Inner-shell Photoionization Research Articles

Low-energy photoelectron dynamics in Kr3d5/2photoionization followed by Auger decay

The effects induced during the process of inner-shell photoionization followed by Auger decay are investigated for the photoionization of Kr $3{d}_{5/2}$ in an extremely low photoelectron energy regime. Both the photoelectron energy and relative angular distributions are calculated with a recently developed quantum-mechanical method where all the Coulomb interactions between each pair of particles are fully considered. The photoelectron energy distribution is asymmetric with respect to the initial photoelectron energy, and its peak shifts to a lower energy. As the absorbed photon energy is reduced, the peak of the photoelectron energy distribution moves to the negative energy range. As a manifestation of these dynamics, peak structures are observed in the relative angular distribution. We also simulate the process with a classical-trajectory Monte Carlo method, obtaining similar results to the quantum-mechanical ones.

PCI effects and the gradual formation of Rydberg series due to photoelectron recapture, in the Auger satellite lines upon Xe 4d−15/2 photoionization

The Xe (N5O2,3O2,3) Auger electron spectra originating from 4d−15/2 inner-shell photoionization were measured, with photon energy tuned close to the ionization threshold. As the photon energy approaches the threshold from above the 4d−15/2 photoionization threshold, Rydberg series structures are formed within the Auger electron peak by the recapture of the photoelectron into high-lying ion orbitals. They emerge in the tail on the higher energy side of the post–collision interaction (PCI) profile of the Auger electron. Discrete Rydberg peaks replace the continuous PCI tail and gradually form a series with intensity distribution emulating the intensity profile of the continuous tail. Structures due to the Xe+5p4(1S0, 1D2, 3P2,1,0) ml series were observed and assigned.

Nanoisland formation of small Ag -clusters on HOPG as determined by inner-shell photoionisation spectroscopy

Abstract XPS (3d) and Auger spectra (MNN) of deposited Ag n -clusters on a non-sputtered HOPG surface have been measured. Most of the XPS and Auger spectra appear very similar to the corresponding bulk spectrum, caused by a high mobility and agglomeration of the clusters on the inert carbon surface, independent of the initial cluster size. The metallic fingerprint of the cluster-agglomerated islands is sustained under UHV conditions for several days. Oxidised Ag-islands reveal a positive binding energy shift in distinct contrast to the negative XPS shift usually observed for silver oxide bulk compounds. The XPS binding energy shift has been used for estimation of the diameter of the cluster-assembled nanoislands (6–9 nm) using an electrostatic model.

Generalized atomic processes for interaction of intense femtosecond XUV- and X-ray radiation with solids

Generalized atomic processes are proposed to establish a consistent description from the free-atom approach to the heated and even up to the cold solid. It is based on a rigorous introduction of the Fermi-Dirac statistics, Pauli blocking factors and on the respect of the principle of detailed balance via the introduction of direct and inverse processes. A probability formalism driven by the degeneracy of the free electrons enables to establish a link of atomic rates valid from the heated atom up to the cold solid. This allows to describe photoionization processes in atomic population kinetics and subsequent solid matter heating on a femtosecond time scale. The Auger effect is linked to the 3-body recombination via a generalized 3-body recombination that is identified as a key mechanism, along with the collisional ionization, that follows energy deposition by photoionization of inner shells when short, intense and high-energy radiation interacts with matter. Detailed simulations are carried out for aluminum that highlight the importance of the generalized approach.

