L-shell Ionization Research Articles

M- and L-shell ionization in near-central collisions of 5.5-MeV/amu 16O ions with Mo atoms deduced from theoretical analysis of high-resolution K x-ray spectra.

The K\ensuremath{\alpha} and K\ensuremath{\beta} x-ray spectra of molybdenum bombarded by 5.5-MeV/amu $^{16}\mathrm{O}$ ions were measured with high resolution. In such heavy-ion--atom collisions, multiple ionization of the M and L shells of the target atoms is extremely likely to occur, resulting in a complex structure of the observed spectra. The ${\mathit{L}}^{\mathit{n}}$-satellite structure was resolved, whereas M vacancies produce only a shift and a broadening of the lines so that it is not possible to obtain in a direct way information about the M-hole distribution. We thus propose a method for the analysis of x-ray spectra of multiply ionized atoms. In this method the measured K\ensuremath{\alpha} and K\ensuremath{\beta} spectra are simultaneously analyzed as a sum of K\ensuremath{\alpha}${\mathit{L}}^{\mathit{n}}$${\mathit{M}}^{\mathit{m}}$ and K\ensuremath{\beta}${\mathit{L}}^{\mathit{n}}$${\mathit{M}}^{\mathit{m}}$ components, respectively, with theoretically determined profiles. A binomial distribution of holes in the M shell is assumed and the M-shell ionization probabilities are treated as adjustable parameters. The profiles are constructed as sums of Voigt functions, whose positions and heights are determined by extensive multiconfiguration Dirac-Fock method calculations. If, in a simultaneous fit to the K\ensuremath{\alpha}${\mathit{L}}^{0}$ and K${\mathrm{\ensuremath{\beta}}}_{1,3}$${\mathit{L}}^{0}$ lines, just one parameter ${\mathit{p}}_{\mathit{M}}^{\mathit{X}}$ describing the M-shell ionization at the moment of the K x-ray transition is used, the experimental data are not reproduced in an entirely satisfactory way (${\mathit{p}}_{\mathit{M}}^{\mathit{X}}$=0.19\ifmmode\pm\else\textpm\fi{}0.02). Therefore, two parameters, one common for the 3s and 3p subshells ${\mathit{p}}_{3\mathit{s}\mathit{p}}^{\mathit{X}}$ and the other for the 3d subshells, were introduced and a much better fit to both lines (${\mathit{p}}_{3\mathit{s}\mathit{p}}^{\mathit{X}}$=0.17\ifmmode\pm\else\textpm\fi{}0.02, ${\mathit{p}}_{3\mathit{d}}^{\mathit{X}}$=0.23\ifmmode\pm\else\textpm\fi{}0.02) was obtained.The importance and influence of alternative ionizing processes as electron-capture and rearrangement processes are discussed and the M-shell ionization probability at the moment of the collision (${\mathit{p}}_{\mathit{M}}$=0.18\ifmmode\pm\else\textpm\fi{}0.02) is deduced from the ${\mathit{p}}_{\mathit{M}}^{\mathit{X}}$ value. It is shown that the differences between ${\mathit{p}}_{3\mathit{s}\mathit{p}}^{\mathit{X}}$ and ${\mathit{p}}_{3\mathit{d}}^{\mathit{X}}$ are mainly due to Coster-Kronig transitions, rather than due to different subshell ionization probabilities. The spectra were also analyzed with respect to the L-shell ionization on the basis of the calculated K\ensuremath{\alpha}${\mathit{L}}^{\mathit{n}}$${\mathit{M}}^{0}$, K${\mathrm{\ensuremath{\beta}}}_{1,3}$${\mathit{L}}^{\mathit{n}}$${\mathit{M}}^{0}$, and K${\mathrm{\ensuremath{\beta}}}_{2}$${\mathit{L}}^{\mathit{n}}$${\mathit{M}}^{0}$ components. The M-shell ionization was taken into account by taking a larger Gaussian width and by shifting the positions of the lines. The intensity yields of the L satellites were determined, the primary vacancy distribution was deduced, and the results from K\ensuremath{\alpha} and K\ensuremath{\beta} spectra are compared.

