Total Ionization Cross Research Articles

L-subshell ionization studies of Au for alpha -particle and lithium-ion bombardment

L-subshell ionization of Au has been investigated for alpha -particle and lithium-ion bombardments with energies 0.54-1.74 MeV u-1 and 0.65-1.44 MeV u-1, respectively. Comparison of experimental X-ray production cross sections with the predictions of the ECPSSR and SCA theories shows reasonably good agreement for Lalpha and Lbeta X-rays, whereas for Lgamma and Lgamma 1+5 the ECPSSR theory underestimates the cross sections by about 60% for both alpha -particle and lithium-ion impact, and the SCA theory agrees reasonably well. The ECPSSR theory underestimates the L1- and L2-subshell ionization cross sections and gives good agreement for the L3-subshell, whereas the SCA theory overestimates the L3-subshell ionization cross sections and gives good agreement for L1 and L2. The experimental data for the total-ionization cross sections are within 25% of the predictions of both the theories.

Energy and angular distributions of electrons from ion impact on atomic and molecular hydrogen. I. 20–114-keVH++H2

Apparatus and procedures are described for the measurement of absolute cross sections, differential in ejected electron energy and angle, for ionization of atomic and molecular hydrogen by ion impact. A hemispherical electrostatic energy analyzer, rotatable from 15[degree] to 165[degree] with respect to the direction of the incident ion beam, was used to measure energy spectra of secondary electrons from 1.5 to 300 eV. Cross sections at ten angles (nine at some energies) and five incident-ion energies from 20 to 114 keV for H[sup +]+H[sub 2] collisions are given. The doubly differential cross sections were integrated over angle and electron energy to obtain singly differential and total-ionization cross sections. The uncertainty in the doubly differential cross sections is 21% at a secondary energy of 1.5 eV decreasing to 18% at 10 eV and above. The total cross sections have a rms deviation of 12% from recommended values. A broad peak at 6 eV in the energy spectrum of electrons from low-energy H[sup +]+H[sub 2] collisions is attributed to autoionization.

Doubly differential cross sections of secondary electrons ejected from atomic hydrogen by electron impact.

We have measured the doubly differential cross sections, in angle and energy, of secondary electrons ejected from atomic hydrogen by electron impact. The incident energies used were from 25 to 250 eV. The energy and angular range of secondary electrons measured were from 1.0 eV to one-half of the difference between the incident energy and the ionization potential of hydrogen atom and from 12{degree} to 156{degree}, respectively. Singly differential cross sections and total-ionization cross sections were obtained from the doubly differential cross sections. The present total-ionization cross sections were compared with previous measurements and theoretical results. Discrepancies have been found between the present results and those measurements and theoretical results.

Application of a time-dependent, distorted initial-state wave function in calculations of K-shell ionization

Initial- and final-state distorted wave functions, which in the semiclassical approximation depend upon projectile position and, hence, also upon time, are considered in the context of K-shell ionization. The use of such time-dependent wave functions can lead to the occurrence of nonorthogonal basis states. Issues associated with constructing the scattering amplitude in the presence of such states are identified and discussed. The incorporation of a time-dependent, distorted initial-state wave function along with a time-dependent binding energy is shown to give rise at low velocities to a large enhancement in the total K-shell ionization cross section over what is obtained with a time-independent wave function. The system studied is that of 3He incident on Ti in the energy region 0.2 to 1.6 MeV/u. This enhancement effectively cancels a corresponding large reduction which is found to occur from the use of a time-dependent vs a time-independent binding energy.

Distorted-wave calculations of the electron-impact ionization of highly ionized Na-like ions.

We report on distorted-wave calculations of the electron-impact ionization cross sections and rate coefficients for the Na-like ions ${\mathrm{Ti}}^{11+}$, ${\mathrm{Cr}}^{13+}$, ${\mathrm{Fe}}^{15+}$, and ${\mathrm{Ni}}^{17+}$. The cross sections for direct ionization out of the 3s, 2p, and 2s subshells were determined using the configuration-average, distorted-wave approximation. In addition, excitation-autoionization contributions originating from inner-shell excitations of the type 2${s}^{2}$2${p}^{6}$3s\ensuremath{\rightarrow}2${s}^{2}$2${p}^{5}$3snl (n=3,4,5) and 2${s}^{2}$2${p}^{6}$3s \ensuremath{\rightarrow}2s2${p}^{6}$3snl (n=3,4) were calculated by employing the distorted-wave approximation and configuration-interaction, bound-state wave functions. Near threshold, the total-ionization cross sections in all four ions are enhanced by approximately a factor of 5 through excitation autoionization. These excitation-autoionization contributions are large, even though they have been substantially reduced by radiative decay to the bound states of the initial ion. Furthermore, although electron correlation has a minimal effect on the excitation cross sections, the magnitude of the branching ratios for autoionization is quite sensitive to configuration interaction in the autoionizing levels. This is especially true for the 2p\ensuremath{\rightarrow}3d excitations, where configuration interaction between 2${p}^{5}$3s3d and 2${p}^{5}$3${p}^{2}$ can increase the magnitude of the autoionizing rates for levels within the 2${p}^{5}$3s3d configuration by as much as three orders of magnitude. Thus, for example, in the case of the 2p\ensuremath{\rightarrow}3d excitation in ${\mathrm{Ni}}^{17+}$, correlation increases the overall branching ratio for autoionization from 0.27 to 0.53.

