Low-energy Positron Collisions Research Articles

Low-energy positron collisions with CH2O⋯ H2O complexes

We present calculated elastic integral cross section (ICS) for low-energy positron scattering by formaldehyde-water complexes (hydrogen-bonded pairs CH2O⋯ H2O). We considered four different structures for the complex obtained by classical Monte Carlo simulations in water environment at room temperature. We also present results for formaldehyde in gas phase. The calculations used the Schwinger multichannel method and were carried out in the static-polarization level of approximation, for energies from 0.5 eV to 10 eV. We also employed the Born-closure procedure to account for the long range potential due to the permanent dipole moment of the complexes and the formaldehyde molecule. We compared our results obtained in gas and liquid phases to investigate the effect of microsolvation in the ICS. We also compared our results for the formaldehyde molecule with the results available in the literature.

Positron-electron correlation-polarization potentials for the calculation of positron collisions with atoms and molecules*

We present correlation-polarization potentials for the calculation of scattering cross sections of positrons with atoms and molecules. The potentials are constructed from a short-range correlation term and a long-range polarization term. For the short-range correlation term we present four different potentials that are derived from multi-component density functionals. For the long-range polarization term we employ a multi-term expansion. Quantum scattering calculations are presented for low energy collisions of positrons with two atomic targets (argon and krypton) and two molecular targets (nitrogen and methane). For collision energies below the threshold for Positronium formation our calculations of scattering cross sections are in good agreement with recent data sets from experiments and theory.Graphical abstract

Positron collisions with acetylene calculated using the R-matrix with pseudo-states method

Eigenphase sums, total cross sections and differential cross sections are calculated for low-energy collisions of positrons with C2H2. The calculations demonstrate that the use of appropriate pseudo-state expansions very significantly improves the representation of this process giving both realistic eigenphases and cross sections. Differential cross sections are strongly forward peaked in agreement with the measurements. These calculations are computationally very demanding; even with improved procedures for matrix diagonalization, fully converged calculations are too expensive with current computer resources. Nonetheless, the calculations show clear evidence for the formation of a virtual state but no indication that acetylene actually binds a positron at its equilibrium geometry.

Low-energy positron collisions with water: elastic and rotationally inelastic scattering

Differential, integral and momentum transfer cross sections for the vibrationally elastic and rotationally inelastic scattering of positrons from water at low collision energy (E ⩽ 10 eV) are reported. Several models within the R-matrix method are used to compute the body-fixed T-matrices, while the scattering calculations are performed within the fixed-nuclei approximation corrected with the standard Born-closure formula. These calculations are compared with experimental results for elastic scattering, and the best model gives reasonable agreement with the most recent measurements (Zecca et al 2006 J. Phys. B: At. Mol. Opt. Phys. 39 1597) but not with some earlier studies. The relative contribution of the rotationally inelastic processes is investigated and comparisons are made with the equivalent electron scattering cross sections.

Metastable trapping of low-energy positrons by cubane: A computational experiment

Quantum-mechanical calculations of low-energy collisions of positrons with cubane $({\mathrm{C}}_{8}{\mathrm{H}}_{8})$ molecules are carried out using a model interaction potential and solving the corresponding scattering equations. The possible presence of low-energy resonances is analyzed in detail and one of them is found to reside largely inside the carbon frame of the cubane. The consequences of this finding are discussed and analyzed.

Elastic collisions and rotational excitation in positron scattering fromCO2molecules

Full quantum calculations are carried out for the elastic (rotationally summed) integral and differential cross sections in low-energy collisions of positrons with rigid rotor ${\mathrm{CO}}_{2}$ molecules. The interaction includes an expansion of the exact Coulomb potential on a single-center (SCE) and a parameter-free correlation-polarization (${\mathrm{V}}_{\mathrm{CP}}$) potential given in local form via density-functional theory (DFT) with gradient corrections. State-to-state rotationally inelastic cross sections (integral and differential) are also presented and their behavior is discussed in relation to what is known from similar experiments with electrons as projectiles. The agreement found between computed and measured elastic integral cross sections is rather good and confirms the realistic quality of a DFT treatment for short-range dynamical correlation effects.

