Positron scattering from neon and argon

We report results for positron scattering by ground-state atomic hydrogen in the energy range 0 to 80 eV. The calculations have been performed in an 18-state Ps(1s, 2s, 3s, 4s, 2p, 3p, 4p, 3d, 4d)+H(1s, 2s, 3s, 4s, 2p, 3p, 4p, 3d, 4d) approximation where the pseudostates (denoted by a bar) have been taken from Fon et al. (1981). Cross sections are presented for elastic scattering, positronium formation, total scattering and ionization. The elastic scattering results are in good agreement with accurate variational numbers at low energies and with other sophisticated, but very different, theoretical approximations at higher energies. We estimate that the elastic cross section is now known to better than 10%. The cross section for positronium formation is dominated by capture into the 1s state and is in fairly good agreement with the measurements of Weber et al. (1994). The results for the total cross section are generally consistent with the upper and lower bounds of Zhou et al. (1994) but are a little larger than other theoretical estimates at the higher energies. There is also a general theoretical problem concerning the degree to which the total cross sections for electrons and positrons merge at energies above 31 eV. The calculated ionization cross section is in agreement with the measurements of Jones et al. (1993).

Coupled-state calculations in a K(4s,4p,5s,5p,3d) + Ps(1s,2s,2p,3s,3p,3d) approximation are reported for positron scattering by ground-state potassium in the energy range 0.5 - 60.0 eV. Comparison is made with the earlier work of Hewitt and co-workers in a K(4s,4p,5s,5p) + Ps(1s,2s,2p) approximation. For the first time cross sections for positronium formation in n = 3 states are obtained. We support the conclusion of Hewitt et al that above about 4 eV positronium formation is mainly into excited states, although we disagree significantly on the distribution of the excited state population. Fairly good agreement is obtained with the measurements of Zhou and co-workers for total positronium formation when scaling is used to estimate the amount of positronium in states with . Our cross sections for elastic scattering, 4s - 4p, 4s - 5s and 4s - 5p excitation of potassium are in relatively good agreement with those of Hewitt and co-workers but our calculations also emphasize the importance of the 4s - 3d excitation which is omitted in their approximation. Atom excitation to the n = 5 level is found to be a relatively minor process. Good agreement is obtained with the total cross section measurements of Kwan and co-workers and Parikh and co-workers when these are normalized upwards by 10% and corrections made for near forward elastic scattering. This upward normalization lies within the quoted overall 21% error on the measurements. Below 4 eV the calculated total cross section is dominated by elastic scattering while at high energies 4s - 4p excitation is the main contributor. In the vicinity of 6 eV, and consistent with the measurements of Parikh and co-workers, the total cross section displays a pronounced peak which derives largely, although not totally, from positronium formation. Positronium formation is not important above about 30 eV.

The equations for positron-hydrogen scattering are written as a set of coupled momentum-space Lippmann-Schwinger equations. An explicit expression for the positronium formation matrix element that is suitable for large-scale computation is derived. Elastic and positronium formation cross sections are obtained in the coupled static model as a validation of the method. Calculations at energies below the hydrogen n = 2 threshold are used to study the convergence of observable quantities with increasing numbers of physical Hand Ps states. At higher energies, a series of calculations including multiple hydrogen and positronium states are performed over the energy range from 1�0 to 4�0 Ryd. Cross sections for elastic scattering, excitation of the 2s and 2p hydrogen levels and positronium formation in the Is, 2s and 2p levels are reported.

The formation of positronium (Ps) in different states by the impact of intermediate energy (20--500 eV) positrons on atomic hydrogen is calculated using the distorted-wave Born approximation along with the first Born approximation. Differential and integrated cross sections for the formation of Ps(1{ital s}), Ps(2{ital s}), Ps(2{ital p}{sub 0}), and Ps(2{ital p}{sub 1}) have been evaluated individually and then 1/{ital n}{sup 3} behavior for charge-transfer cross sections is used for {ital n}{gt}=3 to obtain the total cross sections for positronium formation. The present results for positronium formation cross sections are in agreement with the results of other available investigations.

