Inner-shell Photoionization Research Articles

X-ray radiation damage cycle of solvated inorganic ions

X-ray-induced damage is one of the key topics in radiation chemistry. Substantial damage is attributed to low-energy electrons and radicals emerging from direct inner-shell photoionization or produced by subsequent processes. We apply multi-electron coincidence spectroscopy to X-ray-irradiated aqueous solutions of inorganic ions to investigate the production of low-energy electrons (LEEs) in a predicted cascade of intermolecular charge- and energy-transfer processes, namely electron-transfer-mediated decay (ETMD) and interatomic/intermolecular Coulombic decay (ICD). An advanced coincidence technique allows us to identify several LEE-producing steps during the decay of 1s vacancies in solvated Mg2+ ions, which escaped observation in previous non-coincident experiments. We provide strong evidence for the predicted recovering of the ion’s initial state. In natural environments the recovering of the ion’s initial state is expected to cause inorganic ions to be radiation-damage hot spots, repeatedly producing destructive particles under continuous irradiation.

Inner-shell photoelectron spectroscopy unveils the interplay between hydrogen bonds and π−π stacking in clusters of biomolecules in the gas phase: hypoxanthine clusters as a case study

We present a joint theoretical-experimental study on the inner-shell photoionization of hypoxanthine clusters in the gas phase. Simulations were performed using a computational strategy that combines molecular dynamics to explore the conformers of the clusters, density functional theory for geometry optimization and inner shell photoionization calculations. Two main intermolecular interactions are observed: hydrogen-bonds (HB) and π−π stacking. When increasing the cluster size, a combination of both kinds of interaction occurs. We show that such intermolecular interactions play a role in the chemical shift observed in X-ray photoemission spectroscopy experiments. In particular, we highlight the interplay between charge depletion and charge accumulation in regions where HBs stabilize the clusters.

Postcollision interaction in sequential x-ray radiative and Auger decays after atomic inner-shell photoionization

Postcollision interaction in sequential x-ray radiative and Auger decays after atomic inner-shell photoionization

Revealing and controlling nuclear dynamics following inner-shell photoionization of N2

In this work, we apply the Monte Carlo wave packet method to study the ultrafast nuclear dynamics following inner-shell photoionization of N2 exposed to an ultrashort intense X-ray pulse. The photon energy of the X-ray pulse is large enough to remove a 1s electron from the N atom in N2. The intermediate state in N+2 is highly excited so that autoionization takes place from this state to the dissociative or non-dissociative electronic states of ungerade and gerade symmetries in N++2. The possible vibrational resonances allowed by the non-dissociative states prevents a direct extraction of the nuclear kinetic release (KER) spectrum from the nuclear wave packets in N++2. Therefore, we propose a hybrid technique by combining the advantages of two energy analysis strategies to obtain the final nuclear KER spectrum of the process. A femtosecond IR probe pulse, which couples the electronic states in N++2 together, is applied to achieve a time-resolved imaging and controlling of the ultrafast dynamics that takes place during double ionization of N2. The influence of the laser parameters including the peak intensity, pulse duration and pump-probe delay, on the nuclear dynamics is also investigated.

Interaction of short wavelength photons with atomic and molecular ions: 50 years some examples

Steve Manson has been involved with the study of the photoionisation of atomic ions since the early 70’s. Over the half century since, much has happened in terms of theory and experiment. This short paper provides some examples of the inner-shell photoionisation of atomic ions, which demonstrate different experimental approaches and the important ongoing symbiotic nature of combined theoretical and experimental studies. Recent initiatives in the study of the photoionisation of small molecular ions are also briefly described.

Shortening X-Ray Pulse Duration via Saturable Absorption.

To shorten the duration of x-ray pulses, we present a nonlinear optical technique using atoms with core-hole vacancies (core-hole atoms) generated by inner-shell photoionization. The weak Coulomb screening in the core-hole atoms results in decreased absorption at photon energies immediately above the absorption edge. By employing this phenomenon, referred to as saturable absorption, we successfully reduce the duration of x-ray free-electron laser pulses (photon energy: 9.000keV, duration: 6-7fs, fluence: 2.0-3.5×10^{5} J/cm^{2}) by ∼35%. This finding that core-hole atoms are applicable to nonlinear x-ray optics is an essential stepping stone for extending nonlinear technologies commonplace at optical wavelengths to the hard x-ray region.

