Direct Coulomb Ionization Research Articles

K-shell ionization cross sections for Al, Ti, V, Cr, Fe, Ni, Cu, and Ag by protons and oxygen ions in the energy range 0.3-6.4 MeV.

Absolute K-italic-shell ionization cross sections have been measured for thin targets of Al, Ti, and Cu for protons in the energy range 0.3--2.0 MeV and for thin targets of Ti, V, Cr, Fe, Ni, Cu, and Ag for oxygen ions in the energy range 1.36--6.4 Mev. The experimental results are compared to the perturbed-stationary-state (PSS) approximation with energy-loss (E), Coulomb (C), and relativistic (R) corrections, i.e., the ECPSSR approximation (Brandt and Lapicki), to the semiclassical approximation (Laegsgaard, Andersen, and Lund), and to a theory for direct Coulomb ionization of the 1s-italicsigma molecular orbital (Montenegro and Sigaud (MS)). The proton results agree within 3% with empirical reference cross sections. Also, the ECPSSR provides best overall agreement for protons. For oxygen ions, ECPSSR and MS predict experimental results satisfactorily for scaled velocities xi> or =0.4. For lower scaled velocities, the experimental cross sections become considerably higher than theoretical predictions for Coulomb ionization. This deviation increases with increasing Z-italic/sub 1//Z/sub 2/; it cannot be explained by electron transfer to the projectile or by ionization due to target recoil atoms.

K-shell cross sections for ions penetrating solids at very low velocities

Absolute projectile K X-ray yields have been measured as a function of foil thickness for Siq+ ions penetrating carbon at velocities 2.0v0<or=v<or=5.6v0. The data are analysed on the basis of radiation cross sections and fluorescence yields estimated from previously measured charge state distributions. Cross sections for the creation and quenching of projectile K vacancies are extracted at all velocities down to a detection limit, where the K-vacancy fraction falls below 5*10-6. The results may be explained by direct Coulomb ionisation and collision-perturbed spontaneous decay of L-shell electrons. Correspondingly, the measured change in the fluorescence yield of the ion inside or outside the foil is explained solely in terms of the very high collision frequency inside the solid.

1s sigma molecular orbital ionisation in asymmetric ion-atom collisions by direct Coulomb interaction

Adiabatic perturbation theory is applied to the ionisation of the 1s sigma molecular orbital (MO) and extended to less adiabatic collisions by imposing an asymptotic matching with the semiclassical approximation. The transient molecular state wavefunction is modelled using the concept of an 'effective' charge for the colliding system. Through this procedure the transition amplitudes are evaluated simulating both the relaxation of the passive electrons and the modification of the active electron wavefunction, as well as their connection with the motion of the centre of charge during the collision. The direct Coulomb ionisation of the 1s sigma MO is calculated for projectiles following hyperbolic paths, in terms of this 'effective' charge, using the current-vector formalism. Comparison is made with a large amount of experimental data and good agreement is found.

Projectile Dependence of KLl Vacancy Production Cross Sections of Al by H and He Ion Bombardments

Spectra of Al K α X rays induced by H and He ion bombardments in the energy range of 0.2–0.4 MeV/amu have been measured with a Bragg spectrometer and Z 1 dependence of single K- plus multiple L-shell (KL l ) ionization has been investigated. The result shows that the L-shell ionization probability of Al by light ion bombardments follows the scaling law of direct Coulomb ionization considering the binding energy correction.

Triple-center treatment of ionization inp−Hcollisions

A triple-center atomic-state method is proposed for treating ionization in low-energy ion-atom collisions. This method intrinsically accounts for the Wannier mechanism, in which the electron will not have been removed in a slow collision unless it is asymptotically at the point of unstable equilibrium between the nuclei (the chosen third center). When applied to p-H collisions, the method shows that most of the ionized electrons stay near the third center and that direct Coulomb ionization and charge transfer to the continuum: accounted for by normal double-center methods: are less important: The triple-center method dramatically raises the cross section above that obtained using a double-center pseudostate basis, bringing it closer to the single early experimental curve at energies below 15 keV.

