Inner-shell Ionization Research Articles

Time-resolved Auger-Meitner spectroscopy of the photodissociation dynamics of CS2

Abstract The photodissociation dynamics of UV excited CS2 are investigated using time-resolved Auger-Meitner spectroscopy. Auger-Meitner decay is initiated by inner-shell ionisation with a femtosecond duration X-ray (179.9 eV) probe generated by the FERMI free electron laser. The time-delayed X-ray probe removes an electron from the S(2p) orbital leading to secondary emission of a high energy electron through Auger-Meitner decay. We monitor the electron kinetic energy of the Auger-Meitner emission as a function of pump-probe delay and observe time-dependent changes in the spectrum that correlate with the formation of bound, excited-state CS2 molecules at early times, and CS + S fragments on the picosecond timescale. The results are analysed based on a simplified kinetic scheme that provides a time constant for dissociation of approximately 1.2 ps, in agreement with previous time-resolved X-ray photoelectron spectroscopy measurements (Gabalski, et al. J. Phys. Chem. Lett. 2023, 14, 7126–7133).

L- and M-subshell ionization cross-sections of heavy atoms by electron and proton impact

Inner-shell ionization has many applications related to material analysis. However, full theoretical calculations in heavy multielectronic atoms are scarce. In this contribution, we assess proton and electron impact Li and Mi-subshell ionization cross-sections of heavy targets (72≤Z≤83) using two theoretical models. These calculations are based on the Distorted Plane Wave Born Approximation for electron impact and the shellwise local plasma approximation for the proton impact, combined with relativistic calculations of these deep subshells wave functions and binding energies. We analyze the L-shell ionization of W, Au, and Bi and the M-shell of Hf, Ta, W, Re, Os, Pt, Au, Pb, and Bi. Present values are compared with available experimental data from the literature, with disparate agreement. The convergence of proton and electron impact cross-sections at high-impact velocities is also discussed.

Inner-shell ionization cross sections of atoms by positron impact

The relativistic binary-encounter-Bethe model with Wannier-type threshold law is employed to obtain the inner-shell ionization cross sections of multi-electron atoms (Ni, Cu, Y, Ag, Au, Yb, Ta, and Pb) for positron impact energies from the thresholds up to 105 keV. There is good agreement between the present calculations and the experimental data. The constant in the acceleration term derived from the Wannier law is determined to be 0.2 and 0.5 for the K- and L-shells, respectively.

Directly imaging excited state-resolved transient structures of water induced by valence and inner-shell ionisation

Real-time imaging of transient structure of the electronic excited state is fundamentally critical to understand and control ultrafast molecular dynamics. The ejection of electrons from the inner-shell and valence level can lead to the population of different excited states, which trigger manifold ultrafast relaxation processes, however, the accurate imaging of such electronic state-dependent structural evolutions is still lacking. Here, by developing the laser-induced electron recollision-assisted Coulomb explosion imaging approach and molecular dynamics simulations, snapshots of the vibrational wave-packets of the excited (A) and ground states (X) of D2O+ are captured simultaneously with sub-10 picometre and few-femtosecond precision. We visualise that θDOD and ROD are significantly increased by around 50∘ and 10 pm, respectively, within approximately 8 fs after initial ionisation for the A state, and the ROD further extends 9 pm within 2 fs along the ground state of the dication in the present condition. Moreover, the ROD can stretch more than 50 pm within 5 fs along autoionisation state of dication. The accuracies of the results are limited by the simulations. These results provide comprehensive structural information for studying the fascinating molecular dynamics of water, and pave the way towards to make a movie of excited state-resolved ultrafast molecular dynamics and light-induced chemical reaction.

Neural-network–based algorithm for the inverse problem of measuring K-shell ionization cross-sections of Si induced by 3–25 keV electrons and 4.5–9 keV positrons using the thick-target method

In this study, a neural network method is proposed for solving the inverse problem in the measurement of inner-shell ionization cross-sections using the thick-target method. It was applied to calculate the K-shell ionization cross-section of silicon (Si) from positron impacts in the energy range from 4.5 to 9 keV, using a Monte Carlo simulation program called PENELOPE to construct a comprehensive characteristic X-ray yield and cross-section database, serving as a foundation for training the neural network. The experimental values are compared with those obtained using regularization, yield differential, and distorted-wave Born approximation (DWBA) theoretical models. Our findings reveal that the cross-section results obtained from all three algorithms are in good agreement with the theoretical DWBA values within the error range. Moreover, our study highlights the superiority of the neural network algorithm in solving ill-posed problems, compared with traditional regularization algorithms and the yield differential method. Furthermore, we re-analyse the experimental data of electron-induced ionization cross-sections on a pure thick Si target in the energy range from 3 to 25 keV, which were originally obtained by Zhu et al. who used a regularization method. The reprocessed cross-sections obtained in this study exhibit good agreement with the reported results within the error range. To the best of our knowledge, this is the first experimental report of the K-shell ionization cross-sections of Si from positron impact.