Inner-shell photoionization of free and confined Mg in the region of the 2p thresholds

Photoionization of the free and confined Mg (Mg@C60) have been calculated in the vicinity of the inner-shell 2p thresholds using R-matrix at the Breit–Pauli R-matrix level with a particular emphasis on the various resonance series leading up to the two lowest inner-shell thresholds and the changes resulting from the confinement. For free Mg, the calculated resonance positions agree well with NIST values, and a number of new resonances are identified. For confined Mg, the calculation was done for various depths of the confining potential well in order to follow the trajectories of the resonances as a function of well depth. The results for the well depth corresponding to Mg@C60 showed that most of the resonances moved to higher photon energy, as a result of the confinement, but a few moved to lower energies. This phenomenology was found to result from the fact that the 3p, 3d and 4s excitations are trapped by the confining potential well, while the higher excitations were not.

Inner-shell photoabsorption and photoionisation cross-sections of valence excited states from asymmetric-Lanczos equation-of-motion coupled cluster singles and doubles theory

The asymmetric-Lanczos-based equation-of-motion coupled cluster formalism to compute photoabsorption and photoionisation cross-sections of valence excited states [B.N.C. Tenorio, M.A.C. Nascimento, A.B. Rocha, and S. Coriani, J. Chem. Phys. 151 (18), 184106 (2019). doi:10.1063/1.5125125] has been adapted to enable the calculation of photoabsorption and photoionisation spectral signatures from inner-shell electrons of such states. Since excited-state properties depend on both the electronic character of the molecular wavefunction and on the nuclear dynamics, we computed the photoionisation spectra using both ground and excited state optimised geometries. The total cross-section profiles were generated for the first two electronically excited states of water, ammonia, ethylene, and uracil by two different methodologies: an analytic continuation procedure based on the Padé approximants and by the Stieltjes imaging procedure. A comparison with literature results, whenever available, is presented. Remarkable differences were observed between the results of the core-ionisation cross-sections of the valence excited states yielded by the two quadrature approaches, at variance from previous studies on the valence photoionisation cross-sections of ground states and of valence excited states. Their origin remains unclear.

Maxwell-Bloch modeling of an x-ray-pulse amplification in a one-dimensional photonic crystal

We present an implementation of the Maxwell-Bloch (MB) formalism for the study of x-ray emission dynamics from periodic multilayer materials whether they are artificial or natural. The treatment is based on a direct Finite-Difference-Time-Domain (FDTD) solution of Maxwell equations combined with Bloch equations incorporating a random spontaneous emission noise. Besides periodicity of the material, the treatment distinguishes between two kinds of layers, those being active (or resonant) and those being off-resonance. The numerical model is applied to the problem of $K\alpha$ emission in multilayer materials where the population inversion could be created by fast inner-shell photoionization by an x-ray free-electron-laser (XFEL). Specificities of the resulting amplified fluorescence in conditions of Bragg diffraction is illustrated by numerical simulations. The corresponding pulses could be used for specific investigations of non-linear interaction of x-rays with matter.

Relaxation effects on the magnetic cross section and fluorescence radiation polarization after 4d inner-shell photoionization of Ba-like ions

On the basis of the multi-configuration Dirac–Fock method and the density matrix theory treatment of photoionization (PI) dynamics, a systematic investigation for 4d inner-shell PI of exemplary Ba atom and Ba-like Nd4+, Gd8+ and Yb14+ ions is performed. Starting from the general framework of the relativistic, the relaxation effects are considered by using a set of relaxed one-electron orbitals in solving the coupled Dirac equations. Our results uncover for the first time that the relaxation effects, arising from the overlap integrals between orbitals of the initial state and the relaxed final state, may play a paramount role in determining the magnetic sublevel cross sections, the photoelectron angular-distribution asymmetry parameters, and the fluorescence polarizations of the subsequent x-ray radiation, particularly for neutral atoms close to the threshold regime. Yet the importance of inclusion of the orbital relaxation weakens as the incoming photon energy and/or the degree of atom ionization increases. The nature of these influences is explored, which allows us to get more insight on the physical process and mechanism involved. Our results are consistent with those obtained by the experiments and other theoretical predictions.

Differentiating and Quantifying Gas-Phase Conformational Isomers Using Coulomb Explosion Imaging.

