F. Folkmann

Selective electron capture into highly stripped Ne and N target atoms after heavy-ion impact

Auger electron and X-ray spectra from Ne and N gas targets excited with 1.4 MeV amu-1 Ar12+, Kr15+, Xe24+, and Pb36+ ions are measured, varying the target pressure and mixing other gases into the target volume. A dramatic change of line intensities from outer-shell configurations having a KL two-electron core and a third electron in the n=4, 5, 6 shell is observed, depending on the target pressure and systematically on the target ionisation potential. This effect is explained by highly selective electron capture from neutral target atoms or molecules into outer-shell orbitals of slowly (Er<10 eV) recoiling and highly stripped target atoms with metastable core states. It turns out that selective electron capture is a dominant mechanism for the population of H-, He- and Li-like states of low atomic number (Zt<or=10) target atoms after bombardment with heavy-ion projectiles. The capture process is discussed in the framework of a simple classical model and a capture cross section in the order of 10-14 cm 2 is estimated from the experiment.

Measurement of the Absolute Photoionization Cross Section of C+ near Threshold

The absolute photoionization cross section of the ground-state C+ ion has been measured from the ionization threshold at 24 eV to 31 eV using a merged ion-photon beam setup with synchrotron radiation from an undulator. The experimental results have been compared with nonrelativistic theoretical data from the Iron Project and the agreement is generally good in the near-threshold region, but differences in the magnitude of the continuum cross section of 15%-35% at higher energies and several deviations in the resonance structure are observed. The present measurements are a part of an experimental program to test the extensive theoretical photoionization cross section calculations performed within the framework of the Opacity Project and the Iron Project.

Absolute photoionization cross section of K+ ions from the 3p to the 3s threshold

The absolute photoionization cross section of ground state K+ ions has been measured from the 3p threshold to above the 3s threshold (31-49 eV) by VUV radiation using a merged ion-photon beam set-up. The cross section, which reaches a maximum value of 32 Mb ±10% at 31.8 eV and then decreases with energy, is in overall good agreement with absolute measurements by Peart and Lyon, but the present investigation also reveals the 3s3p6np 2P (n = 4-6) resonances and eliminates a previously unassigned structure at 35.5 eV. The energies, widths and shape parameters of the autoionizing 2P resonances are determined and compared with recent data from dual-laser plasma measurements and with theoretical predictions. The present experimental set-up will allow absolute photoionization cross sections 0.1 Mb to be measured, making it possible to measure the absolute photoionization cross sections of ions of astrophysical importance.

Absolute, State-selective Measurements of the Photoionization Cross Sections of N+ and O+ Ions

The absolute single-photoionization cross sections of singly charged nitrogen and oxygen ions have been measured from 29-80 eV and 30-150 eV, respectively, by merging a synchrotron-radiation beam from an undulator with a 2 keV ion beam. In the case of O+, separate data sets were obtained for the 4S ground state and the 2D/2P metastable states by attenuating the target-ion beam with N2 gas, exploiting the fact that cross sections for charge exchange between O+ ions, in the 4S or in the 2D/2P states, and N2 are different. The spectral structures were very different for the ground and the metastable states, whereas the cross sections for photoionization into the continua were nearly identical. The agreement between the experimental data and data calculated using the model function by Verner et al. or obtained from R-matrix calculations performed within the Opacity and the Iron projects is rather good. Finally, double photoionization (direct) was observed for O+ ions, with an approximately linear onset at 90 eV and a maximum cross section of 0.04 Mb.

Photoionization study of Kr+ and Xe+ ions with the combined use of a merged-beam set-up and an ion trap

Photoionization cross sections of halogen-like Kr+ and Xe+ ions have been measured in the photon energy range extending up to 15 eV above the threshold. Two different devices were used, a merged-beam set-up and an ion trap. Combination of the two techniques allows for the extraction of the pure ground state ionization cross section on an absolute scale. Multiconfiguration Dirac–Fock calculations reproduce the magnitude of the direct photoionization cross sections well.

Absolute cross sections for the photoionization of 4d electrons in Xe+ and Xe2+ ions

The partial and total absolute cross sections for the photoionization of Xe+ and Xe2+ ions, respectively, have been measured in the 4d excitation region (50-130 eV). The experimental cross sections are compared with calculated values obtained by the random phase approximation method taking exchange and rearrangement into account. The maximum total cross section values are 27(3) Mb for both ions, a value also reported for the photoionization of the Xe atom. The reliability of the experimental cross sections has been tested by measuring the absolute photoionization cross section for the He+ ion.

Excited States of Few-Electron Recoil Ions from Ion Impact on Neon Studied by X-Ray and Electron Measurements

Neon K Auger-electron and X-ray spectra have been measured for various incident ions (Cl to U) and for different velocities (0.3-9 MeV amu-1). The degree of ionization of the L shell increases with increasing charge of the ion and decreases with increasing velocity, and it is represented as function of the projectile velocity divided by charge. Electron capture into metastable, highly charged neon recoil atoms proceeds in a highly selective way to specific states, which are studied by measuring delayed emission from gas mixtures. Delayed neon X-ray spectra induced by 0.6-0.7 MeV amu-1 Cl10+ are presented. A classical model, using the ionization potential for the captured electron and the subshell specific binding energy in highly charged ions, explains the Rydberg state and estimates the cross section for the observed electron capture.

Angular distribution of particle-induced X-ray emission

Abstract The angular distribution of the continuum radiation in the energy range 2–15 keV for angles 30–160° has been investigated, both experimentally for 1.5, 2, 3, and 4 MeV protons on Al, and theoretically for secondary electron bremsstrahlung, emphasizing the electron bremsstrahlung angular distributions. The limitations imposed alone by the electron bremsstrahlung are mentioned. There is a reduction in the continuum at backward angles relative to that at 90° which is measured at the characteristic radiation energy T m to be a factor 1.6 in 135° and 2.6 in 160°, rising for higher radiation energies to a factor 3 to 4.

High resolution ion induced X-ray spectroscopy for chemical state analysis

Satellite lines of K X-ray lines from K and Cr, and L lines from In are observed for impact by 1.4 MeVu1H+, 4He2+, 12C4+, 16O4+ and 35Cl8+. A curved crystal spectrometer was used with a position sensitive proportional counter with improved linearity of the backgammon readout. The distribution of Cr Kα satellite lines depends on the chemical state of Cr, being peaked at lower energy for the toxic valency 6 compounds than for valency 3 and metallically bound Cr. Fluorescence excitation of the Cr diagram line is observed for a stainless steel sample from enhancement by Fe X-rays.

High-resolution X-ray spectrometer for PIXE work

Abstract A spectrometer is described with a curved Si(111) crystal and a position-sensitive detector set up in fixed geometry to measure X-rays of energy 3.5 ± 0.28 keV with a resolution of 6 eV. The position result, obtained by division, is processed like an energy into a multichannel system. Positioning the target area significantly closer to the crystal than the detector assures that areas extending over several mm produce similar results. The efficiency is only a few counts per μC per μg/cm 2 .