Shell Vacancies Research Articles

Postcollision interaction effects induced by Auger cascades following Xe L3-shell ionization.

Yield spectra of multiply charged ions have been measured in coincidence with threshold electrons near the Xe ${\mathit{L}}_{3}$-shell ionization region. The ion-yield spectra exhibit characteristic profiles of postcollision interaction effects induced by Auger cascades following Xe ${\mathit{L}}_{3}$-shell ionization. The peak shifts of the spectral profiles increase gradually along with the increase of the charge states, but the peak widths are almost independent of the charge states. The spectral profiles are reproduced by a simple model based on the assumption that Coster-Kronig and Auger shake-off decays play a major role in the Auger cascades following the Xe ${\mathit{L}}_{3}$-shell vacancy creation. \textcopyright{} 1996 The American Physical Society.

Characteristics of porous sintered tantalum: T. Oki et al. (Nagoya University, Nagoya, Japan). J. Japan Inst. Metals, vol 59, no 12, 1995, 1286–1291. (In Japanese.)

The Lι, Lβ1,4,6, Lβ2,15,3, Lγ1, Lγ2,3,6 and Lγ4,4′, X-ray intensities relative to the Lα, caused by the impact of protons of energy 1 to 4.6 MeV and Si-ions of 70 to 98 MeV on Ta targets, have been measured. The results show that the intensity ratios drop significantly for all transitions except Lγ2,3,6Lα with Si-ions of the same energy/amu as compared to those of protons. The experimental results have been compared with those based on the ECPSSR theoretical values. From the energy shift and change in the intensity ratios of various transitions caused by Si-ion impact, the number of outer shell vacancies in the M, N and O-shells simultaneous to that of L-shell have been estimated.

Measurement of L x-ray production cross sections by 60 keV photons and average L shell fluorescence yields of lanthanides

L x-ray production cross sections have been measured for lanthanides with 60 keV. The measured L x-ray production cross section values for the lanthanides are in good agreement with the theoretical ones evaluated using Li subshell fluorescence yields ωi, Coster-Kronig transition probabilities ƒij based on the Relativistic-Hartree-Slater theory, K to Li subshell vacancy transfer probabilities nKLi, fractions of the ratiative width of the subshell Fny and Li subshell photoionisation cross section σPi. The average L shell fluorescence yields ωL have also been derived using the presently measured total L x-ray production cross section values and the theoretical K to L shell vacancy transfer probabilities. These results are compared with theoretically predicted values.

Measurement of L X-ray intensity ratios in tantalum by proton and Si-ion impact

The Lι, L β 1,4,6, L β 2,15,3, L γ 1, L γ 2,3,6 and L γ 4,4′, X-ray intensities relative to the Lα, caused by the impact of protons of energy 1 to 4.6 MeV and Si-ions of 70 to 98 MeV on Ta targets, have been measured. The results show that the intensity ratios drop significantly for all transitions except Lγ 2,3,6 Lα with Si-ions of the same energy/amu as compared to those of protons. The experimental results have been compared with those based on the ECPSSR theoretical values. From the energy shift and change in the intensity ratios of various transitions caused by Si-ion impact, the number of outer shell vacancies in the M, N and O-shells simultaneous to that of L-shell have been estimated.

Full scale relativistic ab initio time dependent calculations for the L-K vacancy transfer in 208 MeV Ni 23+ on a Ge solid target

We present full scale ab initio relativistic calculations for the L-K vacancy transfer in collisions of 208 MeV Ni 23+ on a Ge solid target. Our Dirac-Fock-Slater method allows us to achieve a very accurate quantitative explanation for the experimental impact parameter-dependent vacancy probabilities in terms of dynamic creation and annihilation of Ni n = 2 shell vacancies.

Nonlinear screening effects in the interaction of slow multicharged ions with metal surfaces

Density functional theory is used to treat the problem of screening of highly charged ions in an electron gas when inner shell vacancies are present. Results are presented for N and Ne ions in various electronic configurations. These include information about screening charge densities, induced densities of states in the conduction band, bound energy levels and KLL Auger lines.

Negative ion production in multicharged ion-surface interactions

Measurements are presented of energy and angular resolved reflected charge state distributions for O q+ , q = 1–7, and N q+ , q = 2–7, incident projectiles scattered from a clean A1(110) single crystal surface. The measurements were carried out at 16 keV incident ion energy, and at 1.2° grazing incidence. O − reflected ions were observed for all incident O projectile charge states. The energy loss of O − reflected particles was found to be consistently larger than for reflected O + particles, which was in turn larger than for higher reflected charge states. The magnitude of the energy loss was found to be independent of the incident projectile charge. The reflected charge state distributions were also found to be independent of the incident charge state, except when projectile K shell vacancies were initially present.

