Ionization Cross Section Ratios Research Articles

The effective charge effect in partially stripped ion-helium collisions

The double and single ionization cross section ratios of helium by partially stripped carbon, oxygen and fluorine ions are measured for projectile charge states ranging from +1 to +4 and impact energies from 1.5 MeV to 7.5 MeV. The effective charge effect in partially stripped ion-helium collisions is studied. It is found that the effective charge qeff increases as the impinging energy increases and qeff shows a modest dependence upon the projectile charge state in the present energy range. The projectile charge state, projectile energy, projectile and target electronic state dependences of the effective charge effect may be explained using orbital interpenetrating.

Ratio of double to single ionization of helium: The relationship between ionization by photons and by bare charged particles.

It is well known that cross sections for ionization of atoms by fast charged particles and by photons are related by the Bethe-Born theory. We employ this relationship to derive a corresponding relation for the ratio R of double to single ionization including the first two terms of the Bethe expansion. For sufficiently fast charged particles, where the second term can be ignored, the ratios as a function of \ensuremath{\Delta}E---the energies transferred to the atom by the projectile---for ionization by charged particles ${\mathit{R}}_{\mathit{z}}$(\ensuremath{\Delta}E) and by photons ${\mathit{R}}_{\ensuremath{\gamma}}$(\ensuremath{\Delta}E) are equal: ${\mathit{R}}_{\mathit{z}}$(\ensuremath{\Delta}E)=${\mathit{R}}_{\ensuremath{\gamma}}$(\ensuremath{\Delta}E). In addition, a relation for the ratio of total charged-particle impact ionization cross sections to the photon ratio is derived. The results are compared with recent experimental data and various discrepancies are found and discussed.

L-shell ionization studies of Pb and Bi with alpha particles.

Ionization cross sections for the L subshells of Pb and Bi by \ensuremath{\alpha}-particle bombardment (2.2--8.2 MeV) have been determined from the experimental data and the currently available radiative transition probabilities, fluorescence yields, and Coster-Kronig factors. The measured ionization cross sections and their ratios are compared with the results of ECPSSR calculations [ECPSSR denotes perturbed-stationary-state (PSS) theory with energy-loss (E), Coulomb deflection (C), and relativistic (R) corrections]. The measured individual cross sections for ${\mathit{L}}_{1}$ and ${\mathit{L}}_{2}$ subshells deviated in opposite directions from the theory, whereas their sum shows good agreement. The ${\mathit{L}}_{3}$ and total ionization cross sections obtained from the data also show good agreement with the ECPSSR theory. The ionization cross-section ratios ${\mathrm{\ensuremath{\sigma}}}_{\mathit{L}1}$/${\mathrm{\ensuremath{\sigma}}}_{\mathit{L}2}$ and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{L}3}$/${\mathrm{\ensuremath{\sigma}}}_{\mathit{L}2}$ show large deviations from the ECPSSR theory. The experimental x-ray production cross-section ratios are found to be in better agreement with the theoretical results obtained from using ECPSSR ionization cross sections and the decay yield data of Xu and Xu [J. Phys. B 25, 695 (1992)] rather than those obtained from using the decay yield data of Krause [J. Phys. Chem. Ref. Data 8, 307 (1979)]. The x-ray production cross sections, however, are in better agreement with the theoretical results obtained from using the decay yield data of Krause. The measured centroid energy of the L\ensuremath{\gamma} lines of Pb shows large deviations at high projectile energy, whereas for Bi large deviations are found at the low-energy region.

Charge asymmetry of ionization cross sections of atomic hydrogen by antiproton and proton impact

Large-scale coupled-channel calculations are carried out for the evaluation of the ionization cross section of atomic hydrogen by antiproton impact. The dependence of the ionization cross sections on the projectile charge is investigated in comparison with the case of proton impact. It is demonstrated that continuum states of the antiproton, which were not taken into account satisfactorily in previous calculations, play an important role for the ionization. The ionization cross section ratio of the antiproton and proton impact obtained by the present coupled-channel calculations is close to the classical trajectory Monte Carlo (CTMC) value at variance with the result of Ermolaev [Phys. Lett. A 149 (1990) 151].

