Positron scattering by potassium

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).

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.

Positronium Formation by Positron Impact on Atoms at Intermediate Energies

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.

The relativistic optical potential method has been used to calculate a complete set of cross sections for positron scattering from neon and argon in the energy range from zero to 1 keV. Elastic, positronium formation, excitation, direct ionization, and grand total cross sections are presented and compared to the recommended results of Ratnavelu et al. (J Phys Chem Ref Data 48:023102, 2019). Here, positronium formation is modeled as an ionization process by successively lowering the ionization thresholds. This process was carried out for the four lowest energy levels of positronium.Graphical abstract

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.

Calculated total, differential, and momentum-transfer cross sections are reported for the vibrationally elastic scattering of electrons from ${\mathrm{H}}_{2}$O and ${\mathrm{NH}}_{3}$ molecules in the range of energy 0.5--20 eV. The total cross sections are approximated by a sum of the rotationally elastic ones and the rotational transition from ground to first excited state (00\ensuremath{\rightarrow}10). The first-order Born approximation with the rotating-molecule model is used in the calculation of the first rotational excitation cross section. The experimental value of the dipole moments of ${\mathrm{H}}_{2}$O and ${\mathrm{NH}}_{3}$ are used in this step of the calculation. Spherically approximate molecular wave functions are applied to calculate the spherical part of the interactions of the incident electron with the molecule and the rotationally elastic-scattering cross section, in which the static and exchange interactions are calculated exactly within the accuracy of the molecular wave function and a parameter-free correlation-polarization potential is used to account for the polarization effect. The agreement of the differential cross sections with the experimental data is good at small angles and reasonably good at intermediate and large angles. Good agreement with measured values is obtained for the momentum-transfer cross sections. Total cross sections are reported with theoretical results on the e-${\mathrm{NH}}_{3}$ system below 10 eV. The possible sources of the discrepancies between the current theoretical and experimental results are analyzed for the total cross sections at very low energies by a quantitative consideration of the contributions of very strong forward scattering at near-zero angles to the total cross sections.

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.

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.

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.

We report calculated total elastic cross sections Qel, total ionisation cross sections, Qion, summed total excitation cross sections ∑Qexc and total cross sections QT for CH3SH upon electron impact for energies from ionisation threshold to 5 keV. We have employed Spherical Complex Optical Potential (SCOP) formalism to calculate total elastic cross section Qel, and total inelastic cross section Qinel and used Complex Scattering Potential – the ionisation contribution (CSP-ic) method to extract the ionisation cross sections, Qion, from the calculated Qinel. The calculated total cross sections are examined as functions of incident electron energy and are compared with available data wherever possible and overall good agreement is observed. In this work Qel, Qion, and ∑Qexc are reported for the first time for CH3SH in this energy range.

The molecular beam apparatus used for the measurements of total collision cross sections of potassium and rare gases is shown in detail. A new type of the scattering chamber with a variable length is used to eliminate the error of the effective scattering length. To measure the pressure of the rare gases in the scattering chamber an ionization gauge is connected directly to the scattering chamber. The sensitivities of the ionization gauge for the rare gases are calibrated by a McLeod gauge. To eliminate the mercury drag effect in the calibration, the cut-off position of the McLeod gauge is cooled with dry ice. The magnitudes of the effect are in good agreement with those calculated by Takaishi's theory. Effective total cross sections for the scattering of potassium by He, Ne, Ar, Kr, and Xe have been measured in the velocity range, 400-1200 m/sec, of potassium beam. Corrections for the angular resolution are calculated by a general method derived by the present authors. The correction values are compared with those calculated with Busch's formula. To estimate true total cross sections, the effective total cross sections are corrected for the thermal motion of the scattering gases by means of the method of Berkling et al. The velocity dependences of the total cross sections in the potassium-argon, potassium-krypton, and potassium-xenon systems are shown to be explained by the Van der Waals potential, —C/γ5, where s=6. Glory undulations are observed for the potassium-xenon system. For neon the velocity dependence can not be explained by s=6. For helium the correction factors for the thermal motion of the scattering gas are large and the value of s can not be determined unambiguously. To estimate the Van der Waals constants from the total cross sections, the SLL approximation formula is used and the values of 231, 337, and 515×10-60erg·cm6 are obtained for CK-Ar, CK-Kr, and CK-Xe, respectively. These values are in good agreement with the theoretical ones of Dalgarno and Davison. For the potassium-xenon system the potential parameters of L-J(12, 6)are estimated from the glory undulations, giving 2.3×10-14 erg and 4.7Aforeandγm, respectively.

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.

We report electron impact total cross sections, Q(T), for e-N(2)O scattering over an extensive range of impact energies approximately from 0.1 eV to 2000 eV. We employ an ab initio calculation using R-matrix formalism below the ionization threshold of the target and above it we use the well established spherical complex optical potential to compute the cross sections. Total cross section is obtained as a sum of total elastic and total electronic excitation cross sections below the ionization threshold and above the ionization threshold as a sum of total elastic and total inelastic cross sections. Ample cross section data for e-N(2)O scattering are available at low impact energies and hence meaningful comparisons are made. Good agreement is observed with the available theoretical as well as experimental results over the entire energy range studied here.

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).