Photoionization cross sections of the aluminumlikeSi+ion in the region of the2pthreshold (94–137 eV)

We present measurements of the absolute photoionization cross section of the aluminumlike ${\mathrm{Si}}^{+}$ ion over the 94--137 eV photon energy range. The measurements were performed using the merged-beam setup on the PLEIADES beamline at the SOLEIL synchrotron radiation facility. Signals produced in the ${\mathrm{Si}}^{2+}$ and ${\mathrm{Si}}^{3+}$ photoionization channels of the $2p$ subshell of the ${\mathrm{Si}}^{+}$ ion from both the $1{s}^{2}2{s}^{2}2{p}^{6}3{s}^{2}3p{\phantom{\rule{0.16em}{0ex}}}^{2}{P}_{1/2,3/2}$ ground levels and the $1{s}^{2}2{s}^{2}2{p}^{6}3s3{p}^{2}{\phantom{\rule{0.16em}{0ex}}}^{4}P$ metastable levels were observed. Absolute cross sections were determined. Calculations of the $2p$ inner-shell photoionization cross sections were carried out using the multiconfiguration Dirac-Fock and Dirac-Coulomb R-matrix theoretical approaches and are compared with experiment.

Polarization Transfer in the 2p3/2 Photoionization of Magnesium-Like Ions

The inner-shell 2p3/2 ionization of Mg-like Fe14+, Cd36+, W62+ and U80+ ions by linear polarized light and the subsequent radiative decay are studied theoretically with the multiconfiguration Dirac—Fock (MCDF) method and the density matrix theory. Special attention is paid to exploring the influence of the polarization properties of the incident photon and the non-dipole term which arises from the multipole expansion of the electron-photon interaction on the properties of the subsequent x-ray radiation. The results show that there is a linear relationship between the degree of linear polarization of the radiative decay and ones of the incident light, which can be used for diagnosing the polarization of a light source. In addition, with the increasing photon energies, the non-dipole contribution to the alignment of the residual ions, the degree of linear polarization, as well as the angular distribution of the radiative decay following the inner-shell photoionization will also be increased.

Ionization dynamics and radiative behavior of a betatron driven gold atom

The study of inner-shell transitions induced by an intense (>1019 W/cm2) ultrashort (∼5 fs) x-ray pulse provides a challenging opportunity to investigate the behavior and dynamics of hollow atoms and to explore the feasibility of creating population inversions in some of the inner-shell states that may lead to a variety of amplifications and gains in the x-ray regime. In this paper, we investigate the interaction through inner-shell photoionizations of a spectrally broad femtosecond pulse of betatron x-ray radiation incident on a gold atom. The level populations of Pt-like Au and Ir-like Au are described by non-(local thermodynamic equilibrium) inner-shell dynamics and compared and contrasted with the level populations created by a "single" frequency x-ray laser pulse. Gain coefficients for a variety of transitions are calculated. It is found that long wavelength x-rays must be filtered from the betatron spectrum before any population inversions can be generated.

A COMPREHENSIVE X-RAY ABSORPTION MODEL FOR ATOMIC OXYGEN

An analytical formula is developed to represent accurately the photoabsorption cross section of O I for all energies of interest in X-ray spectral modeling. In the vicinity of the Kedge, a Rydberg series expression is used to fit R-matrix results, including important orbital relaxation effects, that accurately predict the absorption oscillator strengths below threshold and merge consistently and continuously to the above-threshold cross section. Further minor adjustments are made to the threshold energies in order to reliably align the atomic Rydberg resonances after consideration of both experimental and observed line positions. At energies far below or above the K-edge region, the formulation is based on both outer- and inner-shell direct photoionization, including significant shake-up and shake-off processes that result in photoionization-excitation and double photoionization contributions to the total cross section. The ultimate purpose for developing a definitive model for oxygen absorption is to resolve standing discrepancies between the astronomically observed and laboratory measured line positions, and between the inferred atomic and molecular oxygen abundances in the interstellar medium from XSTAR and SPEX spectral models.

Electron reemission processes following photoelectron recapture due to post-collision interaction in inner-shell photoionization of water molecules

Electron reemission following photoelectron recapture due to post-collision interaction has been studied at 0.7 eV the inner-shell photoionization threshold of water molecules, using a multi-electron coincidence method. Electron reemissions after single Auger decay occur from O and OH fragments which are produced by the dissociations of high-n Rydberg H2O(+) states populated through photoelectron recapture. In addition, electron reemissions after double Auger decay are identified in triple coincidence events, where autoionization lines from O and O(+) fragments are observed.

