L-shell Vacancies Research Articles

Study of population dynamics toward the development of atomic X-ray lasers in the 3–5 keV photon energies

X-ray free-electron lasers (XFELs) are powerful tools for characterizing and probing the properties of matter at atomic resolution on the ultrafast timescale. However, they have certain limitations such as spectral fluctuation and poor temporal coherence. Atomic X-ray lasers offer the narrow bandwidth, longitudinal coherence, and spectral stability that can overcome these limitations. In this paper, we study the interaction of inner-shell vacancy states with high-intensity XFEL pulses. We show that it is possible to achieve population inversion between K-shell and L-shell vacancy states in calcium and titanium when pumped by high-intensity XFEL pulses. These states can be used to generate atomic X-ray laser emission in the 3–5 keV photon energy range.

Effect of L-shell spectator vacancies on the competition of filling the K, L1 and L23 vacancies and the natural widths of K-shell hole states of argon ions

The radiative and Auger decay of K-shell hole states of argon ions with configuration 1s2sm2pn3 s23p6 (m=0–2; n = 0–6) are studied theoretically using the flexible atomic code. The effect of L-shell spectator vacancies on the competition of filling the K, L1, and L23 vacancies is discussed for the first time. We find the K or L1 vacancy is preferred to be filled first when there is 0 or 1 vacancy in the L23 shell, but the L23 vacancy becomes the priority when the number of the L23 vacancies is larger than 2, and the total branching ratio of the L23 vacancy is as high as 66.98%, 74.66%, and 80.95% when the L23 shell has 4 vacancies and there are 2, 1 and 0 vacancies in the L1 shell, respectively. In addition, the natural widths of the K, L1, and L23 vacancies with different L-shell vacancy distributions are also calculated. The results show that they all first increase and then decrease with the number of L23 vacancies. It is expected that the present study will be useful for the qualitative analysis of the decay processes of multiple hole states, the understanding of ion yields and the production of satellites in x-ray and Auger spectra.

Gain dynamics of inner-shell vacancy states pumped by high-intensity XFEL in Mg, Al and Si.

High-intensity X-ray free-electron laser (XFEL) beams create transient and non-equilibrium dense states of matter in solid-density targets. These states can be used to develop atomic X-ray lasers with narrow bandwidth and excellent longitudinal coherence, which is not possible with current XFEL pulses. An atomic kinetics model is used to simulate the population dynamics of atomic inner-shell vacancy states in Mg, Al, and Si, revealing the feasibility of population inversion between K-shell and L-shell vacancy states. We also discuss the gain characteristics of these states implying the possibility of atomic X-ray lasers based on inner-shell vacancy states in the 1.5 keV region. The development of atomic X-ray lasers could have applications in high-resolution spectroscopy and nonlinear optics in the X-ray region.

Monitoring Argon L-Shell Auger Decay Using 250-eV Attosecond X-ray Pulses.

Electron correlation describes the interaction between electrons in a multi-electron system. It plays an important role in determining the speed of relaxation of atoms and molecules excited by XUV/X-ray pulses, such as the argon decay rate. Most research on electron correlation has centered on the role of correlation in stationary states. A time-resolved experimental study of electron correlation is a grand challenge due to the required temporal resolution and photon energy. In this research, we investigated Auger decay in argon using 200-attosecond X-ray pulses reaching the carbon K-edge. At such a high photon energy, ionization occurs not only from the outer most levels (3s and 3p), but also from the 2p core shells. We have measured a lifetime of 4.9 fs of L-shell vacancies of argon in pump–probe experiments with a home-built high-resolution time-of-flight spectrometer.

Auger electron wave packet interferometry on extreme timescales with coherent soft x-rays

Wave packet interferometry provides benchmark information on light-induced electronic quantum states by monitoring their relative amplitudes and phases during coherent excitation, propagation, and decay. The relative phase control of soft x-ray pulse replicas on the single-digit attosecond timescale achieved in our experiments makes this method a powerful tool to probe ultrafast quantum phenomena such as the excitation of Auger shake-up states with sub-cycle precision. In this contribution we present first results obtained for different Auger decay channels upon generating L-shell vacancies in argon atoms using Michelson-type all-reflective interferometric autocorrelation at a central free-electron laser photon energy of 274.7 eV.

Multiple ionization X-ray satellites of Ti and its binary oxides in alpha particle PIXE

High energy resolution X-ray emission spectroscopy was used to study the multiple ionization satellites in the K X-ray spectra of titanium induced by target irradiation with MeV alpha particles. Beam energies of 3–5 MeV were used to explore the beam energy dependence. In addition, several titanium binary oxides excited with 5 MeV beam were measured to study also the chemical bonding effects on the multiple ionization satellites. Energy shifts and intensities of the satellites corresponding to one and two L-shell vacancies were determined from the measured spectra. The results will be used to refine the existing experimental alpha particle induced multiple ionization satellite database which is needed to improve the accuracy of PIXE analysis using alpha particle beams, including the PIXE mode of the Mars alpha particle X-ray spectrometers.

