Post-collision Interaction Effects Research Articles

Analysis of electron induced single ionisation differential cross-section of CO2 molecules in different kinematics

Theoretical investigations of electron-impact ionisation for carbon dioxide (CO2) in low to intermediate energy ranges are presented. In addition to the energy range, we have estimated triple differential cross-section (TDCS) in coplanar and perpendicular plane geometry. We have calculated the TDCS for electron impact using the modified distorted wave formalism up to the second order. Post-collision interaction effects have also been introduced to describe the collision dynamics accurately. There is a good correlation between the distorted-wave Born approximation results and the experimental results in terms of forward and backward scattering regions.

Electron impact triple differential cross sections of Xe atoms for coplanar to perpendicular plane single ionisation at 60 eV, 80 eV and 100 eV above ionisation

The evolution of the triple differential cross section (TDCS) for the electron impact ionisation of xenon atoms is reported for the variation of momentum of the projectile electron from a coplanar geometry to a perpendicular plane through intermediate angles with the detection plane. The TDCSs have been calculated for the Xe atoms at 60 eV, 80 eV and 100 eV above the ionisation potential. We have calculated the TDCS using distorted wave Born approximation, utilising both the first and second Born terms. Effects of target polarisation and post-collision interaction have also been included in the complete description of the collision dynamics. The TDCS results are compared with recent measurements by Patel et al (2022 Phys. Rev. A 105 032818) showing a dependence between the TDCS and the scattering geometry and kinematics of the collision. Second-order effects have been found significant, particularly in the description of perpendicular plane ionisation at a low energy and the effect of target polarisation has been found important in describing the coplanar ionisation of the xenon target. With an overall good agreement with the recent measurements, there are points of disagreement which are the motivation for further theoretical effort in the near future, as the present attempt is the first of its kind to analyse these measurements.

Generalization of the post-collision interaction effect from gas-phase to solid-state systems demonstrated in thiophene and its polymers

We demonstrate experimentally and theoretically the presence of the post-collision interaction (PCI) effect in sulfur ${\mathrm{KL}}_{2,3}{\mathrm{L}}_{2,3}$ Auger electron spectra measured in the gas-phase thiophene and in solid-state organic polymers: polythiophene (PT) and poly(3-hexylthiophene-2,5-diyl), commonly known as P3HT. PCI manifests itself through a distortion and a blueshift of the normal Auger S ${\mathrm{KL}}_{2,3}{\mathrm{L}}_{2,3}$ spectrum when S $1s$ ionization occurs close to the ionization threshold. Our investigation shows that the PCI-induced shift of the Auger spectra is stronger in the solid-state polymers than in the gas-phase organic molecule. Theoretical modeling within the framework of the eikonal approximation provides good agreement with the experimental observations. In a solid medium, two effects influence the interaction between the photoelectron and the Auger electron. In detail, stronger PCI in the polymers is attributed to the photoelectron scattering in the solid, which overcompensates the polarization screening of electron charges which causes a reduction of the interaction. Our paper demonstrates the general nature of the PCI effect occurring in different media.

Electron-impact ionization of the 4p6 subshell in strontium

Electron-impact ionization of the 4p6 subshell in Sr atoms has been studied by observing the excitation dynamics of the electron spectra arising from the Auger decay of the 4p states. The measurements were carried out in the energy range of incident electrons from the 4p6 ionization threshold at 26.92 to 600 eV. At near-threshold impact energies, the effect of post-collision interaction was found for most of the Auger lines. The identification of 21 lines observed between 10 and 14 eV electron energy showed that the 4p6 Auger spectrum of Sr atoms is completely described by the configurations 4p54d5s, 4p55s2 and 4p54d2. The cross sections for the lowest quartet and doublet states in 4p54d5s and 4p55s2 configurations have been obtained with the maximum values of ( cm2 at 52 eV for the (4p54d5s)4P state and ( cm2 at 112 eV for the (4p55s2)2P state. Configuration interaction and excitation-autoionization processes determine the energy behavior of the excitation cross sections of doublet states. The lower limit of the total ionization cross section of the 4p6 subshell in Sr has been obtained with the maximum value of ( cm2 at 92 eV.