We report positron scattering cross sections of fluorine and fluorine-containing molecules across an energy range from 1 to 5000 eV. Targets considered in the present study are F, F2, HF, CF4, C2F6, C6F6, CHF3, C2H3F3, and C2H2F2. Elastic, inelastic, total, and positronium formation cross sections were calculated using the optical potential approach, and the ionisation cross sections are estimated employing the complex scattering potential-ionisation contribution method. The computed results are in satisfactory agreement with available theoretical data. Comparison with experiments shows excellent agreement in the high-energy range; however, discrepancies are observed at lower energies, attributed to deficiencies in the polarisation potential model used and, in part, to the difficulty in resolving forward-angle scattering effects in experimental measurements. Positron interactions with these molecules are important in plasma, astrophysics, and radiation physics, where ionisation, energy losses, and the formation and destruction of positronium are significant. The positron scattering cross-sections are primary inputs for modelling in astrophysics, materials science, and medical imaging, such as positron emission tomography.

Positronium Formation by Positron Impact on Atoms at Intermediate Energies

We report on the extension of the single-center convergent close-coupling method to positron scattering on multielectron atoms, along with the development of a technique for separating the direct ionization and positronium-formation channels in single-center calculations. We have performed calculations of positron scattering on carbon and present total elastic, momentum transfer, excitation, direct ionization, total ionization, total inelastic, positronium-formation, stopping power, and total cross sections from threshold to 5000 eV. We also present oscillator strengths, the scattering length, the hidden Ramsauer-Townsend minimum, the energy of the positron-carbon virtual state, and the mean excitation energy. Agreement with electron-scattering experiment and positron-scattering theory for several cross sections has been demonstrated for high energies. However, discrepancies exist between different theoretical methods at lower energies and for the ionization and positronium-formation processes.

The hyperspherical hidden-crossing method (HHCM) has been applied to s-, p-, and d-wave positronium formation in positron-lithium collisions in the energy range 0-1.8 eV, using a model potential to describe the atomic core. The calculations have provided a test of the HHCM. The s-, p-, and d-wave positronium formation cross sections are reported and compared with variational and close-coupling calculations. A minimum is obtained in the s-wave positronium formation cross section which is due to the Stuckelberg phase having a value of {pi}. The cross sections have also been computed including the correction term which emerges from the one-Sturmian theory.

Quantitative information about positron and electron collisions with nucleobases is required in charged-particle track simulations to accurately model and assess any radiation damage at the subcellular level in biological systems. However, scattering experiments have so far been restricted to the pyrimidine nucleobases. In this paper we report on total-cross-section measurements for positron impact on indole, a parent molecule of the purine nucleobases, at impact energies between 1 and 25 eV. We also present theoretical cross sections for elastic and total scattering, positronium formation, electronic excitations, and direct ionization between 1 and 500 eV, as calculated with the independent-atom model with the screening-corrected additivity rule. Rotational excitation cross sections are additionally calculated within a Born framework over that same energy range. A significant discrepancy is found between the measured and computed total cross sections, which cannot be entirely accounted for by the lack of forward-angle-scattering discrimination in the experiment. The present results are also compared to the available theoretical cross sections for the purine nucleobases. In addition, total cross sections for electron-indole collisions computed within our theoretical formalism are provided.

New measurements for the total, single ionization of B/sup 2+/ and O/sup 5+/ by electron impact are reported with particular attention directed to the indirect contribution of excitation-autoionization. For B/sup 2+/ these are the first measurements to our knowledge while for O/sup 5+/ the present data improve on previous measurements. In both cases the direct total ionization cross sections are in good agreement with recent distorted-wave predictions of Younger, and the excitation-autoionization contribution is consistent with measurements for other Li-like ions. For O/sup 5+/ an additional small increase in the cross section is observed at an energy well below the excitation-autoionization threshold; this feature may be attributable to resonance recombination followed by auto-double-ionization. For the Li-like ions Be/sup +/, B/sup 2+/, C/sup 3+/, N/sup 4+/, and O/sup 5+/, the ionization theory of Younger is used as a basis to subtract the direct ionization component from the total measured ionization cross sections in order to extract the 1s/sup 2/2s..-->..summation/sub l/1s2s2l excitation cross section. The deduced inner-electron excitation cross sections are quantitatively compared with excitation theory including new close-coupling calculations for Be/sup +/ and B/sup 2+/ presented here. For all cases except Be/sup +/, the close-coupling theory is in agreement with the more » deduced experimental excitation cross sections. These new results and analysis improve confidence in excitation theory for highly charged ions. « less