Systematics of single and doubleK-shell-vacancy production in titanium bombarded by heavy ions

The systematics of single and double $K$-shell-vacancy production in titanium has been investigated in the limit of zero target thickness (\ensuremath{\sim} 1 \ensuremath{\mu}g/${\mathrm{cm}}^{2}$) for incident C, N, O, F, Mg, Al, Si, S, and Cl ions over a maximum energy range of 0.5 to 6.5 MeV/amu. This corresponds to collision systems with $0.27\ensuremath{\le}\frac{{Z}_{1}}{{Z}_{2}}\ensuremath{\le}0.77$ and $0.24\ensuremath{\le}\frac{{v}_{1}}{{v}_{2 K}}\ensuremath{\le}0.85$, where ${v}_{1}$ is the projectile nuclear velocity and ${v}_{2 K}$ is the mean velocity of an electron in the target $K$ shell. The present work is divided into four major sections. (1) Single $K$-shell-vacancy production has been investigated by measuring $K\ensuremath{\alpha}$ and $K\ensuremath{\beta}$ satellite x-ray-production cross sections for projectiles incident with no $K$-shell vacancies. For incident ions with ${Z}_{1}\ensuremath{\ge}9$, the contribution due to electron-transfer processes from the target $K$ shell to outer shells of the projectile has also been noted. (2) Single $K$---shell-to-$K$-shell electron-transfer cross sections have been obtained indirectly by the measuring of the enhancement in the $\mathrm{Ti}K$ x-ray production cross section for bare incident projectiles over ions incident with no initial $K$-shell vacancies. (3) Double $K$-vacancy production has been investigated by measuring the $K\ensuremath{\alpha}$ hypersatellite intensity in ratio to the total $K\ensuremath{\alpha}$ intensity. (4) Double $K$---shell-to-$K$-shell electron-transfer cross sections have been obtained indirectly with the use of a procedure similar to that used for single $K$ to $K$ transfer. The measured cross sections have been compared to theoretical models for direct Coulomb ionization and inner-shell electron transfer and have been used to investigate the relative importance of these mechanisms for $K$-vacancy production in heavy-ion---atom collisions.

Inner-shell alignment of atoms in ion-atom collisions. III. Light target atoms

For pt.II see ibid., vol.14, p.2635, 1981. A systematic analysis of experimental data on the alignment of the L3 vacancy in light atoms formed in collisions with fast ions has been made. Two vacancy production mechanisms were considered: direct Coulomb ionisation and electron capture. The first Born approximation and the Oppenheimer-Brinkman-Kramers approximation, both employing realistic atomic wavefunctions, were used for the calculation of the alignment due to direct ionisation and electron capture, respectively. The results of the calculations are in good agreement with experimental data.

L X-ray anisotropy and L3-subshell alignment of heavy atoms induced by ion impact

The anisotropy emission of the Ll X-ray line from heavy atoms (54&lt;or=Z2&lt;or=92) has been investigated experimentally for ion impact (1&lt;or=Z1&lt;or=79) in the energy range 0.15 to 10 MeV amu-1. In the case of single ionisation, the collisionally induced alignment of the L3subshell can be derived. The data for H and He projectiles are reasonably well described by direct Coulomb ionisation theories. With increasing Z1, the measured anisotropy becomes considerably smaller than for H impact. No pronounced dependence on the projectile nuclear charge is found although for different projectiles different ionisation mechanisms (direct ionisation, promotion of molecular orbitals) dominate.

Inner-shell alignment of atoms in ion-atom collisions. II. Electron capture

For pt.I see ibid., vol.13, p.1601 (1980). The alignment of ions with an inner-shell vacancy resulting from electron capture in ion-atom collisions is treated theoretically. The general expressions for the alignment are obtained in the Oppenheimer-Brinkman-Kramers and Born approximations. Simple analytical formulae for the alignment are derived within the framework of the hydrogenic model. The dependence of the degree of alignment on the incident particle velocity and the ratio of charges of the colliding particles is analysed. The alignment due to electron capture is shown to differ essentially from the alignment due to direct Coulomb ionisation. The sensitivity of the degree of alignment to the form of captured-electron wavefunctions and to the type of perturbing interaction is investigated. The experimental data on the angular distribution of Auger electrons in the ionisation of the 2p3/2subshell of Mg by protons and He+ ions are reanalysed taking into account the electron capture mechanism.