Manifestation of postcollision interaction in Krypton LMN Auger spectra following K -shell photoionization

We report on an experimental and theoretical study of postcollision interaction (PCI) effects on ${L}_{2}\ensuremath{-}{M}_{4,5}{N}_{2,3}$ Auger electrons measured above the Kr $K$-edge in which ${L}_{2}$ vacancies are primarily generated by $K{L}_{2}$ x-ray emission. Such cascade processes, in which a deep inner-shell vacancy decays first by x-ray emission followed by Auger electron emission, is a strong decay mode in heavy atoms. The ${L}_{2}\ensuremath{-}{M}_{4,5}{N}_{2,3}$ Auger electron peak is observed to become increasingly asymmetric with a shifting peak maximum as the absorbed x-ray energy approaches the $K$-shell ionization threshold. This is attributed to PCI energy exchanges of the Auger electron with the $1s$ photoelectron. To model the PCI effects, we have applied a semiclassical approach modified to account for the combined lifetimes of the $K$ and ${L}_{2}$ hole states. In addition, our analysis treats several closely spaced Auger transitions with final ionic states having different terms and angular momenta.

Time-dependent post-collision-interaction effects in THz-field-assisted Auger decay

The effects of postcollision interaction (PCI) in the time evolution of photoinduced Auger decay, assisted by a terahertz (THz) field, is theoretically investigated. We propose a time-dependent model which is based on the quantum-mechanical description of the photoinduced Auger process and quasiclassical description of the PCI. The suggested model is used for calculating the PCI distortion of the Auger spectrum at different temporal overlaps of the exciting extreme ultraviolet pulse and the streaking THz pulse.

Post-collision interaction effect in THz-assisted Auger decay of noble gas atoms

Auger electron spectra were simulated in the presence of a terahertz streaking field to study post-collision interaction (PCI) effects in the time-evolution of photoinduced Auger decays. The PCI is characterized by the ratio of the spectral line width for Auger electron emission in opposite directions with respect to the THz-field. These calculations have been performed using the analytical semiclassical model developed by Bauch and Bonitz (2012 Physical Review A 85 053416). The results are shown for Ne(KLL), Ar(LMM), Kr(MNN) and Xe(NOO) Auger transitions whereby different possible experimental conditions were evaluated.

The M4,5-NN Auger and M2,3-M4,5N Coster-Kronig spectra of krypton induced by electron impact

The Kr M4,5-NN Auger transitions are studied in the kinetic energy region 24 - 59 eV by using non-monochromatic electron beam and a high resolution electrostatic analyzer. Among the sixty four features observed, fifty five are assigned, while nine stay unassigned. The post-collision interaction (PCI) effects in the M5-N1N1(1S0) and M5-N2,3N2,3(1S0) Auger peaks are studied as a function of electron incident energy approaching the M5 edge (95.04 eV). The PCI shifts are found to be well represented by an exponential function of the excess energy with exponents β = (-0.99±0.05 and -3.08±0.05) for the N2,3N2,3(1S0) and N1N1(1S0) peaks, respectively. The M2,3-M4,5N Coster-Kronig transitions are studied in kinetic energy region 65 - 110 eV and at incident energies from 120 to 2019 eV. Owing to a high resolution twelve well resolved satellites in the region bellow 90 eV are observed, while in the region 90 - 110 eV two broad features are assigned to the M2,3-M4,5N2,3 C-K peaks with intensity ratio (M3/M2 = 2.37) are observed. Comparison is made with available data from literature.

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

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

Fluorescence Time Delay in Multistep Auger Decay as an Internal Clock.

Differences in postcollision interaction (PCI) effects on Kr L_{3}M_{4,5}M_{4,5} Auger electron spectra were observed, depending on whether the initial photoionization occurred slightly above the K threshold or slightly above the L_{3} threshold. For the former, KL fluorescence emission most likely happens and then Auger processes due to the L_{3} hole follow. The time delay due to fluorescence causes a reduced shift of the Auger peak and tailing toward lower energy, since the Auger overtaking of the photoelectron happens later in time and at a location farther away from the ionic core, compared to the case for the simple one-step L_{3}M_{4,5}M_{4,5} Auger decay after L-shell photoionization. Time-dependent theory for PCI in multistep processes agrees well with experiment, illustrating the effect as an internal clock for the time-sequence of the dynamical process.