Chemi-ionisation of Xe in thermal collisions with He*(21S) and He*(23S) metastable atoms

Integral total ionisation and collision cross sections of He*(21S)-Xe and He*(23S)-Xe have been measured in the CM collision energy range 3-220 meV. Optical potentials are fitted to the data. The potential form of Siska (1984) is used for the real part of the potential. The imaginary part of the potential for the triplet system is purely exponential, whereas the pronounced decline of the high energy ionisation of the singlet system requires a drastic reduction of the imaginary part at small distances. Physical reasons for this behaviour are discussed.

Investigation of the impact-parameter dependence of electrons emitted in 30-, 100-, 350-keV H+ and 100-keV 3He

Energy distributions of electrons ejected in collisions of light ions with Ar atoms were measured as a function of projectile scattering angle. For this purpose the ion-electron coincidence technique was combined with the time-of-flight method for electron momentum analysis. Thus, the simultaneous detection of electrons with energies from a few hundred meV up to some keV and ions at different projectile scattering angles could be performed. The electron analyzer could observe electrons at ejection angles of 40/sup 0/--140/sup 0/ with respect to the beam axis. A time resolution of less than 400 ps was achieved with a recently developed position-sensitive ion detector. For the first time, absolute double differential yields for electron emission were measured as a function of impact parameter and electron energy for the system H/sup +/+Ar at collision energies of 30, 100, and 350 keV. Also L-shell ionization yields were determined for 30-, 100-, and 350-keV H/sup +/ and 100-keV He/sup 2+/ impact on Ar. The results are compared with predictions of the plane-wave Born approximation, an atomic orbital expansion, and the semiclassical approximation. Total L-shell ionization cross sections were deduced and compared to other measurements.

Polarization and binding effects in K-shell ionization by 16O and 32S ions.

The theoretical model of Montenegro and Sigaud is used for the description of the experimental total K-shell ionization cross sections for 2.2-MeV/u /sup 16/O and /sup 32/S on 24 targets from /sub 21/Sc to /sub 83/Bi obtained by Seidel et al. The predictions of the model, which considers both binding and static polarization effects within the framework of adiabatic perturbation theory, are very good for the majority of the measured data in the region of validity of the model. The electron-capture process that can contribute in the region of the largest values of relative velocity xi/sub K/ is also analyzed.

Threshold behaviour of L-shell ionisation in slow symmetric ion-atom collisions with 32⩽Z⩽54

Total L-shell ionisation cross sections have been investigated for the collision systems Ge-Ge, Kr-Kr, In-In, Mo-Mo, and Xe-Xe by measuring the X-ray yield in the impact energy range 50 keV<Elab<3 MeV. Energy steps between 2.5 and 40 keV were selected in order to get detailed information about the ionisation threshold behaviour. The influence of multiple collisions on the ionisation process was studied by comparing the X-ray yield obtained from gaseous and solid targets. The experimental data was compared with theoretical results deduced from coupled-state, as well as simplified two-state, calculations. The authors found that in addition to the well known 4f phi -4f delta -4f pi -4f sigma rotational coupling with the initially unoccupied 4f phi MO, further couplings with orbitals originating from the M shell have to be included in order to explain satisfactorily the observed structure in the threshold cross sections.

Investigation of L-subshell ionization of gold by low velocity proton bombardment

Proton induced X-ray emission was used to measure absolute L-subshell and total L-shell ionization cross sections of gold in the energy range 0.2–0.6 MeV. The experimental results were compared with the predictions of two versions (RPWBA-BC, CPSSR) of the plane wave Born approximation.

Positron-impact ionization of hydrogen atoms

The ionization cross section of hydrogen atoms by positron impact has been calculated by using a distorted-wave polarized-orbital method. We have employed two models depending on the choice of the final-channel wave function, and the two sets of results differ dramatically. We have also found that below 100 eV the total positron-impact ionization cross section including positronium formation is appreciably larger than the corresponding results for electron-impact ionization.

Trajectory method for the solution of the time-dependent Schrodinger equation in atomic physics and application to H+-H scattering

A trajectory method has been developed for the solution of the one-electron Schrodinger equation in atomic scattering problems. The method is based on the fluid dynamical interpretation of the quantum mechanical probability density and current. The time evolution is described by trajectories of particles, which represent the probability distribution. The trajectories are calculated by solving classical equations of motion with forces depending on the probability density. Results are obtained for the differential and total ionisation and charge exchange cross sections for the scattering of H+ on H.

