Information about TS-1 and TS-2 double ionization mechanisms for positron and electron impact ionization of argon

Ejected electron angular distributions are measured for single and double ionization of argon by 500 eV positron and electron impact. Double to single ionization ratios show marked differences as a function of projectile charge. Combinations of the positron and electron data and a simple double ionization model are used to obtain differential information about the TS-1 and TS-2 mechanisms. Our analysis suggests that both mechanisms contribute roughly equally to the emission of two electrons and that interference between the two mechanisms significantly alters the ejected electron spectra.     

Near threshold vibrational excitation of molecules by positron impact: A projection operator approach

We report vibrational excitation (ν_i=0→ν_f=1) cross-sections for positron scattering by H₂ and model calculations for the (ν_i=0→ν_f=1) excitation of the C-C symmetric stretch mode of C₂H₂. The Feshbach projection operator formalism was employed to vibrationally resolve the fixed-nuclei phase shifts obtained with the Schwinger multichannel method. The near threshold behavior of H₂ and C₂H₂ significantly differ in the sense that no low lying singularity (either virtual or bound state) was found for the former, while a e⁺-acetylene virtual state was found at the equilibrium geometry (this virtual state becomes a bound state upon stretching the molecule). For C₂H₂, we also performed model calculations comparing excitation cross-sections arising from virtual (-iκ₀) and bound (+iκ₀) states symmetrically located around the origin of the complex momentum plane (i.e. having the same κ₀). The virtual state is seen to significantly couple to vibrations, and similar cross-sections were obtained for shallow bound and virtual states.     

Threshold effects in the ionization of atoms by positron impact

We employ different theoretical models, both classical and quantum-mechanical, to explore the recoil-ion momentum distribution in positron-atom collisions. We pay special attention to the vicinity of the kinematical threshold between ionization and positronium formation. We demonstrate that it is intertwined by dynamical constraints to the formation of highly excited and low-lying continuum electron-positron states. Finally we discuss how the study of recoil-ion momentum distribution, which is characteristic of a reaction microscopy technique, might represent an alternative approach to the standard spectroscopy of electrons and positrons.     

Current status of kinematically complete studies of basic fragmentation processes in atomic systems

Recent developments on kinematically complete experiments on basic atomic fragmentation processes are reviewed. Comparisons between theoretical and experimental fully differential cross sections for single ionization of light atoms by charged particle impact are analyzed. Furthermore, a method developed very recently, four-particle Dalitz plots, is discussed in context of double ionization. The extraordinary power of these plots is their capability to provide a comprehensive picture of the momentum exchange between all four final-state particles in a single spectrum.     

Near-threshold positron impact ionization of hydrogen

The hyperspherical hidden crossing method (HHCM) is used to investigate positron impact ionization of hydrogen near threshold. An important feature of this method is that it can provide valuable insight into scattering processes. In the calculation of positron-hydrogen ionization, the adiabatic Hamiltonian is expanded about the Wannier saddle point; anharmonic corrections are treated perturbatively. The S-wave results are consistent with the Wannier threshold law and with the extended threshold law that was previously derived using the HHCM. We have extended the previous HHCM calculation to higher angular momenta L and have calculated the absolute ionization cross-section for L = 0, 1 and 2. The HHCM calculation confirms that the S-wave ionization cross-section is small and provides the reason why it is small. The HHCM ionization cross-section (summed over the lowest partial waves) is compared with a convergent close-coupling calculation, a 33-state close-coupling calculation and experimental data.     

ECC study in positron impact ionization of helium

A 3C approximation which was employed in the study of the ECC phenomenon in positron impact ionization of H₂ is now applied to positron impact ionization of helium. Our absolute triple differential cross sections are used to calibrate the existing experimental data.     

Convolution approximation for the energy loss, ionization probability and straggling of fast ions

In this work we describe an extension of the convolution approximation for the ionization probability and energy-loss straggling as a function of the impact parameter for swift ions. Analytical formulas for these quantities are derived and compared to full first-order Born calculations. The physical inputs of the model are the electron density and oscillators strengths of the target as well as the screening function of the projectile (in the case of dressed ions). A very good agreement is obtained for all impact parameters. In addition, we propose a general schema to add contributions from distant and close collisions. In this way physical processes arising from large and small impact parameters can be easily included into a single expression valid for all impact parameters. This model is then used to investigate the projectile-charge q dependence of ionization, stopping and straggling cross-sections.     