K shell X-ray production of eight elements from Ti to Ge by14N-ion bombardment

The energy shifts of Kbeta X-rays and the Kbeta /Kalpha intensity ratios by 2.8 MeV N+-ion bombardments have been measured for eight elements from Ti to Ge. The Kbeta /Kalpha ratios are expressed as relative values to the ratios for 2 MeV proton impact. The experimental results indicate single K- plus multiple L- and M-shell ionisation, and strongly reflect the relative importance of M-shell vacancies in the Kbeta /Kalpha ratios at the time of K X-ray emission. These data are compared with the Hartree-Fock-Slater calculations and used to deduce information about the vacancy configurations of ionised atoms. The present results can be qualitatively explained within the framework of the direct Coulomb ionisation theory.

On the role of electron capture in the inner-shell alignment of atoms in ion-atom collisions

Abstract The alignment of the 2p 3 2 vacancy in Mg resulting from collisions with protons is calculated. Two possible mechanisms of ionisation are considered: direct Coulomb ionisation and electron capture by protons. The inclusion of electron capture leads to a marked improvement of the agreement between theory and experiment.

Plane-Wave Born Approximation Calculations of Cross Sections for Direct Coulomb Ionization of M-Subshells by Bare Charged Particles and the Subsequent Emission of M-Shell X Rays

Results of calculations performed in the plane-wave Born approximation for the atomic states of the M-shell are presented in the form of the excitation function I31(η,W), the universal function F31(η/θ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ,θ), and the direct Coulomb ionization cross sections. A description of vacancy filling leading to x-ray production cross sections from incident carbon ions on 79Au, 82Pb, and 83Bi are compared with the present calculations.

The Origin and Limitation of the Universal Nature of the Direct Ionization of Atoms

The dependence of the direct ionization cross section on a single scaled velocity variable makes ionization of a given subshell a universal phenomenon common to all particle-atom Coulomb ionization collisions. Constructing universal plots of a wide range of data has led to a refined description of the direct Coulomb ionization process. The sources of this universality lie in the PWBA, the lower limit of the momentum transfer and a description of the atomic states which has a scaling length. Universality is exact in the low velocity limit but the cross section develops a parametric dependence on the target binding energy as velocity increases. This dependence is usually ignored, but for some cases it is significant.

Cross sections for atomicK-shell ionization by ion impact in the single-particle Glauber approximation

Total cross sections have been computed for $K$-shell ionization in selected multielectron atoms by proton impact using the single-particle Glauber approximation with hydrogenic wave functions. Our single-particle Glauber approximation corresponds to an independent-particle model where explicit coupling with target electrons not directly involved in the interaction is omitted. In principle, the Glauber approximation is valid over a wider region of scattering parameters than is the Born approximation. However, our Glauber results are not in significantly better agreement with observation than simpler Born predictions. For all targets considered at projectile velocities near the peak of the cross section, single-particle Glauber results tend to lie beneath observed cross sections by proton impact, in contrast to previous results for electron impact where, as expected, Glauber results are in better agreement with observations than Born results for low-${Z}_{2}$ targets. Some explanations of the discrepancy and possible extensions of our methods are considered. It is also found that, as the atomic number of the target, ${Z}_{2}$, increases, the relative contribution of Glauber corrections to corresponding Born calculations decreases at projectile velocities near and above the peak of the $K$-shell ionization cross section. For ${Z}_{2}\ensuremath{\gtrsim}5$, the Glauber scattering corrections to Born results for $K$-shell ionization at projectile velocities near and above the peak of the cross section are less than a few percent, e.g., less than errors due to our use of hydrogenic target wave functions. For ${\mathrm{O}}^{+8}$ + Ne, unlike ${p}^{+}$ +Ne, significant differences are apparent between Glauber and Born predictions for direct Coulomb ionization.