The K-shell ionisation of64Gd,74W and90Th induced by heavy, charged particles

The total K-shell ionisation cross sections were measured relative to nuclear Coulomb excitation cross sections for 64Gd, 74W and 90Th targets bombarded by 4He and 16O projectiles. In the case of 74W, projectiles of 12C and 19F were also applied. The experimental results are compared with the predictions of the SCA-LAL model, and the overall agreement is very good. The data are presented as reduced cross sections and compared with a 'universal' cross section function.

Investigation of threshold behaviour of ionisation cross sections

The threshold behaviour of differential and total electron-atom ionisation cross sections is investigated by means of the limit procedure in the integral expression of the ionisation amplitude. The case of total angular momentum L=O is considered for which the integral expression of the amplitude is derived. The semiclassical final-state wavefunction is investigated by means of numerical integration. The results confirm the non-linear threshold law found by Wannier and uniform distribution of electron energies.

K-shell ionization probability in close collisions of 7-12-MeV protons with targets ofZ=24−83

K-shell ionization probabilities were measured by the particle--x-ray coincidence technique for 7-MeV protons of 15 to 39 fm impact parameter on 24 targets from Cr to Bi. For 22 of these targets, total K-shell ionization cross sections are also reported. Additional K-shell ionization probabilities were measured for 7-, 10-, and 12-MeV protons on Mo, Cd, and Sn, at impact parameters down to 2 fm. While the total K-shell cross sections closely follow relativistic semiclassical (RSCA) calculations supplemented by minor binding-plus-polarization corrections, significant discrepancies of up to 70% occur for the K-shell ionization probabilities at small impact parameters, particularly for the targets from Cr to Zr. They can be traced back to the use of hydrogenic wave functions in the RSCA calculation, whereas nuclear recoil may account only for a few percent of the discrepancies. The data on both K-shell total cross sections and ionization probabilities exhibit the pattern characteristic for small distortions of the K-electron binding energy by the projectile charge: adiabatic binding for xi/sub K/ 1. The perturbed-stationary-state approximation somewhat underestimates these distortion effects.

Low-energy total K-shell ionisation cross sections in the plane-wave Born approximation: an analytical result

Starting from the general formulation of the plane-wave Born approximation (PWBA) an analytical expression for the low-energy total K-shell ionisation cross section is obtained. The reduced cross section is given as a power series in the adimensional parameters xi 2 and theta which reproduces the values obtained by numerical integration within the precision inherent in the available tables. The universal part of the reduced cross section in the PWBA (that part of F( xi 2, theta ) that does not depend explicitly on theta ) is expressed in terms of a few elementary analytical functions. The exact physical limits of integration are taken into account through the introduction of an effective parameter xieff.

Investigation of inner-shell ionization by electron impact in the 60–600 keV energy region

Total L X-ray production and total K-shell ionization cross sections have been measured for Se, Ag and Sb by electron impact ionization in the 60–600 keV bombarding energy region. Experimental results have been compared with the theoretical predictions of the PWBA, those of the BEA and in the case of K-shell those of the virtual photon method as well. In the investigated energy and atomic number region the relativistic PWBA gave the best agreement with our experimental data.

Role of transitions with different changes of the electron orbital angular momentum projection in the proton ionisation of the 1s and 2s states of hydrogen-like ions

In the first Born approximation the ionisation probabilities of the ns states (n=1 and 2) of hydrogen-like ions by nuclei corresponding to the transitions with the different changes in the orbital angular momentum projection of the relative motion h(cross) mu have been calculated at a fixed impact parameter rho . It has been shown within the 5% accuracy that the total ionisation probability and cross section are determined by the transitions with mod mu mod <or=2 n.

Thick and thin Au and Pb L-subshell ionisation cross sections for protons

L1,L2,L3 and total L-shell ionisation cross sections were measured for thick and thin Au and Pb targets over the proton energy range 0.6 to 3.0 MeV. The results have been compared with the plane-wave Born approximation (PWBA), the relativistic RWBA (PWBAR) and the Coulomb deflection corrected and perturbed stationary states relativistic PWBA (CPSSR) of Brandt and Lapicki (1979). The experimental subshell cross sections have also been used to produce the L alpha , L beta , L gamma ratios for the ten strongest X-ray peaks observed for Au and Pb with modern high resolutions Si(Li) detectors.

Ionization of alkali-metal atoms by He+ impact

The total single ionization cross sections of alkali-metal atoms (Li, Na, K, Rb, and Cs) have been calculated in the binary-encounter approximation (BEA) using the Hartree–Fock velocity distribution function for the bound electron. The calculated cross sections are compared with the experimental data. The BEA results agree with the observations to within a factor of 2–3.