Investigation of the influence of inner-shell photoionization and photoexcitation on the Heα spectrum in photoionized plasmas

The present paper investigates the influence of inner shell photoionization and photoexcitation on the Heα, i.e., the 1s2 to 1s2p transition in He-like ions, and the associated satellite spectra in photoionized plasmas. A comparison of the importance of these processes is made relative to other atomic processes as a function of the electron temperature and irradiation conditions. For the formation of the Heα and the satellite spectra, the K-shell photoionization is found to have significant contribution under low radiation temperature and/or intensity, when lithium- and beryllium-like ions have high abundance, but highly ionized H-like ions are rare.

Influence of the partial temporal coherence of short FEL pulses on two-colour photoionization and photoinduced Auger decay of atoms

The influence of the partial temporal coherence of free electron laser (FEL) radiation on the sidebands arising in the electron spectra of laser-assisted photoionization and photoinduced Auger decay of atoms is theoretically analysed. A simple model is developed which describes the inner-shell photoionization by a short (femtosecond) FEL pulse and the following Auger decay in a strong field of an infrared laser. The model is based on the time-dependent approach and uses the strong field approximation for both photo- and Auger electrons. Particular calculations have been carried out for Ne 1s photoionization and KLL Auger emission. We demonstrate that the temporal coherence of FEL pulses influences the line widths in the photoelectron spectrum. For a small coherence time the sidebands in this spectrum cannot be resolved. On the other hand, our calculations show that in the Auger electron spectrum the sidebands are practically independent of the coherence time of the ionizing pulse.

Small molecule photoelectron spectroscopy: Recoil effects, stoichiometric surprises, and double-core-hole ionization

Three features of small-molecule photoelectron spectroscopy are considered (1) the atom from which a photoelectron is emitted must have a recoil momentum equal to that of the emitted electron. This is shared among the various modes of motion of the ion, leading to rotational and vibrational excitation. Furthermore, any initial velocity of the atom (due to either translational, rotational, or vibrational motion) will lead to Doppler broadening. These effects are observable and can, in general, be accounted for by simple models. In some cases, however, the simple models fail and a deeper insight is necessary. (2) Inner-shell photoionization is essentially an atomic process, and it is expected that the intensity for emission of a photoelectron from the core of an atom in a molecule will be independent of its chemical environment. Recent measurements on the carbon 1s photoelectron spectra of three chloroethanes show that this is not the case. At energies not far above the ionization threshold there are strong oscillations of the intensity ratio (CCl/CH) with increasing photon energy. These are similar to those seen in EXAFS and can be accounted for by considering backscattering of the photoelectrons from the chlorine atoms. Moreover, even at high energies the cross section for ionization has been found to depend on the chemical environment of the atom. These results have important consequences for the use of inner-shell electron spectroscopy for quantitative analysis. (3) Single-core-hole ionization energies have long been used as a tool for investigating chemical phenomena. Double-core-hole ionization energies provide additional chemical information. By combining the single-hole and double-hole ionization energies it is possible to determine the effects of the initial-state charge distribution and final-state charge rearrangement on the chemical shifts and on other chemical properties. Until recently double-core-hole ionization energies have not been experimentally accessible for first-row elements. New experimental techniques have, however, made it possible to measure these not only for single sites in a molecule, but also for two different sites in the same molecule. The chemical information that can be obtained from such measurements is discussed.

Multiphoton Ionization of Xenon at the LCLS Free-Electron Laser

With the first X-ray free-electron laser (FEL), the Linac Coherent Light Source (LCLS), multiphoton ionization has been pushed to a new regime, where atoms and molecules are not just ionized by a series of valence ionizations but "from the inside out". At unprecedented high intensities and short pulse durations in the soft X-ray regime, a series of inner-shell photoionizations followed by cascades of Auger decays was observed to lead to highly charged final states in rare gases such as Ne, Ar, Kr, and Xe. Ion time-of-flight and fluorescence spectra were recorded for different FEL pulse energies and pulse lengths and compared to theoretical models to explain the underlying processes that lead to unexpectedly high charge states in Xe.