Differential energy-loss cross sections for ionization of atomic hydrogen by 25-200-keV protons

Ionization continuum cross sections, differential in energy loss, have been determined from the energy-loss spectra of 25-, 50-, 75-, 145-, and 200-keV protons scattered from atomic hydrogen. The theoretical differential cross sections in the Glauber and Born approximations that are presented show that the theory and the experimental results for 200-keV proton impact agree very well, but below 200 keV there are differences. These differences, which are more pronounced at the lower projectile energies, may be explained in terms of charge transfer to the continuum, which is not treated in our Born and Glauber calculations of direct Coulomb ionization. The total ionization cross sections obtained by integrating the differential cross sections are in reasonable agreement with ionization cross-section measurements that have been obtained from crossed-beam experiments. (AIP)

Inner-shell ionization ofZ=25?32 elements by 5-48 MeV32S ions

TheK-shell ionization cross sections of Mn, Ni, Cu, Zn and Ge under32S bombardment have been measured in the energy range from 5 to 48 MeV. The cross sections are compared with available theories based on a direct Coulomb ionization mechanism and with the predictions of theK-vacancy sharing process. This last process can reasonably account for the measured cross sections at high bombarding energies. The energy shifts of theKα- andKβ-lines and theKα/Kβ-intensity ratios have also been measured. This information is used to deduce the defect configuration of the atoms. The mechanisms responsible for the multiple vacancy production are discussed.

L-shell x-ray production cross sections forO16ions on Ce, Pr, Sm, Eu, Dy, and Ho: 0.50 to 2.25 MeV/amu

L-shell and individual x-ray production cross sections for 8--36-MeV oxygen ions on thin solid targets of Ce, Pr, Sm, Eu, Dy, and Ho are measured and compared with predictions of theories of direct Coulomb ionization. In comparing theory and experiment, we use, with reasonable justification, fluorescence yields, Coster-Kronig yields, and radiative widths obtained from single-hole initial configurations. The theoretical treatment which provides the best overall fit to the data is the plane-wave Born approximation with corrections for Coulomb deflection of the projectile and increased binding of the target electrons. The effects of multiple ionization are examined through measurement of x-ray energy shifts as measured with Si(Li) detectors and by vacuum crystal- spectrometer measurements of L x-ray spectra produced by 3-MeV proton and 25-MeV oxygen-ion bombardment of a thick La target.

Integral representation for the Glauber scattering amplitude for direct Coulomb ionization by charged particles

An integral representation is developed for the scattering amplitude for ionization by charged particles in the Glauber approximation. Unlike previous results based on a series expansion in the momentum of the ejected electron, ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{e}$, the present results are rigorously convergent for all physical values of ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{e}$. Results calculated by this method agree with earlier results for ionization of atomic hydrogen by electron impact using Pad\'e approximants to approximately sum the series. Total cross sections for the ionization of atomic hydrogen by proton impact are presented and compared to experimental data. This technique is applicable to the ionization of neutral atoms whose participating electron is represented by a hydrogenic wave function, and the remaining electrons are ignored.

Outers-shell ionization of neon, argon, and krypton by H 2 + and He+ impact (l00 keVto 1000 keV)

The outers-shell ionization of Ne, Ar, and Kr by H 2 + and He+ impact has been studied in the energy range of 100 keV to 1000keV. The observed structures in the ionization functions are explainable by molecular processes and by direct Coulomb ionization.

CharacteristicLX-Ray Spectra from Proton,α-Particle, and Oxygen Bombardment of Sn

Spectra have been measured [using a crystal (LiF;200) spectrometer] of $L$ x rays produced by 2.0-MeV proton, 3.2-MeV $\ensuremath{\alpha}$-particle, and 30.0-MeV oxygen bombardment of thick tin targets. $L$ x-ray lines are observed in the $\ensuremath{\alpha}$-particle-produced spectrum from initial configurations with up to four additional $M$-shell electron vacancies. The energy resolution of 8 eV (full width at half-maximum) did not allow any detailed structure to be resolved in the oxygen bombardment; an almost continuous structure is observed from multiply ionized tin. The ratios of x-ray yields from $L+M$ vacancies to yields from $L$ vacancies are shown to have the correct order of magnitude for a simultaneous direct Coulomb ionization process for proton and $\ensuremath{\alpha}$-particle impact.