Features of the Excitation of Principal Series Lines of Atoms of the Zinc Subgroup by an Electron Impact: I. Cadmium

Using the technique of intersecting beams of slow electrons and atoms of zinc, excitation functions of four spectral lines of Zn I principal series corresponding to the 4snp $${}^{1}P_{1}^{^\circ }$$ → 4s2 1S0 transitions for n = 4, 5, 6, and 7 are measured. Their wavelengths are 213.9, 158.9, 145.8, and 140.4 nm, respectively. Within the energy range of 11–18 eV, the effect of postcollision interaction of electrons (slow scattered and fast ejected ones) has been revealed on these functions with the decay of the autoionizing state. At incident electron energies of ∼11–13 and ∼15–18 eV, such interaction leads to an additional population of the initial levels for these lines and, correspondingly, to the formation of maximum on their excitation functions due to the capture of a scattered electron into these excited levels. The terms of atomic autoionizing states responsible for the observed maximums are determined. The combined influence of the decay of these autoionizing states and postcollision interaction is expressed in the energy shift of the maximums on the excitation functions relative to positions of the corresponding autoionizing states. Using a straightforward calculation in the classical approximation, effective widths of the electron decay of the autoionizing states are estimated. The estimates involve an exact formula and approximation formulas. The latter are valid for different relations between the value of the postcollision shift of maximums on the excitation functions and electron bonding energy at the atomic level. They are used for the determination of effective widths when approximating experimental shifts by the least squares method.

Target dependence of postcollision interaction effects on fully differential ionization cross sections

We have measured and calculated fully differential cross sections (FDCS) for ionization of helium by 75-keV proton impact. Ejected electrons with a speed close to and above the projectile speed were investigated. This range of kinematics represents a largely unexplored regime. A high sensitivity of the FDCS to the details of the description of the few-body dynamics, reported earlier for ionization of ${\mathrm{H}}_{2}$, was confirmed. A peak structure was found in an electron angular range between the regions where the so-called binary and recoil peaks are usually observed. The need for nonperturbative calculations using a two-center basis set is demonstrated.

Single ionization of helium by fast proton impact in different kinematical regimes

We present ultrahigh-resolution data on fully differential cross sections for single ionization of helium induced by 1 MeV proton impact. In the present work we explore a different regime of kinematic conditions in terms of momentum transfer and electron energies than previously published data. These data are compared with different theoretical calculations. Reasonable agreement between the first Born approximation and experiment is obtained in the kinematic regime close to the Bethe ridge. Far from this region the calculated binary peak is shifted with respect to experiment. In order to resolve this problem, we analyze several theoretical mechanisms beyond the customary first Born approximation theory. These mechanisms include the 3C model (three Coulomb functions), effective charges, off-shell pair $T$ matrices instead of pair potentials, and semiclassical postcollision interaction. We find that a combination of the 3C model with a semiclassical postcollision interaction effect may explain the observed discrepancy.

Selectivity of the Br 3d−1 Auger decays in HBr

The Auger decay of the spin-orbit and molecular-field split Br $3{d}^{\ensuremath{-}1}$ core holes in HBr is investigated, both by a photoelectron--Auger-electron coincidence experiment and by ab initio calculations based on the one-center approximation. The branching ratios for the Auger decay of the five different core-hole states to the $4p{(\ensuremath{\sigma},\ensuremath{\pi})}^{\ensuremath{-}2}$ dicationic final states are determined. Experimental and theoretical data are in good agreement and conform to results for the $4p{\ensuremath{\pi}}^{\ensuremath{-}2}$ final states from a previous analysis of the high-resolution conventional Auger-electron spectrum. The branching ratios for the Br $3{d}^{\ensuremath{-}1}$ Auger decay to the $4p{(\ensuremath{\sigma},\ensuremath{\pi})}^{\ensuremath{-}2}$ with \ensuremath{\Sigma} symmetry follow the propensity rule of ${L}_{2,3}VV$ Auger decay [S. Svensson, A. Ausmees, S. J. Osborne, G. Bray, F. Gel'mukhanov, H. \AA{}gren, A. Naves de Brito, O.-P. Sairanen, A. Kivim\aki, E. N\~ommiste, H. Aksela, and S. Aksela, Phys. Rev. Lett. 72, 3021 (1994)] stating that the oriented core holes decay preferentially by involving a valence electron from an orbital with the same spatial orientation. For the ${M}_{4,5}VV$ decay in HBr this propensity rule has to be supplemented by the requirement that the Auger-electron channel and the other valence orbital have the same preferential orientation. We also probe the influence of the Auger kinetic energy on the distortion of the photoline caused by the postcollision interaction effect. For small kinetic energies, differences between experimental results and theoretical predictions are identified.