Site-specific Auger electron spectra of ethyl trifluoroacelate molecules studied by magnetic bottle electron spectrometer

We performed multielectron coincidence measurements for inner-shell photoionizations of ethyl trifluoroacelate molecules (C4H5F3O2) using a magnetic bottle electron spectrometer. From a two dimensional coincidence map between a photoelectron and Auger electron for C 1s ionizations, we extracted site-specific Auger electron spectra for each carbon site and corresponding binding energy of doubly charged states.

Theoretical treatment of molecular photoionization based on the R-matrix method

The R-matrix method was implemented to treat molecular photoionization problem based on the UK R-matrix codes. This method was formulated to treat photoionization process long before, however, its application has been mostly limited to photoionization of atoms. Application of the method to valence photoionization as well as inner-shell photoionization process will be presented.

EMISSION SPECTRUM OF HELIUM-LIKE IONS IN PHOTOIONIZED PLASMAS

The aim of the present paper is to investigate the influence of inner-shell photoionization and photoexcitation on He? and its satellite's spectra in photoionized plasmas. An analysis is carried out on the relative importance of the various atomic processes in photoionized plasmas as a function of the electron temperature and irradiation conditions. In particular, we investigate the influence of K-shell photoionization of Li-like ions on the He? spectrum and of Be-like ions on the He? satellites. It is found that in photoionized plasmas these inner-shell processes contribute significantly under low radiation temperature and/or intensity, when Li- and Be-like ions are highly abundant but highly ionized H-like ions are rare. A short discussion is presented about the parameter space in which the excited 1s2p state has statistical or non-statistical distributions, and how such distributions affect the emission spectrum.

Post collision interaction probed by multi-electron coincidences: Application to the Ar 2s inner-shell photoionization

Post-collisional interaction following inner-shell ionization of atomic systems has been investigated by multi-coincidence electron spectroscopy, with a special emphasis on the decay by emission of two Auger electrons. The case of the Ar 2p hole is first reviewed. After Ar 2s inner-shell photoionization, the dominant decay channel is cascade Coster–Kronig/Auger decay. The distortion of the photoelectron peak shows a sensitivity to both the energy of the Coster–Kronig electron and to the lifetime of the intermediate Ar2+ (2p−13l−1) state that is well reproduced by a theoretical model based on the eikonal approach. The dynamics of cascade double Auger decay can be clearly understood thanks to this study.

Laser-intensity requirements for generating enhanced kilovolt bremsstrahlung emission in intense laser-cluster interactions