The electron-impact ionization of Ar and Kr revisited: a critical analysis of double-to-single ionization cross section ratio measurements using the fast-atom-beam technique

We report new measurements of the absolute electron-impact double ionization cross sections for Ar and Kr and of the ratios of double-to-single ionization for impact energies from threshold to 200 eV using the crossed electron-beam — fast-atom-beam technique. The work was motivated by the recently highlighted spread of about 30% in the Ar2+/Ar+ ionization cross section ratios obtained by several groups using different experimental techniques. Such a spread is inconsistent with statistical uncertainties of typically 3% or less that were quoted for the various reported ratios. A similar situation exists for Kr where the spread among the recently published Kr2+/Kr+ ionization cross section ratios is about 15%. We made an attempt to identify all potential systematic errors inherent to the fast-beam technique that could affect the measurement of cross section ratios with special emphasis on those systematic errors that could influence the detection of singly and doubly charged product ions differently. We found Ar2+/Ar+ and Kr2+/Kr+ cross section ratios of, respectively 0.066 ±0.007 and 0.087 ±0.008 at 100 eV which confirm earlier measurements using the same experimental technique. The error limits on cross sections ratios measured in our fast-beam apparatus were determined to be at least ±9% for cross section ratios of multiple-to-single ionization for the same target atom and at least ±10% for ratios of single ionization cross sections for different target species. Our error limits are dominated by systematic uncertainties of the apparatus which do not cancel when cross section ratios are measured, since the ratios are obtained under similar, but not identical experimental conditions.

Problems in the measurement of the Ar2+/Ar+ partial ionization cross-section ratio by use of the pulsed-electron-beam time-of-flight method

In order to clarify a long standing problem in mass spectroscopy we have carried out a careful reanalysis of our earlier work on the ratio of the Ar2+ to Ar+ partial ionization cross-section ratio using the pulsed-electron-beam time-of-flight method. Of all the possible error sources investigated only three were found to have a significant effect on the ratio but all three acted in the same direction. We have thus revised our previous estimates for the value of the ratio downward and increased our estimates of the uncertainy in our measurements. Our new results agree with those recently published in this journal as determined from a careful re-examination of the fast neutral beam technique.

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.

Double and single ionization of helium by high velocity N7+ ions.

Beams of fully stripped nitrogen ions have been used to investigate the behavior of the double-to-single ionization cross-section ratio of helium in the 10--30-MeV/amu velocity region. The measured ratio was found to remain nearly constant over this velocity range at a value of 0.01, which is about 4.5 times higher than the high-velocity limit established previously for {ital q}=1 projectiles.

Velocity dependence of the cross section for Penning and associative ionization of argon atoms by metastable neon atoms

Relative cross sections for Penning and associative ionization in Ne*(3P2,0)–Ar collisions have been measured, in a crossed beam experiment, as a function of the collision velocity, in the thermal energy range. The total ionization cross sections have been analyzed, together with other experimental results, obtaining a best fit resonance width function. The analysis of the associative to Penning ionization cross section ratios shows that, in the high collision energy range, the ionization occurs predominantly through the 2Σ1/2 ground state of NeAr+ ion. Some considerations on the role played by the interaction anisotropy in these ionization processes are reported.

Absolute partial electron impact ionization cross sections of Xe from threshold up to 180 eV

Partial electron ionization cross section ratios and functions of Xe were determined in the low energy regime (≤180 eV) using a refined mass spectrometric technique. The experimental results are compared with previous measurements and calculations.

Higher order processes in L-shell ionization

The time-dependent perturbation theory has been applied to the description of the L-shell ionization of atoms by heavy charged particles in the independent-particle model approximation. A second order correction factor to cross sections calculated in a first order theory (e.g. PWBA) has been derived considering transitions at an average impact parameter and with minimum energy transfer as dominant ionization processes in low-velocity collisions. Numerical calculations have been performed for light and heavy ( Z 1 ⩽ 8) ion impact ionization of gold in the energy range 0.15—2.0 MeV u . The results are in satisfactory agreement with the experimental data for the L 3-to-L 2 and L 1-to-L 2 subshell ionization cross section ratios. The model seems to account also for the anomalous behaviour of the L 3-subshell alignment observed recently at heavy-ion impact.