Revealing excitonic processes and chemical bonding in MoS2 by x-ray spectroscopy

Electron core-hole processes at absorption edges sport several signatures, primarily in the form of bound states and/or excitonic enhancement. Through analysis of the resonant-Raman Auger effect for the two-dimensional transition-metal dichalcogenide $\mathrm{Mo}{\mathrm{S}}_{2}$, we demonstrate that these effects have significant manifestations at the S $K$ edge. We characterize two asymmetric excitonically enhanced edges: one at the fundamental band edge and one several electron volts above it following a second band gap that lies entirely within the unoccupied states, with the latter revealing an anomalous postcollision interaction effect. Our interpretation is supported by comparison of both the photoemission valence-band spectrum and the S $K$-edge x-ray-absorption spectrum with electronic structure calculations and an ab initio solution of the Bethe-Salpeter equation that accurately accounts for the electron-core-hole interaction.

Theoretical investigation of the double differential cross section for electron impact ionization of methane molecule

SynopsisWe present theoretical calculations of double differential cross sections (DDCS) for methane molecule. Angular distributions of secondary electrons have been measured after ionizing electron collisions with methane at primary energy of 350 eV using a conventional electron spectrometer. The theoretical model used is the first and second Born approximations with Gamow factor. It is found that these theoretical calculations are successful to describe the post-collision interaction effects due to the Coulomb long-range interaction between the outgoing electrons in the final state.

Valence Auger decay following3sphotoionization in potassium

We have studied photoionization in the inner valence $3s$ subshell of K and the spectroscopic properties of the two $3{s}^{\ensuremath{-}1}\phantom{\rule{0.28em}{0ex}}(^{1}S)$ and $(^{3}S)$ resulting states. Similar to the Rb and Cs cases, the lifetime widths of the $(^{1}S)$ and $(^{3}S)$ states are found to be markedly different, due to the electron correlation effects. The main part of the study deals with the subsequent Auger decay of the $3{s}^{\ensuremath{-}1}$ states, which have the particularity to involve low energy $(\ensuremath{\sim}5\phantom{\rule{0.16em}{0ex}}\mathrm{eV})$ Auger electrons. A magnetic bottle spectrometer with a multicoincidence technique has been used to observe and filter the Auger spectra with respect to the ${\mathrm{K}}^{2+}$ final state. The evolution of these Auger spectra has been investigated near the ionization threshold. They show strong post-collision interaction (PCI) effects, which are well reproduced by semiclassical and eikonal models. They reveal the importance of the photoelectron--Auger-electron interaction associated with these low energy Auger electrons.

Postcollision interaction effects inKLLAuger spectra following argon1sphotoionization

Postcollision interaction effects on the Auger decay of a deep core hole are studied both experimentally and theoretically. KL2,L-3(2),(3) decay spectra of the Ar 1s vacancy are measured with high- ...

Comparison of different quantum mechanical methods for inner atomic shell photo-ionization followed by Auger decay

A numerical time-dependent quantum mechanical approach was developed previously for simulating the process of photoionization followed by Auger decay for cases where the photoelectron energy is not very large; the method accurately calculates the interaction between the two active electrons, but simplifies their interaction with the core electrons. More established theoretical methods, which take account of postcollision interaction effects, allow an accurate description of this process when the photoelectron energy is not too low. We demonstrate that using the time-dependent method (although with some simplifications that are needed for its numerical implementation) for low energy photoelectrons and more established methods for higher energy allows accurate calculations for nearly all possible combinations of electron energy. This is confirmed by performing calculations of the photoelectron energy and angular distributions for the 1 s photoionization of Ne, with a subsequent KLL Auger transition. By computing the energy and angular distributions for energies where the two groups of methods should agree and where they should disagree, we demonstrate their consistency and range of accuracy. For the regions where the methods disagree, we discuss the reasons for any discrepancies and the trends in the differences. In addition, some of our calculations are compared with existing experimental data for the same system. The agreement found in the comparison confirms the reliability of the theoretical approaches.

Fully differential cross sections for low to intermediate energy perpendicular plane ionization of xenon atoms

Triple differential cross section (TDCS) results are reported for the perpendicular plane ionization of Xe (5p) at incident electron energies 5eV, 10eV, 20eV, and 40eV above ionization potential. The modified distorted wave Born approximation formalism with first as well as the second order Born terms has been used to calculate the TDCS. Effects of target polarization and post collision interaction have also been included. We compare the (e, 2e) TDCS results of our calculation with the recent available experimental data and theoretical results and discuss the process contributing to structure seen in the differential cross section. It has been observed from the present study that the second order effect and target polarization make significant contribution in description of collision dynamics of xenon at the low and intermediate energy for the perpendicular emission of electrons.