Multiple ionization X-ray satellites of magnesium, aluminum and silicon in alpha particle PIXE

Abstract Wavelength-dispersive spectroscopy was used to study the multiple ionization satellites in the K X-ray spectra of magnesium, aluminum and silicon bombarded by alpha particles having energies of 3, 4 and 5 MeV. The measured energy shifts and intensities relative to the diagram lines of the groups with one, two and three L-shell vacancies were extracted from the spectra. These results compare well with the values in an interim database which was recently assembled from the limited existing literature. The present work is the first step in refining this database so that it may be used to improve the accuracy of PIXE analysis using alpha particle beams, including the PIXE mode of the Mars alpha particle X-ray spectrometers. The effects of oxide bonding are also determined and compared to those observed when excitation is effected by photons and by electrons.

Structure of the Ru, Ag and Te L X-ray emission spectra

The emission of X-rays in atomic transitions from L-shell vacancy states of Ru, Ag and Te induced by electron incidence was studied.

Energy shifts of Kα,β,γ X-ray satellites for atoms with 13 ⩽ Z ⩽ 90: A systematic calculation and a new analytical description

The transition energy shifts of the Kα, Kβ and Kγ X-rays resulted from addition vacancies in L-shell for atoms with 13⩽Z⩽90 have been calculated systematically, by using the Flexible Atomic Code (FAC), in which the Breit interaction, QED corrections and nuclear finite mass effects are also considered. It is shown by means of concrete figures that the overall agreement among the present results, theoretical results of other authors as well as a large body of measured ones was excellent. Contrary to other discussions, we found that the energy shifts of the Kα and Kβ satellites did not varies absolutely linearly with Z, the contributions of the Z2 term cannot be neglected, and the energy differences between neighboring L satellites were not equal. Three simple empirical formulae for estimating the energy shifts of the Kα, Kβ and Kγ X-ray satellites for any atom with any number of L-shell vacancies were put forward. They will be useful in qualitative interpretation of various K X-ray spectra and better understanding of the collision processes.

Molybdenum L-shell X-ray production by 350–600keV Xeq+ (q=25–30) ions

Molybdenum L-shell X-rays were produced by Xeq+ (q = 25-30) bombardment at low energies from 2.65 to 4.55 keV/amu (350-600 keV). We observed a kinetic energy threshold of Mo L-shell ionization down to 2.65-3.03 keV/amu (350-400 keV). The charge state effect of the incident ions was not observed which shows that the ions were neutralized, reaching an equilibrium charge state and losing their initial charge state memory before production of L-shell vacancies resulted in X-ray production. The experimental ionization cross sections were compared with those from Binary Encounter Approximation theory. Taking into account projectile deflection in the target nuclear Coulomb field, the ionization cross section of Mo L-shell near the kinetic energy threshold was well described. (C) 2010 Published by Elsevier B.V.

Resonant coherent excitation of Li-like Ar15+ Ions in a Thin Si crystal

We investigated resonant coherent excitation (RCE) of 445 MeV/u Li-like Ar15+ under the planar channeling and non-channeling conditions. The K-shell and L-shell excitations are observed as decreases in the survival fraction of the Ar15+ ions after passing through a thin Si crystal. RCE under the planar channeling condition provides the opportunity to study a strong static Stark effect on the Li-like system with K-shell or L-shell vacancy. Under the non-channeling conditions, the observed excitation energies and the reported theoretical calculations were in reasonable agreement.

Channels of Potential Energy Dissipation during Multiply Charged Argon-Ion Bombardment of Copper

The dissipation of potential energy of multiply charged Ar ions incident on Cu has been studied by complementary electron spectroscopy and calorimetry at charge states between 2 and 10 and kinetic energies between 100 eV and 1 keV. The emitted and deposited fractions of potential energy increase at increasing charge state, showing a significant jump for charge states q>8 due to the presence of L-shell vacancies in the ion. Both fractions balance the total potential energy, thus rendering former hypotheses of a significant deficit of potential energy obsolete. The experimental data are reproduced by computer simulations based on the extended dynamic classical-over-the-barrier model.