K-shell vacancies carried by swift O and Si ions inside ferromagnetic hosts.

The single and double K-shell vacancy fractions for swift O and Si ions inside Fe and Gd foils have been measured using the probe layer technique. The measurements are carried out at velocities varying from 7.5 to 13.5${\mathit{v}}_{0}$ for Si ions and at a velocity of 7.8${\mathit{v}}_{0}$ for oxygen ions (${\mathit{v}}_{0}$=\ensuremath{\alpha}c where \ensuremath{\alpha} is the fine-structure constant and c is the speed of light). It is shown that all such available data for light ions fall on a smooth curve when plotted against the reduced velocity of the ion. These values are used along with the existing transient magnetic field data to derive the electron spin polarization acquired by the ions traveling inside ferromagnetic hosts. The degree of polarization is shown to decrease with the atomic number of the ions. This observation is, however, in disagreement with recent theoretical calculations.

Spectra of L x rays from fast highly charged Xe ions traveling in solids.

Spectra of [ital L] x rays emitted by 6- and 8-MeV/u Xe ions interacting with a variety of solid targets have been measured with a curved-crystal spectrometer. The spectra displayed an overall structure consisting of six strong peaks which were fairly well resolved for low-[ital Z] targets ([ital Z][sub 2]=3--6), but broadened into two featureless groups for higher-[ital Z] targets. The main components were identified as [ital L][alpha][sub 1] and [ital L][beta][sub 1] pairs associated with initial-state configurations having one to six [ital L]-shell vacancies. Detailed spectral analysis provided estimates of the average projectile charges and the average [ital L]- and [ital M]-electron populations [ital inside] the solids.

Investigation of M x‐ray emission following photoionization of L and M shell electrons in high‐Z‐elements

AbstractThe cross‐sections of M x‐ray emission following photoionization of L and M shell electrons in Au, Pb, Th and U by 59.57 ke V gamma rays were measured. The experimental values of cross‐sections were found to be higher than those calculated from the theoretical values of L and M shell photoionization cross‐sections, the Li subshell to M shell vacancy transfer probabilities and M shell average fluorescence yields. The transfer of vacancies from the Li subshell to the M shell was found to play a significant role in M x‐ray emission. The present status of the experimental and calculated values of the cross‐sections is discussed.

Damage induced by monochromatic synchrotron X-rays at the resonance absorption energy of intracellular phosphorous in yeast cells

Two key experiments are analysed in terms of track structure parameters to obtain information on the excessive damage observed upon the production of inner shell vacancies at the resonance energy of bound phosphorus (Kobayashi et al., 1991) and for bromine incorporated into DNA (Usami et al., 1991). The relative damage enhancement factor due to resonance in phosphorus is 1.32. Additional Auger electron cascade events induced at resonance contribute about 35% to the total effect cross-section. In the bromine experiments, the intrinsic effectiveness is shown to be less than that for phosphorus. The effects can be quantified by the equilibrium electron fluences generated, combined with a quality specification determined from the net mean free path for ionization.

Direct Hartree-Fock calculation of multiple Xei+ ion production through inner shell vacancy de-excitations

The yields of multiple Xei+ ions produced by the cascade de-excitations of the L2 to N45 inner shell vacancies are calculated with inclusion of every possible pathway of deexcitation (Auger, Coster-Kronig and radiative) at each branching point of cascade deexcitation trees. Ejections of additional electrons through shake processes are included at each step of de-excitation. Good agreement with the experiment is obtained. Simplified de-excitation schemes suggested earlier by other authors for the initial M-shell vacancies are discussed.

Z variations in yields of multicharged ions scattered and recoiled from a silicon surface

The doubly charged and triply charged ion yields from keV ion-silicon surface scattering are found to have a strong dependence on the atomic number Z 1 of the incident ion. For Z 1 < Z 2 the yield of scattered multicharged ions increases with Z 1, so that these ions dominate the recoiling Si 2+ and Si 3+ ions by Z 1 = 9. In contrast, when Z 1 > Z 2, there are large yields of Si 2+ and Si 3+ ions, and the multicharged scattered ion yields are too small to detect. The interaction radius at which shell vacancies are produced is also found to change, suggesting that electron promotion occurs at a different level crossing on either side of Z 1 = Z 2.