On the double ionization of helium by very slow antiproton impact

Here we present experimental information on the cross section for double ionization of helium atoms by slow antiproton impact. It is used to discern between many advanced theoretical calculations. Earlier measurements of the ratio R between the double and single ionization cross sections for antiproton impact on helium show a persistent increase for the projectile energy decreasing from 10 MeV to 10 keV. The present data show that below 10 keV this increase stops and we give an upper limit to R.     

Low energy positron scattering from helium

A new experiment has been developed for high resolution studies of positron scattering from atoms and molecules. Based on the Surko trap technology, a pulsed positron beam has been used to obtain preliminary measurements of low energy, differential elastic scattering cross sections from helium. The operation of the beamline is described and preliminary absolute cross section values for scattering energies of 5, 10 and 15 eV are presented and compared with contemporary theoretical calculations.     

Differential cross sections for positronium formation in positron-hydrogen scattering

Positron scattering by hydrogen atom is an interesting system to test theoretical methods due to its simplicity. Recently, theoretical calculations have reported differential cross sections (DCS) for positronium (Ps) formation for this system. The present work utilises the coupled-channel optical method (CCOM) that allows simultaneous treatment of the target channels and the Ps channels in the close-coupling method and the incorporation of the continuum effects via an optical potential to provide a comparative view of the DCS for Ps(1s) formation and Ps(2s) formation at energies ranging from 20 to 100 eV. A large 12-states and 15-states CCOM calculations have been undertaken and the results compared with other available data.     

Dynamics of collisions with positrons

The measurement of ionization by positron impact reveals that the maximum of the electron capture to the continuum cusp is shifted from its theoretical position. In this work the hypothesis that the observed effect is the result of an anisotropic momentum distribution in the projectile-electron reference system is considered.By elaborating on the ansatz that the cusp asymmetry is qualitatively similar for positron impact than the for ion-atom collisions, we obtain fully differential cross sections that show the same features than those experimentally observed. The present estimation for the position of the maximum agrees well with cross section measurements performed in coincident electron-positron detection experiments.     

Total cross sections for positron scattering on benzene - angular resolution corrections

We compare recent measurements of total cross sections for positron scattering in benzene from Trento laboratory [G.P. Karwasz, R.S. Brusa, Z. Idziaszek, A. Karbowski, Eur. J. Phys. D 144 (2007) 197] with early [O. Sueoka, J. Phys. B 21 (1988) L631], later [O. Sueoka, M.K. Kawada, M. Kimura, Nucl. Instr. Method. Phys. B 171 (2000) 96] and the most recent [C. Makochekanwa, O. Sueoka, M. Kimura, Phys. Rev. A 68 (2003) 32707-1] data from Tokyo laboratory. The latter data are significantly lower than the two other sets. A simple calculation shows that an angular resolution correction, rising strongly in the limit of zero energy, should be applied to measured values if wide apertures in the scattering cell and or strong guiding magnetic fields are used. We show, with the help of the modified effective range theory, that the data from Trento and those of Sueoka (1988) would agree well with those of Sueoka et al. (2000) if the data from Sueoka et al. (2000) were artificially low, as a result of the big angular resolution error due to the experimental conditions present in Sueoka et al. (2000).     

Low energy elastic scattering of positrons by CO: An application of continued fractions and Schwinger variational iterative methods

Iterative Schwinger variational methods and the method of continued fractions, widely used for electron-molecule scattering, are applied for the first time to investigate positron-molecule interactions. Specifically, integral and differential cross sections for elastic positron scattering by CO in the (0.5-20) eV energy range are calculated and reported. In our calculation, a static plus correlation-polarization potential is used to represent the collisional dynamics. Our calculated results are in general agreement with the theoretical and experimental data available in the literature.     