Hard X-ray emission from laser-produced plasmas of U and Pb recorded by a transmission crystal spectrometer

Hard X-ray spectra from laser-produced plasmas were recorded by a transmission crystal survey spectrometer covering the 12– 60 keV energy range with a resolving power of E/Δ E≅100. This emission is of interest for the development of hard X-ray backlighters and hot electron diagnostics. Foils of U and Pb were irradiated at the OMEGA laser facility by 24 beams (12 on each side), each with an energy of ≅500 J , a pulse duration of 1 ns , and no beam smoothing. The beams were focused to a 50 μm diameter spot on the target plane. The spectra typically exhibit a few intense and relatively narrow features in the 12– 22 keV energy range. Initial analysis suggests that these hard X-ray features are inner-shell transitions resulting from L-shell vacancies created by energetic electrons. The observed transition energies are slightly higher than the neutral-atom characteristic X-ray energies. Calculations suggest that the transitions are in the Ni-like or lower ionization stages. The analysis further indicates that opacity effects play an important role in producing the spectra.

Polarization of argonKx radiation following electron-impact ionization

The angular and impact-energy dependence of the characteristic K x radiation of argon produced from collisions of electrons with 10.0--24.0 keV energies has been investigated. The radiation is found to show a net linear polarization of up to $(17\ifmmode\pm\else\textpm\fi{}6)%$ in the investigated range of impact energy. This anisotropy is believed to arise from the strong polarization of $K\ensuremath{\alpha}$ satellite lines of argon produced by the simultaneous decay of a single K- and multiple L-shell vacancy states created in the collisions. Also, the behavior of the net polarization of K radiation as a function of impact energy in the range of 10.0 to 24.0 keV has been studied. Over the range of impact energies, the polarization is found to show a nearly constant value within the experimental uncertainty.

Ar K X-ray production in slow, highly charged Ar q+(q = 8–18) + Ar collisions

In the present work, mechanisms of Ar K X-ray production have been investigated under low-energy Arq+ + Ar collisions in a wide range of the ion-charge states. Ar K X-rays have been observed as a function of the collision energy over (0.75–3.75)q keV/u and of the charge state over q = 8–16 of projectile Arq+ ions. If the L-shell vacancies become available (q = 9), Ar K X-ray yields are found to be enhanced roughly three orders of magnitude, compared with Ar8+ ions with no L-shell vacancy; to show some threshold; and to tend to quickly saturate at higher collision energies. The present analysis, based on the energy correlation diagrams of the quasi-molecules, suggests that Ar K X-rays originate entirely from target Ar atoms through promotion via the rotational coupling between 2pσ –2pπ molecular orbitals. Furthermore, the observed cross sections are found to increase nonlinearly with the projectile Arq+ ion charge q (q = 9). This can be understood because of the combined effects of two contributions: (i) the increased number of the L-shell vacancies of projectile Arq+ ions and (ii) the enhanced fluorescence yields of target Ar atom. In addition, Ar K X-rays have been observed under 2.5q keV/u Ar17+ and Ar18+ + Ar collisions and found to originate from the decay of K-shell vacancies initially present in the projectile Ar ions through electron capture into their highly excited states, followed by cascade down to the ground states. For the first time, though very weak, the X-rays due to the two-electron–one-photon transition, namely, K αα lines, have also been observed in bare Ar18+ ion collisions. PACS Nos.: 34.70+e, 32.80Rm

Time-dependent independent-particle model calculation of multiple capture and ionization processes inp−Ar,p¯−Ar,andHe2+−Arcollisions

Previous work on multielectron transitions in proton, antiproton, and ${\mathrm{He}}^{2+}$-ion impact on neon is extended to the case of argon targets for collision energies in the 5--1000 keV/amu range. Global quantities such as net electron loss from the target, net capture, and net ionization are predicted within experimental errors using a spherically symmetric optimized effective target atom potential with dynamical screening effects based on the time-dependent net ionization probability. The inclusion of target response is crucial in order to obtain correct positions and heights for the peaks in the net ionization cross sections. Effects due to cascading following multiple outer-shell excitation are found to be appreciable at energies between 10 and 100 keV/amu, but are overestimated by the statistical model. L-shell vacancy production is reported to affect recoil charge state production at energies above 200 keV/amu for charge states $q>~3.$ At low and intermediate energies, the independent-particle model is shown to overestimate q-fold recoil ion production significantly for $q>~3$ for proton impact signaling the role of electronic correlations for these channels. For antiproton impact the $q=3$ cross section is consistent with the independent-particle model.