Positron-lithium scattering with the inclusion of positronium formation

Detailed studies have been made of elastic scattering and positronium formation in low energy collisions of positrons with lithium atoms for the two partial wavesl=0,1. For this system, as for all alkali atoms, the positronium formation channel is open even at zero positron energy. A two-channel version of the Kohn variational method is used with trial functions containing many variational parameters, and reasonably well converged results are obtained. The s-wave positronium formation cross section is infinite at zero positron energy but it then falls rapidly to become several orders of magnitude smaller than the elastic scattering cross section which has a maximum value of approximately 100 πα 0 2 at a positron energy of 0.5 eV. For p-wave scattering the positronium formation cross section rises to a value of approximately 10 πα 0 2 at an energy of 0.1 eV, with the elastic scattering cross section rising to a maximum of approximately 60 πα 0 2 just below the first excitation threshold at 1.84 eV.

Low-energy positron collisions with CH4 and SiH4 molecules by using new positron polarization potentials

The collision of low-energy (below positronium formation threshold) positrons with methane (CH4) and silane (SiH4) molecules is investigated by employing two types of parameter-free model polarization potentials within the density functional description of the target correlation. The first potential is based on the correlation energy, in corr, of one positron in a homogeneous electron gas of the target, while the second one is a modified version of the electron correlation potential in which the exchange interaction is absent. In the asymptotic region, both types of polarization potentials have the correct analytic form (- alpha 0/2r4, where alpha 0 is the polarizability of the target). Results for the total and differential cross sections are compared with recent measurements. The authors found that the first polarization potential mentioned above, is better than the second one.

Elastic scattering of low-energy positrons by bound silicon and germanium

Elastic scattering differential and total cross sections for low-energy positron collisions with bound silicon and germanium atoms have been computed using partial wave analysis with static and polarization potentials. The calculations are performed at incident positron energies ranging from 0.2 to 500 eV. For each impact energy, an appropriate number of phase shifts are obtained by numerically integrating the radial wave equation to ensure convergence of the scattering amplitude. A simple parametrization of the total cross section in terms of the screened Rutherford cross section is also presented for use in Monte Carlo codes. Due, in part, to the lack of reliable experimental data, relatively little theoretical work has been done to calculate the inelastic and elastic differential cross sections (DCS) and total cross sections (TCS) for positron scattering. Most of the measurements were made on noble gases where it is observed that low-energy positrons exhibit a Ramsauer–Townsend (RT) minimum for helium, neon, and possibly argon (for electrons, the RT minimum occurs for heavier noble gases).1,2 Calculations of the positron TCS for these atoms agree with the observations. The RT effect has also been identified in a number of other free atoms and molecules for electrons.3,4 Recently, Meredith et al. computed the electron elastic scattering cross sections in solid silicon and germanium.5 In this work we calculate the DCS and TCS for positrons elastically scattered by silicon and germanium atoms which are bound in an amorphous solid using partial waves and the optical model. The results are presented for incident positron energies ranging from 0.2 to 500 eV. Simple fits to each TCS are given for use in Monte Carlo scattering simulations. For the static potential VS, Salvat and Parellada used Dirac–Hartree–Fock–Slater (DHFS) calculations to accurately fit analytic screening functions for atoms.6 They also imposed Wigner–Seitz boundary conditions (WSBC) on their wave functions to simulate atoms bound in a solid. The functional form of the static potential is (in atomic units)

The physics of low-energy positron collisions

The production of low-energy positron beams is described and some motivation for their use in atomic physics scattering experiments is discussed. Selected topics of current interest relating to positron collisions involving positronium formation and ionisation are highlighted.

Elastic Scattering and Rotational Excitation of Molecular Hydrogen by Low Energy Positrons

The adiabatic nuclei approximation is used to study low energy positron collisions with hydrogen molecules. The interaction of the positrons with the hydrogen molecules is represented as a superposition of electrostatic and polarization potentials. Total and differential elastic and momentum transfer cross sections are calculated. The effect of small variations in the potentials, and the convergence of the partial wave expansion are discussed. The annihilation rate of the positrons is calculated with and without incorporating the effect of distortion of the electron cloud by the incoming positrons. Rotational excitation cross sections, both differential and total, are presented. The results are compared, where possible, to recent similar two-center calculations of Hara, and experimental results of Coleman et al.