Positron Scattering and Annihilation in Organic Molecules

A. Karbowski, G. Karwasz, M. Franz , J. Franz ACTA PHYSICA POLONICA B – 2020
In this paper, we address the problem of connecting positron lifetimes in liquids with collision cross sections in gases. We present the analyses of annihilation lifetime spectra of positrons in the liquid benzene, c-hexane, n-hexane, methanol and ethanol and calculations of scattering cross sections of positrons with benzene and c-hexane in the gas phase.

Rotational state-changing collisions of C2H− and C2N− anions with He under interstellar and cold ion trap conditions: A computational comparison

J. Franz , B. Mant, L. González-Sánchez, R. Wester, F. Gianturco – JOURNAL OF CHEMICAL PHYSICS – 2020
We present an extensive range of quantum calculations for the state-changing rotational dynamics involving two simple molecular anions that are expected to play some role in the evolutionary analysis of chemical networks in the interstellar environments, C2H− (X1Σ+) and C2N− (X3Σ−), but for which inelastic rates are only known for C2H−. The same systems are also of direct interest in modeling selective photo-detachment experiments in cold ion traps where the He atoms function as the chief buffer gas at the low trap temperatures. This study employs accurate, ab initio calculations of the interaction potential energy surfaces for these anions, treated as rigid rotors, and the He atom to obtain a wide range of state-changing quantum cross sections and rates at temperatures up to about 100 K. The results are analyzed and compared for the two systems to show differences and similarities between their rates of state-changing dynamics.

Investigation of rotational state-changing collisions of CCN$^{-}$ ions with helium

J. Franz , F. Gianturco – 2019
Molecular anions which contain carbon chains, like C$_{4}$N$^{-}$ and C$_{6}$N$^{-}$, have already been found in the interstellar medium [1]. There is still no evidence for the existence of C$_{2}$N$^{-}$ in interstellar medium. However, it is well known, that the molecule C$_{2}$N can form stable anions [2]. Because of their large dipole moments, these species have large rotational cross sections for rotational inelastic collisions with atomic species, like Helium [3]. Rotational inelastic collisions can accelerate the cooling of atomic gases by converting collisional energy into rotational energy, which can be emitted by radiation [3]. We are presenting results from quantum scattering calculations for the rotational inelastic collision process C$_{2}$N$^{-}$(J) + He \rightarrow C$_{2}$N$^{-}$(J) + He . From our results we try to give some estimates for the cooling efficiency of the species C$_{2}$N$^{-}$, C$_{4}$N$^{-}$, C$_{6}$N$^{-}$. References [1] Millar T. J., Walsh C. , Field T. A. (2017). Chem. Rev. 117, 1765-1795. [2] Garand E, Yacovitch, T. I., Neumark, D. M. (2009). J. Chem. Phys. 130, 064304. [3] Hauser D., Lee S., Carelli F. et al (2015), Nature Physics 11, 467-470.

Low-energy positron scattering from gas-phase benzene

In this paper we are presenting calculations of the elastic cross section of positrons with gas-phase benzene for the energy range from 0.25 eV to 9.0 eV. The calculations are done with the molecular R-matrix method for positron-scattering from poly-atomic molecules using a scaling factor to scale the electron-positron interaction. The scaling factor influences the position of the poles of the R-matrix. We adjust the scaling factor is such a way, that the position of the lowest pole is similar to the proposed binding energy of around 150 meV, given by [J.A. Young, C.M. Surko, Phys. Rev. Lett 99, 133201 (2007)]. Below the threshold for positronium formation, we calculate elastic cross section in good agreement with experimental cross sections. Above the threshold for positronium formation we use the difference between the experimental total cross section data and our computed elastic cross section to provide a first estimation of the cross section for positronium formation.

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.

The POCOBIO Database for Computed Scattering Cross-Sections for Positron Collisions with Biomolecular Systems

The design of a database for positron interactions with biomolecular systems is outlined. The database contains only scattering cross sections, which are derived from theory. The data model is defined in a very flexible way, which facilitates the usage of weakly bound clusters of molecules and molecular systems with many tautomeric forms.

Do positrons measure atomic and molecular diameters?

J. Franz , K. Fedus, G. Karwasz – EUROPEAN PHYSICAL JOURNAL D – 2016
We report on density functional calculations (DFT) of elastic integral scattering cross-sections for positron collisions with argon, krypton, nitrogen and methane. The long-range asymptotic polarization potential is described using higher-order terms going much beyond an induced dipole potential (−α / r 4) while the short-range interaction is modeled by two different forms of electron – positron correlation potential (Boroński-Nieminen and Quantum Monte Carlo potentials). The results of both approaches agree quite well with the recent theoretical and measured values. Based on the present and previous theoretical and experimental data we discuss some systematics observed in integral scattering cross-sections below the positronium formation threshold. In particular we point out on the correlation between the values of scattering cross-sections and atomic dimensions.

Interactions of positrons with atoms and molecules

J. Franz – 2016
The positron is the antiparticle of the electron. It has the same mass as the electron, but opposite charge. The understanding of the interactions of positrons with normal matter, like atoms and molecules, is of interest in various scientific fields, like nuclear medicine, plasma physics and astronomy. In this talk we will give a short introduction to some theoretical methods to describe the interactions of positrons with atoms on molecules. The remainder of the talks will deal with the calculation of cross sections for collisions of positrons with atomic and molecular gases and the comparison with experimental data.

A dipole-driven path for electron and positron attachments to gas-phase uracil and pyrimidine molecules: a quantum scattering analysis

F. Carelli, F. Gianturco, J. Franz , M. Satta – EUROPEAN PHYSICAL JOURNAL D – 2015
Electron and positron scattering processes in the gas-phase are analysed for uracil and pyrimidine molecules using a multichannel quantum approach at energies close to threshold. The special effects on the scattering dynamics induced by the large dipole moments in both molecules on the spatial features of the continuum leptonic wavefunctions are here linked to the possible bound states of the Rydberg-like molecular anions or ‘positroned’ molecules which could be reached via further couplings with molecular internal degrees of freedom.

Dipole-driven dynamics for near-threshold electron/positron interactions with pyrimidinic DNA bases: a path to compound formations

F. Carelli, J. Franz , F. Gianturco – MOLECULAR PHYSICS – 2015
Calculations are reported for electron and positron scattering from isolated cytosine and thymine, where the two pyrimidinic single-ring DNA bases, in the gas-phase at energies near the elastic threshold, they reveal the special features of the dipole-driven scattering states. All molecules examined exhibit, in fact, supercritical (>1.67 D) permanent dipoles which can therefore also support, below threshold, excited bound compound states (M·e− and M·e+) classified as dipole-bound states. The possible paths for going from above to below the energy threshold to form whole, unfragmented bound complexes are discussed for both leptonic projectiles.