L-shell ionization in Au/Ag foils by electron and positron impact

Measurements of relative cross sections of K- and L-shell ionization of silver and gold targets by positron and electron impact at projectile energies of 30–70 keV were performed at the slow positron source TEPOS at the linac of the Strahlenzentrum in Giessen. The experimental equipment will be described shortly and the corresponding results will be presented. These results agree well with independently performed plane wave Born (PWBA) calculations accounting for electron exchange, and the deceleration or acceleration of the projectiles in the nuclear field of the target atom.

Multiple photoionization of Ne in the K-shell ionization region

Multiple photoionization of Ne in the K-shell ionization region (810-1340eV) has been studied using a time-of-flight mass spectrometer. The singly charged ion Ne+ appears only at the 1s → 3p excitation, except for production through direct L-shell ionization. The doubly charged ion Ne2+ shows the most intense yield above the 1s → 3p excitation, and K-shell excited Rydberg states mainly turn into Ne2+ through Auger shake processes. The partial photoionization cross sections of Ne3+ and Ne4+ have maxima at about 130eV and 230eV above the 1s ionization threshold, respectively. These maxima are attributed to shake-off processes at the initial photoabsorption step. Double Auger processes following the 1s photoionization are estimated to happen at about 6% relative to normal Auger processes.

Coupled channel treatment of the L-shell ionization in ion-atom collisions

The deviations of the experimental L-subshell ionization probabilities relative to the semiclassical approximation are discussed in terms of couplings between the target atomic states during the collision by means of a relativistic one-centre coupled channel formalism with hydrogenic wavefunctions. Numerical results are compared with experimental data for systems He+Pt, Ne+Pt, Ne+Yb at 0.9 MeV amu-1 and S+Au at 1 MeV amu-1. A simple inclusion of the screening effect presented in this work yields a major improvement in the description of the L-shell ionization probabilities. The intra-L-shell couplings modify slightly the ionization probabilities for light projectiles but they have a marked effect for heavier projectiles. The couplings between the L and M shells were also included, but they were found to have only a small influence as compared with intra-L-shell couplings.

Multiple photoionization of K and Ca atoms in the L-shell ionization region

The charge state distribution of photoions from K and Ca targets has been measured in the L-shell ionization region (270-400 eV for K, 330-500 eV for Ca) using a time-of-flight technique coupled with synchrotron radiation as a light source. In both the K and the Ca targets, doubly and triply charged ions are the prominent products, and outstanding doublet peaks due to the 2p-3d resonance excitation are observed in the yield spectra. The doubly charged K2+ ion predominates over the K3+ ion throughout the present energy range. On the other hand, the Ca2+ ion is the most intense species below the 2p ionization threshold and the Ca3+ ions dominate over Ca2+ ions above the threshold. These features of the photoion can be understood mainly by successive Auger decays from the 2p-shell vacancy states. The yield of higher charged ions indicates some contribution of shake processes.

L-Shell Ionization by Positron and Electron Impact

Measurements of relative cross sections of K- and L-shell ionization of silver and gold targets by positron and electron impact at projectile energies of 30–70 keV are reported. The experiments were performed at the slow positron source TEPOS at the linac of the Strahlenzentrum. This source and the experimental equipment will be described briefly and the experimental results will be presented in detail. In addition, plane wave Born (PWBA) calculations were performed. They include an electron exchange term and the deceleration or acceleration of the incident projectile in the nuclear field of the target atom. The present experimental results agree well with these calculations; for L-shell ionization, they are at variance with a previous experiment where a different energy dependence was observed.

L-shell ionization by positron and electron impact

Cross section ratios for L-shell ionization of gold by positron and electron impact have been measured at energies of 35−65 keV. Also PWBA calculations were performed which take electron exchange and the deceleration or acceleration of the incedent positron or electron, respectively, in the target nuclear field into account; these calculations are in reasonable agreement with the measurements.

Electron and positron impact experiments on metallic foils at the giessen positron source TEPOS

The sources TEPOS for slow positrons were installed at the Giessen 65 MeV electron linear accelerator. Ratios of K- and L-shell ionization cross-sections by electron and positron impact were measured on Au-/Ag-foils as function of the incident projectile energy. With energies near threshold a departure is observed of the ratio σ(e−)/σ(e+) from unity (increasing with decreasing energy).