Superoxide Production under Soft X-ray Irradiation of Liquid Water

Soft X-rays behave like particles with high linear energy transfer, as they deposit a large amount of their energy in the nanometric range, triggered by inner-shell ionization. In water, this can lead to the formation of a doubly ionized water molecule (H2O2+) and the emission of two secondary electrons (photoelectron and Auger electron). Our focus lies on detecting and quantifying the superoxide (HO2°) production via the direct pathway, i.e., from the reaction between the dissociation product of H2O2+, i.e., the oxygen atom (∼4 fs), and the °OH radicals present in the secondary electron tracks. The HO2° yield for 1620 eV photons, via this reaction pathway, was found to be 0.005 (±0.0007) μmol/J (formed within the ∼ps range). Experiments were also performed to determine the yield of HO2° production via another (indirect) pathway, involving solvated electrons. The indirect HO2° yield, measured experimentally as a function of photon energy (from 1700 to 350 eV), resulted in a steep decrease at around 1280 eV and a minimum close to zero at 800 eV. This behavior in contradiction with the theoretical prediction reveals the complexity hidden in the intratrack reactions.

Application of spectrum smoothing to improve the accuracy of atomic inner-shell ionization cross sections impacted by electron near the threshold energy

In this study, the X-ray spectrum smoothing is applied to improve the reading precision of the experimental X-ray peak net counts. The experimental Sn L-shell X-ray production cross sections (XRPCSs) and Bi-Mαβ XRPCSs before and after smoothing are compared, respectively. The result shows that spectrum smoothing effectively improves the accuracy of the experimental XRPCSs. The after-smoothing present XRPCSs agree well with the distorted wave Born approximation (DWBA) theoretical XRPCSs and are consistent with the previously published XRPCSs within the experimental uncertainty. Finally, the factors affecting the comparison between the experimental and DWBA theoretical XRPCSs are analyzed.

Cascade energy reemission after inner-shell ionization of atomic gold. Role of photo- and cascade-produced electrons in radiosensitization using gold-containing agents

A method of straightforward construction and analysis of the cascade decay trees is applied to calculate photon and electron spectra emitted upon the cascade de-excitation of single inner-shell vacancies in the gold atom produced by photoionization. The energy acquired by the gold atom upon photoionization is split into the following channels: i) energy absorbed by the gold atom itself, ii) energy carried away by the cascade electrons, and iii) energy carried away by the cascade photons. The energies absorbed through channels i–iii are analyzed for the cascade decay of each single vacancy in O to K shells, and presented as functions of incident photon energies based on calculated partial photoionization cross sections. Energy reemitted through channels ii and iii in the cases of O-, N-, M-, L-, and K-ionization make, respectively, 47–60%, 55–69%, 85–89%, 94–96%, and 99% of the initially acquired energy. Cascade-produced electrons and photons eventually transfer their energy to the medium which lays the base for using gold-containing compounds and gold nanoparticles as radiosensitizing agents in radiotherapy. Cascade-produced electrons, as well as photoelectrons, are found to be the principal transmitters of energy to atoms of the environment in the vicinity of the sites of initial ionization. Their relative role in possible radiosensitizing effect is analyzed on the incident photon energy range of 0.01–1000 keV.

Hydrogen migration in inner-shell ionized halogenated cyclic hydrocarbons

We have studied the fragmentation of the brominated cyclic hydrocarbons bromocyclo-propane, bromocyclo-butane, and bromocyclo-pentane upon Br(3d) and C(1s) inner-shell ionization using coincidence ion momentum imaging. We observe a substantial yield of CH3+ fragments, whose formation requires intramolecular hydrogen (or proton) migration, that increases with molecular size, which contrasts with prior observations of hydrogen migration in linear hydrocarbon molecules. Furthermore, by inspecting the fragment ion momentum correlations of three-body fragmentation channels, we conclude that CHx+ fragments (with x = 0, …, 3) with an increasing number of hydrogens are more likely to be produced via sequential fragmentation pathways. Overall trends in the molecular-size-dependence of the experimentally observed kinetic energy releases and fragment kinetic energies are explained with the help of classical Coulomb explosion simulations.