Isobar discrimination in accelerator mass spectrometry by detecting characteristic projectile X-rays

The measurement of long-lived radioisotopes by accelerator mass spectrometry (AMS) is often limited by interferences of isobars, which cannot be separated easily (e.g. 36C1/ 36S, 59Ni/ 59Co, 60Fe/ 60Ni,…). Standard isobar separation techniques using ionisation chambers, gas-filled magnets and complete ion stripping can be applied only at sufficiently high energies. It is not possible to achieve enough discrimination for heavier isobars with small AMS accelerators. Recently, the detection of characteristic projectile X-rays during the stopping of ions in matter has been proposed by H. Artigalas et al. [H. Artigalas et al., Nucl. Instr. and Meth. B 79 (1993) 617.] as another isobar separation technique. In this case the discrimination does not depend on the ion energy. A high X-ray yield can be obtained with targets having a nuclear charge close to that of the projectile because inner shell vacancies are produced by molecular orbital formation. We present first results performed at the ETH/PSI 6 MV EN-tandem accelerator applying this technique in the detection of 59Ni and 60Fe. Efficiency, suppression factors and detection limits are discussed.

New trends in the study of alignment and orientation in ion-atom collisions

An overview of recent experimental and theoretical studies of the alignment of inner shell vacancies produced in fast ion-atom collisions is given. Information about impact parameter dependence of the alignment as obtained from coincidence measurements and from studies of the anisotropy of the X-ray and Auger satellite lines is discussed. The contribution of various mechanisms of inner-shell vacancy production (direct Coulomb ionization, electron capture, shake-off) is discussed. Recent experiments on the alignment in electron capture processes are analyzed. Finally, new types of experiments, namely, measurements of the alignment and orientation of a vacancy produced by proton impact ionization of laser excited and aligned atoms, is theoretically analyzed.

Multiple ionization of copper by electron impact

A pulsed cross beam technique previously developed in this laboratory has been used to study the electron impact ionization of copper. Previous measurements have been very limited in scope and exhibit large discrepancies. Relative cross sections sigma n for the formation of 1 to 5 times ionized copper have been measured with high accuracy within the range 7.8-2100 eV. Individual cross sections have been obtained by normalization to absolute values of sigma 2 obtained by Freund et al (1990) at energies below 200 eV using a fast crossed beam technique. Weak structures in sigma 1 can be attributed to Auger decay processes following the creation of 3s subshell and L shell vacancies but there is a lack of other pronounced structures in sigma n for n>1 where many close-lying subshell vacancies are involved. At 2100 eV cross sections sigma 5 are less than three orders of magnitude smaller than sigma 1.

Production of Li subshell and M shell vacancies following inner-shell vacancy production

The probabilities for transfer of vacancies from the K shell to an L i Subshell ( η KL i ) and to the M shell (η KM), and from the L i Subshell to the M shell ( η L i M) are evaluated for elements with atomic numbers 18 ≤ Z ≤ 96 using the theoretical radiative transition rates of Scofield [Phys. Rev. A 9 (1974) 1041; At. Data Nucl. Data Tables 14 (1974) 121] and radiationless transition rates tabulated by Chen et al. [At Data Nucl. Data Tables 24 (1979) 13; Phys. Rev. A 21 (1980) 442]. The calculated vacancy transfer probabilities are least-squares fitted to polynomials to obtain analytical relations that represent these probabilities as a function of atomic number.

Inner shell photoexcitation of atoms with correlation effects on autoionization : application to the argon 2p shell

For atoms with closed shells in their ground state, a Hartree-Fock (HF) equation is obtained for the electron moving in the field of an inner shell vacancy which decays through autoionization. In this equation, the following processes are taken into account: 1) change of the HF field of the remaining atom as a result of autoionization decay of the inner shell vacancy; 2) interaction of the photoelectron with a virtual Auger electron, thus the radial correlation in their movement is accounted for. The developed method is applied to study the L II,III -photoexcitation in Ar atom

Doubly and triply charged ion production in O +—Si surface scattering

Significant fractions of the scattered and recoil ions from single collisions of keV O + ions with near surface atoms of amorphous silicon are found to be doubly and triply charged. Definite threshold energies are found for each multicharged ion, suggesting that they are produced in close binary collisions with individual surface atoms and that inner shell vacancies are produced by molecular orbital promotion. Characteristic velocities for the decay of O +, O 2+, Si +, Si 2+, and Si 3+ in passing out through the surface are reported.

The role played by fast electrons in heavy-ion-induced desorption and track formation

We have substantial evidence that ion damage tracks and ion-induced desorption are consequences of the electronic excitation that is caused by the incident ion's impact on the target. This is shown by the correlation of the desorption yield/track damage with the energy loss of the ion to electrons during its slowing down process; the extended size of the desorbed molecules and the diameter of the tracks show tht the transport of energy away from the ion's path is mediated by these secondary electrons. Experimental studies show that the distribution of damage along an ion track is not uniform, which suggests that statistical fluctuations in the electronic energy loss are involved. These fluctuations can arise from the number of local ionization events produced, the localization of secondary electrons (multi-hit processes), and the Auger decay of inner shell vacancies created. This paper uses the data available to assess the relative importance of these processes and to suggest the sort of experimental data that would improve our understanding of desorption/track-damage phenomena.