Polarization effects on the cross-section of ion-atom collisions

Coupled-channel cross-sections for electron capture, ionization and electron loss due to polarization effects are calculated. The maximum impact parameter for electron escape is analyzed within the classical framework. The probabilities of ionization and capture are analyzed simultaneously by a semi-empirical method. Differing from the n-body classical trajectory Monte Carlo method, the condition for electron escape is determined by Coulomb forces related to the two nuclei. This method can be used to calculate coupled-channel cross-sections rather than single-channel ones in other methods. Therefore the calculated results can be compared with experimental data directly. In the low energy range, neglecting the ionization effect, the single-capture cross-sections of hydrogen atoms induced by various partially-stripped ions were calculated. In the high energy range, neglecting the capture effect on ionization, the pure-ionization cross-sections of neon atoms induced by Ne^q+ (q = 4, 6, 8) and Ar^q+ (q = 4, 6, 8, 10) at an incident energy E = 1.05 MeV/u were calculated. Good agreement was found between our calculation and experimental data in the literature. This method had been partially applied for intermediate energy successfully.     

Single- and multiple-electron loss cross-sections for fast heavy ions colliding with neutrals: Semi-classical calculations

Extensive calculations of single, multiple and total electron-loss cross-sections of fast heavy ions in collisions with neutral atoms are performed in the semi-classical approximation using the DEPOSIT code based on the energy deposition model and statistical distributions for ionization probabilities. The results are presented for Ar¹⁺, Ar²⁺, Kr⁷⁺, Xe³⁺, Xe¹⁸⁺, Pb²⁵⁺ and U^q+ (q = 10, 28, 39, 62) ions colliding with H, N, Ne, Ar, Kr, Xe and U atoms at energies E > 1 MeV/u and compared with available experimental data and the n-particle classical-trajectory Monte Carlo (nCTMC) calculations. The results show that the present semi-classical model can be applied for estimation of multiple and total electron-loss cross-sections within accuracies of a factor of 2.From calculated data for the total electron-loss cross-sections σ_tot, their dependencies on relative velocity v, the first ionization potential I₁ of the projectile and the target atomic number Z_A are found and a semi-empirical formula for σ_tot is suggested. The velocity range, where the semi-classical approximation can be used, is discussed.     

Fluorescence of polarized atoms excited by polarized electrons

A general expression for the cross section describing properties of the fluorescence radiation from polarized atoms excited by polarized electrons is obtained in the distorted wave (DW) approximation. The expression is presented in the form of multiple expansion over the state multipoles of all particles taking part in the two-step process. It is used to derive the alignment parameters of excited atom and angular distribution of emitted fluorescence. The polarization degree of radiation from the lowest autoionizing states following the excitation of non-polarized Na and K atoms by non-polarized electrons is calculated.     

Properties of Auger electrons following excitation of polarized atoms by polarized electrons

In non-relativistic approximation, the most general expression for differential cross sections describing the properties of Auger-electron emission induced in the excitation of polarized atoms by polarized electrons is obtained for the first time. The ways of the application of the general expressions suitable for the specific experimental conditions are outlined by deriving the expressions for the asymmetry parameters and the magnetic dichroism of the angular distribution of the Auger electrons as well as of the angular correlations between the scattered and Auger electrons.     

Post-collision interaction after electron impact measured by (e,2e) coincidence technique

Auger electron line-shapes after electron impact inner-shell ionization of Argon at 350 eV primary electron energy have been studied by coincidence spectrometry. Emitted Auger electrons are detected in coincidence with the scattered electrons and the energy of the very slow PCI inducer ejected electron was calculated from energy conservation. The background, caused by outer-shell electrons was measured and then removed from the coincident spectrum. The effect of PCI is studied on the background-free diagram Auger spectrum. A systematic deviation was found from the line-shape given by the semi-classical approach.     

Resonant Auger decay of 2p hole in argon induced by electron impact

We present an experimental study of L-MM resonant Auger spectra of argon after electron impact excitation. The electron spectra were measured at ten different electron impact energies between 442.6 eV and 461.7 eV. During (e, 2e) measurement the energy of the second electron was kept fixed at 209.6 eV, corresponding to the energy of one of the strongest resonant Auger transitions from the [2p_3/23d] state. Except for the monopole excitations, the recorded spectral structures are explained on the basis of photon impact data.