A database for KL ionization satellites in PIXE

The experimental database of the intensities of the KαL satellites induced in collisions with protons has been assembled. A detailed analysis of the rearrangement process and the influence of the L-shell spectator vacancy on the Kα fluorescence yield has been carried out, providing the ratios of the double to single ionization cross-section for the collected experimental points. These ratios were compared to the theoretical predictions within the independent particle model employing the semiclassical approximation (SCA). An empirical curve describing the ratio of the experimental and theoretical ionization probabilities has been deduced. This relationship can be used along with computed SCA values within PIXE data-processing packages such as GUPIX, to incorporate double ionization satellites into the model PIXE spectrum.

Monte Carlo simulations and measurement of DNA damage from x-ray-triggered auger cascades in iododeoxyuridine (IUdR).

We investigated the DNA damage from Auger electrons emitted from incorporated stable iodine (127I), following photoelectric absorption of external x-rays. The effectiveness of the Auger electrons in producing DNA double-strand breaks (DSB) was determined theoretically, using Monte Carlo simulations of the radiation physics and chemistry, and was shown to be in reasonable agreement with DNA damage measured using the comet assay. The DSB yields were measured in CHO cells for 60Co (as a non-Auger-promoting radiation) and for tungsten-filtered 100 kVp x-rays capable of producing Auger electron emission. The theoretical study showed that on average, 2.5 Auger electrons were emitted for N-shell orbital vacancies and up to 10 Auger electrons were emitted from L1-shell vacancies. These Auger bursts produced approximately 0.03 DSB per N-shell vacancy and 0.3 DSB per K-shell or L-shell vacancy. The calculated yield of DSB from Auger cascades per unit dose (1 Gy) in water was approximately 1.7 for tungsten-filtered 100 kVp x-rays, assuming 20% IUdR substitution of thymidine. The comet assay yielded an experimental value of 3.6+/-1.6 per 1 Gy for the same conditions. The Monte Carlo simulations also demonstrated a high complexity of DSB produced by Auger cascades with virtually all DSB from inner shell orbitals (i.e. K, L shells) accompanied by compounded strand breakage and base damage, indicating a difficult lesion to repair. This finding agrees well with comet assay results of DNA repair, where an increase in the DSB yield in IUdR-sensitized cells was shown to persist after a time of 24 h. We conclude that Auger cascades in iodine produce a modest increase in the number of initial strand breaks of the order of 10% but the complex nature of these DSB makes them very difficult to repair or potentially prone to misrepair. The accentuated DNA damage may have major consequences for cell survival and may be exploitable in kilovoltage photon activation therapy (PAT) of tumors sensitized with iodine.

Anomalously high yield of doubly charged Si ions sputtered from cleaned Si surface by keV neutral Ar impact

The energy spectra of positively charged and neutral species ejected from the Si(1 1 1) surfaces by keV Ar impact have been measured by means of a combined technique of the time-of-flight (TOF) analysis with the multi-photon resonance ionization spectroscopy (MPRIS). It is shown that positively charged species of Si +, Si 2+ and SiO + are ejected from the as-cleaned 7×7 surface by 11 keV Ar impact. It is also shown that Ar sputter cleaning of the as-cleaned 7×7 surface for 14 min at the flux of 2×10 13/cm 2s removes completely the oxygen impurity and the yields of Si 2+ is comparable to that of Si +. Moreover, the ionization probability of Si atoms sputtered is shown to be expressed as an exponential function of the inverse of their velocity. The production mechanism for the doubly charged Si ion is discussed based on the L-shell ionization of Si atoms due to quasi-molecule formation in the collisions of the surface atoms with energetic recoils and subsequent Auger decay of the L-shell vacancy to doubly ionized Si ions.

ProjectileZdependence of AlK-shell vacancy production in 10-MeV/amu ion-solid collisions

Beams of 10-MeV/amu Ne, Ar, Cr, Kr, Xe, and Bi ions have been employed to examine the dependence on projectile atomic number ${(Z}_{1})$ of Al K-shell vacancy production in nearly symmetric ${(Z}_{1}{/Z}_{2}=0.8)$ to highly asymmetric ${(Z}_{1}{/Z}_{2}=6.4)$ collisions. Aluminum K x-ray yields were measured with a Si(Li) detector as a function of target thickness to establish cross sections for charge equilibrated projectiles. Average target atom L-shell vacancy distributions excited in K-shell vacancy-producing collisions and x-ray yield corrections for contributions from photoionization and secondary electron impact were deduced from high-resolution spectral measurements performed with a curved-crystal spectrometer. Knowledge of the average L-shell vacancy distributions provided the means for estimating the appropriate fluorescence yields needed to convert the x-ray yields to K-shell vacancy production cross sections. Theoretical cross sections greatly overestimate the experimental results for ${Z}_{1}>10.$ The ${Z}_{1}$ dependences of the K-shell vacancy production cross sections for both Al and Cu (from a previous study) are well characterized by a simple empirical scaling law.