Computed corrections for recollision effects on ionization and x-ray production cross sections obtained in positron-impact studies

The accurate experimental measurement of inner-shell ionization cross sections induced by positrons near the threshold energy is of great significance both for theory and practical application. So far, there are few published experimental data in this field, most of which involve a slow positron beam device and acceleration fields to achieve the desired collision energies. Most previous experimental studies, in which a negative acceleration potential was applied to the target, neglected to consider how backscattered positrons might be accelerated back to the target, generating surplus characteristic x-rays and inflating the cross-section estimates derived therefrom. Using Geant4 and Ansys Maxwell to simulate charged-particle transport, the experimental collision process in the negative high-voltage electromagnetic field of the target chamber is simulated realistically in this paper. Through the simulation process, the effect of the negative high-voltage electromagnetic field on backscattered positrons, hence on ionization and characteristic x-ray production cross sections, is obtained. We then estimate corrections for previously published experimental Ti K-shell ionization cross sections and Ag-Lαβγ, Bi-Mαβ, Pb-Mαβ x-ray production cross sections obtained by positron impact. In addition, we use simulation to demonstrate that such undesired effects can be suppressed by adjusting the size of the target and the tilt angle between the target normal and the positron beam axis.

Recent Developments in MaMFIS Technology for the Production of Highly Charged Ions

We present results for the production of highly charged ions in a rippled electron beam propagating in a multi-section drift tube with different electrostatic potentials in an axial magnetic focusing field. The inner-shell ionization of target atoms by electron impact occurs in local ion traps formed at the electron-beam crossovers. The utmost electron current density achieved is assessed at ~10 kA/cm2. The successive ionization of cathode materials and working substances such as xenon and bismuth was investigated as a function of the confinement time. The characteristic X-ray radiation from ions including Ir62+, Ce48+, Xe46+, and Bi60+ was detected. It is shown that it is possible to extract highly charged ions from local ion traps for a certain geometry of the drift tube structure and a certain distribution of the electric potentials.

Calculating the Atomic electron impact Ionization Cross Section for some elements

Inner-shell ionization cross section (ICS) by electron impact are of interest not only to the basic collision physics for complex atoms, but also to various practical applications in material science and electron microscopy. Theoretical difficulties in the past were between the threshold and the peak, which occur usually four to five times the threshold energy. Theories based on classical mechanics are somewhat better than on Born cross section, but such theories usually requires adjustable parameters. We used the Gryzinski formalism to calculate atomic electron impact ionization cross sections for the elements (Fe, Co, Mn, Ti, Zn, and Nb). Good to satisfactory agreement was found for all atoms with the exception of (Nb), where the distance between our cross section and Deutsch-Mark formula become larger at the high values of the overvoltage (U). Moreover, when compared to other to available ionization cross sections for these atoms, calculated using other methods and semiempirical formula, the Gryzinski formalism achieved a level of agreement with experimental data that is as good better than the predictions from the other methods.

Inner-shell ionisation of Xe[formula omitted]–Au and Pb collision systems: MO picture

The generation of collision-induced vacancies and their transfer within the electronic inner shells are the source of several unprecedented phenomena, which have been revealed in recent experimental studies conducted using state-of-the-art next-generation accelerators and detectors. Despite the multitude of studies conducted over the years, our understanding of the atomic vacancy transfer mechanisms remains exiguous. The detection and analysis of collision-induced X-rays is a widely-used powerful technique for analysing atomic-scale phenomena. However, both experimental and theoretical investigations on ion–atom collision-induced X-rays have remained mostly restricted to high-energy light-ion impact on heavy atoms. To date, only a few studies have been reported on the investigation of very-low-energy heavy-ion impact on heavy atoms using X-rays, especially M-shell X-rays of the heavy atom. This study was conducted to evaluate the inner-shell ionisation of Au (135 and 508 μg/cm2) and Pb (107, 157 and 390 μg/cm2) targets due to low-energy (2–5 MeV) 54Xeq+-ion impact. The collision-induced X-ray spectra of both the collision partners were examined, and the corresponding intensity ratios and cross-sections were compared with those calculated using two well-known ionisation theories, viz. PWBA and ECPSSR. The measured cross-sections were underestimated by these theories. Moreover, the cross-sections measured in this study were found to be several orders of magnitude higher than those obtained in other reported studies on low-energy lighter-ion-impact on Au and Pb. The significant discrepancies observed between the experimental and theoretical values have been explained qualitatively within the framework of quasi-molecular orbital formation using level correlations. The insights obtained from the results of this study can be applied to analyse the vacancy transfer mechanisms in very-heavy systems (combined atomic number > 100).