Dynamical subshell coupling effects in L-shell ionization of thorium by heavy ions

Calculations have been made to interpret the L-shell ionization data measured by Berinde et al. (1988) for collisions of Be, C, F, Mg, Si and S ions with Th in the energy range 0.5-2.5 MeV amu-1. The calculations are based on the cross section values obtained by Chen et al. (1985) in the plane-wave Born approximation with the use of Dirac-Hartree-Slater wavefunctions. The cross sections of the first-order theory have been corrected to include higher-order effects due to the dynamical couplings between the L subshells. The coupling effects have been determined by solving a set of coupled equations which govern the time evolution of the L substate amplitudes. A reasonable agreement has been found between the experiment and theory.

L-shell ionization by antiprotons.

Semiclassical coupled-state model calculations have been performed for L-shell ionization of gold induced by protons and antiprotons in the energy range 0.15--3 MeV. The results of the calculations have been compared with the predictions of a simple approach suggested by Brandt and Basbas for description of antiparticle excitation of atomic inner shells. Apart from the range of very low collision velocities, a reasonable agreement has been found between the two models. In addition to the binding and Coulomb-distortion effects discussed by Brandt and Basbas [Phys. Rev. A 27, 578 (1983); 28, 3142(E) (1983)], for the L shell a further effect due to dynamical couplings between the L-substate ionization amplitudes also contributes to the particle-antiparticle differences. While the latter processes have only a negligible effect on the energy dependence of the cross sections, their inclusion has been shown to be unavoidable when ratios of the subshell ionization cross sections are analyzed for the two kinds of excitation.

Extreme ultraviolet amplifier experiments at the LLNL Nova two-beam facility (abstract)

We present the results of our most recent experiments on extreme ultraviolet (XUV) and soft x-ray amplifiers conducted at LLNL’s Nova laser two-beam facility. Of particular interest are high-Z nickel-like amplifiers which have demonstrated gain at photon energies above the carbon K edge, sub-Doppler-resolution spectra of amplified lines from long selenium amplifiers, absolutely timed streaked XUV and x-ray spectra from germanium amplifiers at a variety of L-shell ionization states, and gain measurements from neon-like silver amplifiers. The array of spectroscopic instruments utilized in these experiments will be described in detail. This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract W-7405-ENG-48.

Impact-parameter-dependent multiple ionisation probabilities for fast proton-neon collisions

Absolute multiple ionisation probabilities in dependence of the impact parameter b have been measured for 0.5 and 1.0 MeV proton on neon collisions in an impact parameter regime of b<or approximately=rK, where rK is the expectation value for the Ne K-shell radius. The analysis of the experimental data within the independent-particle model provided b-dependent absolute K and L-shell ionisation probabilities P8K(b) and PL(b). The PL(b) are in fair agreement with results of classical many-body calculations (CTMC), the PK(b) are well described in the semiclassical approximation (SCA).

Light-ion-induced L i ionization of 46 ≤ Z ≤ 60 elements: First-and second-order corrections to PWBA

Recently measured proton- and 4He-ion-induced L i -subshell ionization cross sections for the elements with 46 ≤ Z ≤ 60 have been compared with the ECPSSR predictions. A recalculation of the binding correction, which leads, when the incident energy decreases, to a limit of the binding energy of the electrons concerned corresponding to a United-Atom approximation is suggested. A good agreement between the experimental and predicted L-shell ionization cross sections and a satisfactory one for the single L i -subshell ones is obtained. The introduction of vacancy rearrangement effects gives further better agreement, but not enough to reproduce completely the experimental findings.

The effect of the relativistic Hartree-Fock-Roothaan momentum distribution in the binary encounter approximation for inner shell ionization

K- and L-shell ionization cross sections of gold due to impact of proton and alpha particles have been calculated in the binary encounter approximation incorporating the effects of Coulomb deflection of the projectile, of increase in binding of the target electron, and of polarization and relativity. Relativistic Hartree-Fock-Roothaan (RHFR) momentum distributions for the target electron along with an approximate relativistic correction to the collision dynamics have been used in the present calculations. A comparison with the corresponding calculations using non-relativistic Hartree-Fock-Roothaan wave functions, experimental results and other available calculations is presented.

L-shell ionization of Si, P, S, Cl, and Ar by 0.4-2.1-MeV He+ bombardment.