L-subshell ionization cross sections for proton bombardment of Ag, In, Sn, and I

$L$---x-ray production cross sections were measured for thin targets of Ag, In, Sn, and I in the proton energy range 0.300-5.000 MeV. Subshell ionization cross sections were extracted and compared with existing data and with the predictions of plane-wave Born approximation (PWBA), perturbed---stationary-state theory including energy loss, Coulomb deflection and relativistic corrections (ECPSSR), binary-encounter approximation with corrections accounting for binding effect and Coulomb retardation (BEABC), and semiclassical approximation taking into account binding, energy loss, Coulomb deflection, and relativistic effects (SCABCR). Large disagreement was found with PWBA, but the other theories reproduced the experimental data rather well. Owing to the nodal structure of the $2s$ wave function, deeper insight was achieved by considering the ratios of the sub-shell ionization cross sections. These results were found to be in good agreement with analogous measurements on high-$Z$ elements and favor the ECPSSR over the other theories. A realistic value for the ${\ensuremath{\omega}}_{1}$ fluorescence yield was suggested for indium near the discontinuity at $Z\ensuremath{\sim}50$.

Deuteron- and Alpha-Induced K-Shell Ionization Cross Section Ratios in the Z=12-17 Region

Experimental deuteron- and alpha-induced K-shell ionisation cross section ratios are presented for elements ranging from Mg to Cl (Z=12-17) in the incident energy range 0.2 to 0.45 MeV/u. The experimental ratios R = ??/4?d are generally close but slightly higher than the theoretical ratios calculated according to the procedure of Brandt and Lapicki; the largest differences occur for Al and Si where the experimental values are 15-20% too high. The experimental ratios show roughly the same dependence on reduced projectile velocity, ?= (v1 / vK)(2/?K), as the theoretical ones in the reduced incident velocity range used in the present work, ?= 0.45 to ?=1. This is in opposition to the results previously obtained in the velocity range ?=0.25 to ?=0.55 for elements ranging from Ca to Cu (Z=20-29), where the observed ratios are rather independent of the reduced velocity.

Projectile-charge dependence of inner-shell-ionization cross sections in a high-velocity region

Ratios of ionization cross sections for proton impact to those for $^{3}\mathrm{H}\mathrm{e}\ensuremath{-}\mathrm{i}\mathrm{o}\mathrm{n}$ impact have been measured on $\mathrm{Al}K$ and $\mathrm{Y}L$ shells at projectile energies of 7, 9, 12, 16, and 21 MeV /amu. The ratio tends to unity with an increase in the projectile energy for both the $\mathrm{Al}K$ and $\mathrm{Y}L$ ionizations. This behavior can well be explained in terms of the Binstock-Reading theory combined with the distant-collision theory of Hill and Merzbacher: The polarization effect in distant collisions and the decreasing electron-density effect in close collisions cancel out each other in this high-energy region.

Screening and antiscreening by projectile electrons in high-velocity atomic collisions

The scattering amplitude for a projectile of nuclear charge ${Z}_{1}$ carrying $N$ electrons colliding with an atomic target with charge ${Z}_{2}$, evaluated in the first Born approximation using hydrogenic wave functions, is compared with recent experimental results. In the present approximation where the minimum momentum transfer ${t}_{min}$ is considered to be approximately independent of the final state of the projectile, the differential cross sections separate into a product of one term that depends only on the target times the square ${Z}_{1}^{2}(t)$ of an effective projectile charge. Here an analytic expression for ${Z}_{1}^{2}(t)$ is given for 2, 1, and 0 electrons. Some total ionization cross-section ratios are also given.

Electronic relativistic effect on the ratios of deuteron to alpha-particle induced K-shell ionization cross sections

The ratios of the K-shell ionization cross sections induced by deuterons to those induced by α-particles with the same velocities have been calculated in the plane wave Born approximation using relativistic hydrogenic wave functions for the target electrons. The numerical results are compared with the recent experimental valus of Chang et al. The use of relativistic wave functios considerably improves the agreement with the experimental data.

The binding change effect on the K-shell ionization by 0.3–0.7 [formula omitted] deuterons and alpha particles

The ratios of the K-shell ionization cross sections induced by deuterons to those induced by α-particles for Cu, Ag and Tb targets were measured in the energy range of 0.3–0.7 MeV amu at equal velocities of the deuterons and α-particles. The observed ratios are found to be 15–20% lower than those predicted by the CPSS theory of Basbas et al. The predicted rise of the ratios with decreasing velocities is consistent with our Cu data but not with our Ag and Tb data. The importance of the relativistic effect of the K-shell electrons is discussed.