Experimental determination of atomic alignment of Mo42, Cd48 , and In49 with differential x-ray intensity ratios by 100–250-keV proton impact

This work aims to study the alignment properties of electron vacancies produced by proton bombardment in the low-energy region from 100 to 250 keV. The alignment parameter ${\mathcal{A}}_{20}$ of $_{42}\mathrm{Mo}, _{48}\mathrm{Cd}$, and $_{49}\mathrm{In}$ ions after ${L}_{3}$ subshell ionization has been investigated experimentally. The typical $L$ x-ray spectra are measured for each target at emission angles from ${115}^{\ensuremath{\circ}}$ to ${155}^{\ensuremath{\circ}}$. The angular dependence of differential intensity ratios ${L}_{\ensuremath{\alpha}}/{L}_{\ensuremath{\beta}1}$ and ${L}_{\ensuremath{\beta}2}/{L}_{\ensuremath{\beta}1}$ is studied as a function of the second-order Legendre polynomial ${P}_{2}(\mathrm{cos}\ensuremath{\theta}$). Our result demonstrates that ${L}_{\ensuremath{\alpha}}$ and ${L}_{\ensuremath{\beta}2}$ lines exhibit anisotropic emission spatially. The anisotropy parameter is converted to the alignment parameter by considering the Coster-Kronig (CK) correction coefficient and anisotropy coefficient. The results are compared with theoretical prediction within the framework of the semiclassical theory of inner-shell ionization, and good agreement is found in general. A small discrepancy around the inflection point of the alignment parameter is attributed to the atomic parameters employed only for single ionization in the CK correction coefficient.

Understanding the mechanisms of L-shell x-ray emission from Os atoms bombarded by 4–6 MeV/u fluorine ion

The L-subshell ionization mechanism is studied in an ultra-thin Os target bombarded by 4–6 MeV/u fluorine ions. Multiple ionization effects are considered through the change of fluorescence and Coster-Kronig yields while determining L-subshell ionization cross sections from L x-ray production cross sections. The present experimental values are compared with various theoretical approximations: (i) the relativistic semi-classical approximation (RSCA), (ii) the shellwise local plasma approximation (SLPA), and (iii) the ECUSAR theory. We also take into account the vacancy sharing among the subshells by the coupled-states model (CSM) and the electron capture (EC) by a standard formalism. We find that the ECUSAR-CSM-EC describes the measured excitation function curves the best. However, the theoretical calculations are still about a factor of two smaller than the measured values even though the recent fluorescence and Coster-Kronig yields are considered. Hence, a re-evaluation of these parameters is a challenge for the theoretical works. Whatsoever, this work leads to demonstrate that in the present energy range the heavy-ion induced inner-shell ionization of the heavy atoms can be understood by combining the direct Coulomb ionization, the electron capture, and the vacancy sharing among subshells, together with optimizing the atomic parameters. Optimization of the atomic parameters shows that our experimental results agree with theoretical vacancy production theories if the L1 fluorescence yield is nearly doubled. Such a optimization is validated by the proton induced L-shell ionization data of uranium atoms.

Coupling a magnetic bottle multi-electron spectrometer with a liquid micro-jet device: a comprehensive study of solvated sodium benzoate at the O 1 s threshold

We have developed a magnetic bottle time-of-flight electron-electron coincidence spectrometer to perform measurements on solvated molecules in a liquid micro-jet. We present here the first results obtained after ionization of the oxygen 1s inner-shell of sodium benzoate molecules and show the possibilities to filter out the electron signal arising from the liquid phase from the signal of water molecules in the gas phase. Both photoelectrons and Auger electrons spectra (unfiltered and filtered) are presented.