The effects of ultrahigh-intensity laser radiation on dynamical processes such as electron scattering, bremsstrahlung emission, and pair production, have received growing theoretical interest as laser intensities in the laboratory continue to increase. Recently, for example, a calculation was published that predicted resonant increases of more than four orders of magnitude in bremsstrahlung emission in the presence of intense optical laser radiation [A. A. Lebed and S. P. Roshchupkin, Phys. Rev. A 81, 033413 (2010)]. The analysis in that paper was limited to laser intensities of $\ensuremath{\le}$${10}^{17}$ W/cm${}^{2}$, and it was applied only to bremsstrahlung emissions at the laser frequency. In the present paper, we extend this Lebed and Roshchupkin analysis in order to assess the possibility of achieving some enhancement in bremsstrahlung emissions at significantly higher harmonics of the optical laser photon energies ($\ensuremath{\sim}$6 keV) and thereby to appraise whether or not enhanced bremsstrahlung emissions may have played a hidden role in producing the population inversions and kilovolt x-ray amplifications that have been seen experimentally [A. B. Borisov et al., J. Phys. B 40, F307 (2007)]. In those experiments, light from a KrF laser was focused onto a gas of xenon clusters to intensities $\ensuremath{\gtrsim}$${10}^{19}$ W/cm${}^{2}$. A model of the expansion and ionization dynamics of a xenon cluster when heated by such laser intensities has been constructed [Tz. B. Petrova et al., High Energy Density Phys. 8, 209 (2012)]. It is capable of replicating the x-ray gains seen experimentally, but only under the assumption that sufficiently high inner-shell photoionization rates are generated in the experiments. We apply this model to show that such photoionization rates are achievable, but only if there are enhancements of the Bethe-Heitler bremsstrahlung emission rate of three to four orders of magnitude. Our extended analysis of the Lebed and Roshchupkin work shows that, for there to be emissions (whether or not enhanced by four orders of magnitude) at high-order KrF-laser harmonic energies, laser intensities $\ensuremath{\gtrsim}$${10}^{19}$ W/cm${}^{2}$ must be reached. Thus, further extensions of these calculations (or experimental measusurements) are needed to determine whether the enhancement factors that are predicted for small laser harmonics at laser intensities $\ensuremath{\lesssim}$${10}^{17}$ W/cm${}^{2}$ can be extrapolated to large laser harmonics at laser intensities $\ensuremath{\gtrsim}$${10}^{19}$, which are shown in our work to be needed in order to produce high laser harmonic kilovolt emissions.

X-ray amplification in intense ultrashort KrF laser–Xe cluster interactions

In earlier work, a time-dependent, ionization dynamic model of a cluster of xenon atoms was constructed [2,3] in an effort to determine conditions under which the X-ray line amplification data that was observed experimentally at wavelengths between 2.71 and 2.88 Å[1] could be replicated. Model calculations showed that, at laser intensities greater than 1019 W/cm2, the outermost N-shell electrons of xenon would be stripped away by tunnel ionization in less than a femtosecond. They also showed that L-shell electrons within the resulting cluster of Ni-like ions could be photoionized at a sufficient rate as to generate population inversions between these hole states and the states they radiatively decayed into. These inversions only lasted for several femtoseconds, and they were generated early in time when the cluster was being rapidly heated and the cluster's density was rapidly evolving, but was still high. They were seen to depend on the heating and expansion dynamics of the cluster, which had not been modeled in detail in this early work. In this paper, molecular dynamics calculations are described in which the rapidly evolving temperatures and ion densities of an intensely laser-heated cluster are calculated for different peak laser intensities and for two different sized xenon nano-clusters. This data is then used as an input to the ionization dynamic calculations in order to determine the influence of cluster size and of peak laser intensity on the gain coefficient calculations. In these calculations, inner-shell photoionization rates are shaped by the temperature and density dependence of the bremsstrahlung emissions under the assumption that these emissions drive the photoionizations. This shaping produces calculated gain coefficients that agree well with the measured ones.

Spin-entanglement in a three electron system produced in double Auger decay

The simultaneous emission of two electrons in photoionization, or in the non-radiative spontaneous decay of an inner-shell vacancy, are two of the best known examples of the failure of the independent-particle model of atoms and molecules. The later of these provides also one of the two competitive processes, following inner-shell photoionization, for producing three flying electrons which can, for example, be used in implementing many protocols hitherto developed in quantum information. The correlation properties of the three-particle system consisting of these two electrons plus the photoelectron are analyzed using methods from quantum information theory. The entanglement of the consequent tripartite spin-state is shown to be completely independent of the mechanism(s) which may be responsible for the emission of these three electronic qubits in two different steps in the absence of spin-orbit interaction. Our analysis shows that the tripartite state formed in the present case is more like a |W〉 class of states possessing pairwise entanglement. The experimental characterization of these states is fully achieved merely by the measurements of the energies of three flying electrons, without requiring any entanglement witness or other similar protocols hitherto developed in quantum information. Changes in these entanglement properties of a tripartite state of electronic qubits on the inclusion of the spin-orbit interaction have also been discussed.