Using a Beer–Lambert attenuation approach, we report measured total cross sections (TCSs) for positron scattering from vinyl acetate (C4H6O2) in the incident positron energy range 0.15–50 eV. In addition, we also report an independent atom model with screening corrected additivity rule computation results for the TCSs, differential and integral elastic cross sections, the positronium formation cross section and inelastic integral cross sections. The energy range of these calculations is 1–1000 eV. While there is a reasonable qualitative correspondence between measurement and calculation for the TCSs, in terms of the energy dependence of those cross sections, the theory was found to be a factor of ∼2 larger in magnitude at the lower energies, even after the measured data were corrected for the forward angle scattering effect.

Ionization cross sections for the four combinations of electrons and positrons on H and He/sup +/ have been calculated at energies near the threshold for ionization using the classical trajectory Monte Carlo method. In addition, charge-exchange (positronium formation) cross sections have been calculated for the positron-impact collisions. The ionization cross sections compare well with the predictions of the Wannier and Klar models and experimental observations. Our results indicate that the Wannier-type threshold behavior may extend to higher energies than was previously thought. In addition, we have calculated two of the collision parameters, the angle and the relative separation between the escaping particles, used in the Wannier and Klar models, and find good agreement with the values predicted by these theories.

Integral cross sections (ICSs) for the excitation of 18 excited electronic states, and four composite excited electronic states, in nitric oxide (NO) have been determined for incident electron energies of 15, 20, 30, 40 and 50 eV. These ICSs were derived by extrapolating the respective measured differential cross sections (M J Brunger et al 2000 J. Phys. B: At. Mol. Opt. Phys. 33 783) to 0° and 180° and by performing the appropriate integration. Comparison of the present ICSs with the results of those determined in earlier optical emission measurements, and from theoretical calculations is made. At each incident energy considered, the current ICSs are also summed along with the corresponding elastic and rovibrational excitation ICSs from B Mojarrabi et al (1995 J. Phys. B: At. Mol. Opt. Phys. 28 487) and the ionization cross sections from Rapp and Englander-Golden (1965 J. Chem. Phys. 43 1464), to derive an estimate of the grand total cross sections (GTSs) for e-+ NO scattering. The GTSs derived in this manner are compared with the results from independent linear transmission experiments and are found to be entirely consistent with them. The present excited electronic state ICS, and those for elastic and rovibrational excitation from Mojarrabi et al, appear to represent the first set of self-consistent cross sections for electron impact scattering from NO.

Calculations, using the Kohn variational method, have been made of the d-wave contributions to the elastic scattering and positronium formation cross sections for positron scattering by atomic hydrogen in the Ore gap, 6.8-10.2 eV. Very elaborate trial functions were used and it is believed that the results have converged to within 10% of their exact values. These results have been combined with others by the present authors for s- and p-wave scattering in an investigation of the behaviour of the total elastic scattering cross section in the vicinity of the positronium formation threshold and, also, of the angular distribution of the positronium. The findings of both investigations suggest a possible explanation of the discrepancy between the experimental measurements of the positronium formation cross section by Charlton et al. (1983) and Fornari et al. (1983).

The authors apply the first-order target continuum distorted wave (TCDW1) approximation to electron capture by positrons from helium into the ground state of positronium. Total cross sections for positronium formation are found to be in good agreement with the measurement of Fromme et al. (1986) above 160 eV. This is the energy range in which most theoretical results disagree with the measurements and indeed in previous classical trajectory Monte Carlo (CTMC) calculations it has been suggested that the experiments incorrectly describe large-angle ionization as charge transfer. The authors elaborate on this matter.