W L-shell X-ray emission induced by C<sup>6+</sup>ions in the energy range of several hundred MeV/u

The L-shell X-ray emissions of tungsten has been investigated under the bombardment of C<sup>6+</sup> ions in the high energy region of 154 - 424 MeV/u. Compared to the atomic data, the energy of the X-ray is enlarged, and the relative intensity ratios of Lı, Lβ<sub>1, 3, 4</sub> and Lβ<sub>2, 15</sub> to Lα<sub>1, 2</sub> X-rays are enhanced. The L-subshell and the total X-ray production cross sections were calculated by a well corrected thick target formula and compared with the theoretical estimation of BEA, PWBA and ECPSSR. On the whole, the experimental cross sections are all smaller than the prediction of PWBA and ECPSSR, but in agreement roughly with that of BEA. It is indicated that the inner-shell ionization of W can be considered to be a binary process between the high energy C<sup>6+</sup> ions acting as a point charge and the independent target electrons. With the L-shell ionization, the outer-shells are multiply ionized. The multi-ionization degree is regard to be almost constant in the present work. This results in the X-ray energy blue shift and the enhancement of the relative intensity ratios of Lı and Lβ to Lα X-ray. Using the atomic parameters corrected by multi-ionization, the X-ray production cross section can be estimated by the BEA model.

W L-shell X-ray emission induced by C<sup>6+ </sup>ions with several hundred MeV/u

The L-shell X-ray emission of tungsten is investigated under the bombardment of C<sup>6+</sup> ions in a high energy range of 154—424 MeV/u. Compared with the atomic data, the energy of the X-ray is enlarged, and the relative intensity ratio of Lι, Lβ<sub>1,3,4</sub> and Lβ<sub>2,15</sub> to Lα<sub>1,2</sub> X-rays are enhanced. The L-subshell and the total X-ray production cross section are calculated from a well corrected thick target formula and compared with the theoretical estimation of binary encounter approximation (BEA), plane-wave Born approximation (PWBA) and ECPSSR (PWBA theory modified with Energy-loss, Coulomb-repulsion, Perturbed-Stationary-State and Relativistic corrections). On the whole, the experimental cross sections are all smaller than the prediction of PWBA and ECPSSR, but in rough agreement with that of BEA. It is indicated that the inner-shell ionization of W can be considered as a binary process between the high energy C<sup>6+</sup> ions acting as a point charge and the independent target electrons. With the L-shell ionization, the outer-shells are multiply ionized. The multi-ionization degree is approximately regard as a constant in the present work. This leads the X-ray energy to be blueshifted and the relative intensity ratios of Lι and Lβ to Lα X-ray to be enhanced. Using the atomic parameters corrected by multi-ionization, the X-ray production cross section can be estimated by the BEA model.

Inner-Shell-Ionization-Induced Femtosecond Structural Dynamics of Water Molecules Imaged at an X-Ray Free-Electron Laser

The ultrafast structural dynamics of water following inner-shell ionization is a crucial issue in high-energy radiation chemistry. We have exposed isolated water molecules to a short x-ray pulse from a free-electron laser and detected momenta of all produced ions in coincidence. By combining experimental results and theoretical modeling, we can image dissociation dynamics of individual molecules in unprecedented detail. We reveal significant molecular structural dynamics in H2O2+, such as asymmetric deformation and bond-angle opening, leading to two-body or three-body fragmentation on a timescale of a few femtoseconds. We thus reconstruct several snapshots of structural dynamics at different time intervals, which highlight dynamical patterns that are relevant as initiating steps of subsequent radiation-damage processes.10 MoreReceived 25 March 2021Revised 20 July 2021Accepted 16 September 2021DOI:https://doi.org/10.1103/PhysRevX.11.041044Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAutoionization & Auger processesElectronic excitation & ionizationPhotodissociationPhotoemissionSingle- and few-photon ionization & excitationPhysical SystemsMoleculesTechniquesAb initio calculationsHartree-Fock methodsPhotoionizationX-ray photoelectron spectroscopyAtomic, Molecular & Optical

Suppression of X-ray-Induced Radiation Damage to Biomolecules in Aqueous Environments by Immediate Intermolecular Decay of Inner-Shell Vacancies.

The predominant reason for the damaging power of high-energy radiation is multiple ionization of a molecule, either direct or via the decay of highly excited intermediates, as, e.g., in the case of X-ray irradiation. Consequently, the molecule is irreparably damaged by the subsequent fragmentation in a Coulomb explosion. In an aqueous environment, however, it has been observed that irradiated molecules may be saved from fragmentation presumably by charge and energy dissipation mechanisms. Here, we show that the protective effect of the environment sets in even earlier than hitherto expected, namely immediately after single inner-shell ionization. By combining coincidence measurements of the fragmentation of X-ray-irradiated microsolvated pyrimidine molecules with theoretical calculations, we identify direct intermolecular electronic decay as the protective mechanism, outrunning the usually dominant Auger decay. Our results demonstrate that such processes play a key role in charge delocalization and have to be considered in investigations and models on high-energy radiation damage in realistic environments.