Cross Section Ratios for the Electron Impact Production of Singly and Multiply Ionized Rare Gas Ions

Electron impact ionization of He, Ne, Ar, Kr and Xe has been studied with a double focussing mass spectrometer Varian MAT CH5. Ratios of various multiple ionization cross sections with respect to single ionization cross sections for He, Ne, Ar, Kr and Xe at electron energies of 50, 100 and 150eV are given. These cross section ratios are com­pared with previous determinations.

Quantitative X-Ray Photoelectron Spectroscopic (XPS) Measurement on the Surfaces of GaAs(111), (1̄1̄1̄) and (110) Single Crystals –Determination of Relative Photo-Auger Ionization Cross Sections and Electron Mean Free Paths by Using the Crystal Regularity of Compound Semiconductors–

Quantitative XPS studies were carried out on the surfaces of GaAs (111), (1̄1̄1̄) and (110). Applying the exponential electron attenuation model to the regularly layered structure of the GaAs (111) and (1̄1̄1̄) surfaces, the values of electron mean free paths were determined. Then relative photo-Auger ionization cross sections (ratios of photo-Auger ionization cross sections of Ga i peak to As j peak; τGai/τAsj) were obtained to be 0.97 for τGa2p/τAs2p 0.94 for τGa3p/τAs3p and 0.80 for τGa3d/τAs3d in the case of Al kα excitation. The corresponding results from the GaAs (110) surface were 1.0, 0.94 and 0.82, respectively, and a good agreement was observed between the two independent measurements. Compared with the results theoretically calculated by Scofield, a comparatively good agreement was also observed between experiment and theory.

Thermodynamic study of the vaporization of europium monosulfide by Knudsen-cell mass spectrometry Atomization energies of EuS(g), Eu 2S(g), EuS 2(g), Eu 2O(g), Eu 2O 2(g), Eu 2OS(g), and Eu 2S 2(g)

The vaporization of EuS(s) is studied in the temperature range 1550 to 2500 K. Mass-spectrometric analysis of the vapour leads to the characterization of the gaseous molecules EuS, EuS 2, Eu 2S, Eu 2S 2, EuOS, Eu 2S, Eu 2O 2, and probably Eu 2S 3 and Eu 3S 4. The main vaporization process is EuS(s) = Eu(g) + αS(g) + { (1 − α) 2 } S 2( g) S 2(g), which leads to congruent effusion. The congruently effusing composition varies slightly with temperature, the mole ratio n (Eu) n (EuS) being displaced from 1830 to 2260 K by (7.6±0.9) × 10 −4 towards the sulphur-rich side. EuS(s) exists within a homogeneity range which extends between EuS 1.027 at 1420 K and at least EuS 0.99 at 1935 K, when the congruently effusing composition at 2000 K is set at exact stoichiometry. Partial pressures determined by the mass-spectrometric Knudsen-effusion method are compared with the literature data. Experimental ionization cross-section ratios are determined for Eu(g), S(g) and S 2(g). The atomization energies are D at o(EuS, s, 0) = (217.4 ± 2.6) kcal th mol −1, D o(EuS, g, 0) = (85.7 ± 3.1) kcal th mol −1, D at o(EuS 2, g, 0) = (171.7 ± 5.0) kcal th mol −1, D at o(Eu 2S, g, 0) = (167.3 ± 5.0) kcal th mol −1, D at o(Eu 2S 2, g, 0) = 283.2 ± 5.0) kcal th mol −1, D at o(EuOS, g, 0) = (315.1 ± 5.0) kcal th mol −1, D at o(Eu 2O, g, 0) = (205.1 ± 5.0) kcal th mol −1, D at o(Eu 2O 2, g, 0) = (345.1 ± 5.0) kcal th mol −1, D at o(Eu 2S 3, g, 0) ⩽ (354.3 ± 10.0) kcal th mol −1, and D at o(Eu 2S 4, g, 0) ⩽ (452.7 ± 10.0) kcal th mol −1. The corresponding standard enthalpies of formation are also listed.