Conformational isomerism plays a crucial role in defining the physical and chemical properties and biological activity of molecules ranging from simple organic compounds to complex biopolymers. However, it is often a significant challenge to differentiate and separate these isomers experimentally as they can easily interconvert due to their low rotational energy barrier. Here, we use the momentum correlation of fragment ions produced after inner-shell photoionization to distinguish conformational isomers of 1,2-dibromoethane (C2H4Br2). We demonstrate that the three-body breakup channel, C2H4+ + Br+ + Br+, contains signatures of both sequential and concerted breakup, which are decoupled to distinguish the geometries of two conformational isomers and to quantify their relative abundance. The sensitivity of our method to quantify these yields is established by measuring the relative abundance change with sample temperature, which agrees well with calculations. Our study paves the way for using Coulomb explosion imaging to track subtle molecular structural changes.

Statistical and quantum photoionization cross sections in plasmas: Analytical approaches for any configurations including inner shells

Statistical models combined with the local plasma frequency approach applied to the atomic electron density are employed to study the photoionization cross-section for complex atoms. It is demonstrated that the Thomas–Fermi atom provides surprisingly good overall agreement even for complex outer-shell configurations, where quantum mechanical approaches that include electron correlations are exceedingly difficult. Quantum mechanical photoionization calculations are studied with respect to energy and nl quantum number for hydrogen-like and non-hydrogen-like atoms and ions. A generalized scaled photoionization model (GSPM) based on the simultaneous introduction of effective charges for non-H-like energies and scaling charges for the reduced energy scale allows the development of analytical formulas for all states nl. Explicit expressions for nl = 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, and 5s are obtained. Application to H-like and non-H-like atoms and ions and to neutral atoms demonstrates the universality of the scaled analytical approach including inner-shell photoionization. Likewise, GSPM describes the near-threshold behavior and high-energy asymptotes well. Finally, we discuss the various models and the correspondence principle along with experimental data and with respect to a good compromise between generality and precision. The results are also relevant to large-scale integrated light–matter interaction simulations, e.g., X-ray free-electron laser interactions with matter or photoionization driven by a broadband radiation field such as Planckian radiation.

Photoionization of Astrophysically Relevant Atomic Ions at PIPE

We review recent work on the photoionization of atomic ions of astrophysical interest that has been carried out at the photon-ion merged-beams setup PIPE, a permanently installed end station at the XUV beamline P04 of the PETRA III synchrotron radiation source operated by DESY in Hamburg, Germany. Our results on single and multiple L-shell photoionization of Fe+, Fe2+, and Fe3+ ions, and on single and multiple K-shell photoionization of C−, C+, C4+, Ne+, and Si2+ ions are discussed in astrophysical contexts. Moreover, these experimental results bear witness of the fact that the implementation of the photon-ion merged-beams method at one of the world’s brightest synchrotron light sources has led to a breakthrough for the experimental study of atomic inner-shell photoionization processes with ions.

Validity of the static-exchange approximation for inner-shell photoionization of polyatomic molecules

Author(s): Marante, CA; Greenman, L; Trevisan, CS; Rescigno, TN; McCurdy, CW; Lucchese, RR | Abstract: The simple single-channel static-exchange approximation completely ignores correlation between the continuum and molecular ion electrons. In molecular systems with symmetry equivalent atoms, the single-channel approximation can seriously fail in core ionization when using delocalized orbitals to represent the core hole states. We present cross sections and molecular frame photoelectron angular distributions with both localized and delocalized core orbitals in CF4 F (1s) ionization. We show that only a full coupled-channel calculation can recover an accurate description of the physics of inner-shell photoionization when using delocalized orbitals, whereas nearly the same result can be obtained from independent single-channel static-exchange calculations when localized core orbitals are used. A grid-based variational method described here makes such single-channel calculations possible on larger systems without local-exchange approximations. Illustrative calculations on the core ionization of SF6 are presented to illustrate the power of the grid-based method.

Self-induced splitting of x-ray emission lines

We present a theoretical approach for describing collective x-ray emission processes in extended multilevel targets, based on a combination of propagation equations for the quantum correlation functions and the semiclassical Maxwell-Bloch equations. Such a description overcomes the key deficiencies of the two constituent sets of equations, which have until now been independently used to describe these phenomena. The model developed is employed to study the spectral properties of superfluorescence of $K\ensuremath{\alpha}$ emission in zinc at $\ensuremath{\sim}8630\phantom{\rule{4pt}{0ex}}\mathrm{eV}$ after inner-shell photoionization with intense attosecond free-electron laser pulses. At high pump intensities, the numerical simulations predict a splitting of the $K{\ensuremath{\alpha}}_{1}$ emission line due to a self-induced Autler-Townes effect, which could readily be observed with standard high-resolution x-ray spectrometers. As short duration of the pump pulse is one of the crucial parameters for the manifestation of this phenomenon in the hard-x-ray regime, experimental verification could be possible due to the most recent developments of free-electron laser sources.

