fixed frame. The separation is achieved by comparing the experimental Auger-electron angular distributions at different kinetic-energy release values with theoretical curves calculated for different final dicationic states.