The L-shell ionization cross sections of Si, P, S, Cl, and Ar have been obtained for incident ${\mathrm{He}}^{+}$ ion energies 0.4--2.1 MeV in steps of 0.2 MeV. The cross sections were obtained from the Auger electron yields measured for ${\mathrm{SiH}}_{4}$, ${\mathrm{PH}}_{3}$, ${\mathrm{H}}_{2}$S, ${\mathrm{CH}}_{3}$Cl, and Ar gaseous targets. The present experimental L-shell ionization cross sections are compared to existing experimental L-shell ionization cross sections as well as to previous theoretical predictions.

Production of recoil Nei+ ions accompanied by electron loss and capture of 1.05-MeV/amu Neq+ (q=2, 4, 6, 8, and 10) ions.

The charge distributions and production cross sections of recoil ions accompanied by projectile electron loss and capture have been compared in 1.05-MeV/amu ${\mathrm{Ne}}^{\mathrm{q}+}$ (q=2, 4, 6, 8, and 10) +Ne collisions. While singly charged recoil ions are dominant in pure ionization processes, the production of highly charged recoil ions is strongly enhanced in collisions involving simultaneous charge change (by both electron loss and capture) of projectiles, and the charge distributions of recoil ions are found to be similar for both projectile loss and capture processes. These results have been analyzed with an independent-electron model and indicate that the dominant contribution to the production of higher-charge recoil ions comes from L-shell ionization, with some contribution from K-shell ionization of targets, accompanied by violent multiple ionization of projectiles.

Charge distribution of Ar recoil ions produced in one- and two-electron capture collisions by 16-MeV Oq+ projectiles.

The yields of Ar recoil ions produced in collisions with 16-MeV ${\mathrm{O}}^{\mathrm{q}+}$ (q=3--8) were measured in coincidence with one- and two-electron capture by the projectile. Time-of-flight analysis identified the charges of the Ar recoil ions and enabled the determination of charge-state distributions. The independent-electron approximation was employed along with a simple exponential parametrization of the ionization and capture probabilities to calculate the cross sections for multiple ionization accompanied by one- and two-electron capture. By accounting for direct L-shell ionization and Auger multiplication of L-shell vacancies, good agreement between calculated and measured charge-state distributions was attained.

Electron degradation and yields of initial products: V. Degradation spectra, the ionization yield, and the Fano factor for argon under electron irradiation.

The electron-degradation spectrum is fundamental for describing a variety of quantities bearing on electron slowing-down processes in matter. We calculate the electron-degradation spectrum in Ar gas by solving the Spencer-Fano equation, using a realistic set of cross sections. The influence of Auger electrons on the degradation spectrum is studied in detail. As an application, we study the statistical fluctuations in the ionization yield, which are expressed in terms of the Fano factor F(T) for an electron incident at fixed energy T. The energy dependence of F(T) is greatly influenced by L-shell ionization. The Fano factor approaches an asymptotic value of 0.16 at T=2 keV. Our results are consistent with experimental results.

L-shell ionization and L X-ray emission induced by antiprotons

The ionization of the L shell of atoms by energetic antiprotons has been studied theoretically for the first time. In addition to the effects discussed by Brandt and Basbas for the K shell, for the L shell a further effect has been, shown to contribute to the particle-antiparticle differences in the cross sections. The effect is a result of coupling processes between the different L substates. As a demonstration, model calculations based on the second-order Born approximation have been performed for gold in the energy range 0.2–1 MeV.

Charge multiplication via Auger decay of L vacancies in the production of highly charged Ar ions by collisions with 1-MeV/amu Oq+ and Fq+

Cross sections for the production of Ar recoil ions in collisions with 1-MeV/amu ${\mathrm{O}}^{\mathrm{q}+}$ and ${\mathrm{F}}^{\mathrm{q}+}$ (q=2 to bare nuclei) were measured by the time-of-flight technique. The cross sections for the production, by pure ionization, of recoil ions having charges greater than 6 display an unexpected behavior as a function of q. They rise at low q, reach a maximum in the vicinity of q=5--7, and then decrease at high q. Cross sections calculated using the independent-electron approximation rise continuously in this region and are lower than the experimental cross sections by as much as 4 orders of magnitude for high recoilion charge and low q. The observed turnover in the cross sections, as well as the large discrepancies in absolute magnitude between experiment and calculation, are accounted for by vacancy multiplication resulting from L-shell ionization followed by Auger decay.