Narrow-band hard-x-ray lasing with highly charged ions

A scheme is put forward to generate fully coherent x-ray lasers based on population inversion in highly charged ions, created by fast inner-shell photoionization using broadband x-ray free-electron-laser (XFEL) pulses in a laser-produced plasma. Numerical simulations based on the Maxwell–Bloch theory show that one can obtain high-intensity, femtosecond x-ray pulses of relative bandwidths Δω/ω = 10−5–10−7, by orders of magnitude narrower than in x-ray free-electron-laser pulses for discrete wavelengths down to the sub-ångström regime. Such x-ray lasers can be applicable in the study of x-ray quantum optics and metrology, investigating nonlinear interactions between x-rays and matter, or in high-precision spectroscopy studies in laboratory astrophysics.

Angular distribution and polarization of characteristic x-ray lines following innershell photoionization of tungsten by linearly polarized light

The innershell 2p3/2 photoionization of tungsten by linearly polarized light has been investigated, together with the subsequent Lα1 (3d5/2 → 2p3/2) and Lα2 (3d3/2 → 2p3/2) radiative decays. In particular, a specific density matrix formalism is presented for the photoionization and decay process to explore how the linear polarization of the incident ionizing light influences the photoionization process as well as the angular and polarization properties of the emitted characteristic Lα1, 2 photons. Detailed calculations show that the alignment parameter A20 of the 2p3/2 hole state is independent of the polarization of the ionizing photons, while the parameters A2±2 behave proportional to the polarization. It is found that the Lα1, 2 characteristic lines become stronger polarized for linearly polarized incident light with the degree P=−1, when compared with the results in Ref. [Phys. Rev. A 93, 033409 (2016)] for initially unpolarized light, while the characteristic lines become unpolarized for P=1. It is also found that the Lα1, 2 angular distributions are isotropic for the case of P=−1, while they become anisotropic for P=0 and 1.

Study of chiral asymmetries in the inner-shell photoionization of partially oriented trifluoro-methyloxirane

SynopsisPhotoelectron circular dichroism (PECD) was determined for O(1s) photoionization of methyloxirane (C3H6O) and trifluoro-methyloxirane (C3H3F3O). Several electron energies above the threshold were chosen to compare the partially fixed-in-space electron distributions.

Femtosecond-resolved observation of the fragmentation of buckminsterfullerene following X-ray multiphoton ionization

X-ray free-electron lasers have, over the past decade, opened up the possibility of understanding the ultrafast response of matter to intense X-ray pulses. In earlier research on atoms and small molecules, new aspects of this response were uncovered, such as rapid sequences of inner-shell photoionization and Auger ionization. Here, we studied a larger molecule, buckminsterfullerene (C60), exposed to 640 eV X-rays, and examined the role of chemical effects, such as chemical bonds and charge transfer, on the fragmentation following multiple ionization of the molecule. To provide time resolution, we performed femtosecond-resolved X-ray pump/X-ray probe measurements, which were accompanied by advanced simulations. The simulations and experiment reveal that despite substantial ionization induced by the ultrashort (20 fs) X-ray pump pulse, the fragmentation of C60 is considerably delayed. This work uncovers the persistence of the molecular structure of C60, which hinders fragmentation over a timescale of hundreds of femtoseconds. Furthermore, we demonstrate that a substantial fraction of the ejected fragments are neutral carbon atoms. These findings provide insights into X-ray free-electron laser-induced radiation damage in large molecules, including biomolecules. X-ray pump–probe experiments reveal that the molecular structure of C60 molecules substantially delays their fragmentation following photoionization. This may help to understand X-ray laser-induced radiation damage on molecules.

The effect of elliptical polarization in MSXα calculations of the molecular-frame photoelectron angular distributions of CO C(1s) ionization

We present a theoretical investigation of the molecular-frame photoelectron angular distribution (MF-PAD) resulting from the inner-shell photoionization of fixed-in-space CO molecules with elliptically polarized light. The MSXα output program, originally written for linearly polarized light, was changed in order to calculate the MF-PAD for circularly and elliptically polarized light. The calculations were made for CO C(1s) inner-shell photoionization for 3, 10, and 30 eV kinetic energy electrons. The MF-PADs were calculated from 0° angle of ellipticity (equivalent to linear polarized light parallel to the molecular axis) in steps of 5° (corresponding to different types of elliptical light) up to 90° (equivalent to linear polarized light perpendicular to the molecular axis).