This paper is a review of low-energy positron interactions with atoms and molecules. Processes of interest include elastic scattering, electronic and vibrational excitation, ionization, positronium formation and annihilation. An overview is presented of the currently available theoretical and experimental techniques to study these phenomena, including the use of trap-based positron beam sources to study collision processes with improved energy resolution. State-resolved measurements of electronic and vibrational excitation cross sections and measurement of annihilation rates in atoms and molecules as a function of incident positron energy are discussed. Where data are available, comparisons are made with analogous electron scattering cross sections. Resonance phenomena, common in electron scattering, appear to be less common in positron scattering. Possible exceptions include the sharp onsets of positron-impact electronic and vibrational excitation of selected molecules. Recent energy-resolved studies of positron annihilation in hydrocarbons containing more than a few carbon atoms provide direct evidence that vibrational Feshbach resonances underpin the anomalously large annihilation rates observed for many polyatomic species. We discuss open questions regarding this process in larger molecules, as well as positron annihilation in smaller molecules where the theoretical picture is less clear.

/pdf/low-energy-positron-interactions-with-atoms-and-molecules-1wveosw733.pdf

Low-energy positron interactions with atoms and molecules

Ionization of atoms by particle and antiparticle impact

We reexamine in detail the various processes undergone by positrons in the interstellar medium (ISM) from their birth to their annihilation using the most recent results of positron interaction cross sections with atomic and molecular hydrogen, as well as helium. The positrons' lives are divided into two phases: the "in-flight" phase (between ≈1 MeV and tens of eV) and the thermal phase. The first phase is treated with a Monte Carlo simulation that allows us to determine the fraction of positrons that form positronium and annihilate as well as the characteristics of the annihilation emission as a function of the medium conditions. The second phase is treated with a binary reaction rate approach, with cross sections adopted from experimental measurement or theoretical calculations. An extensive search and update of the knowledge of positron processes was thus undertaken. New reaction rates and line widths have been obtained. We investigate the treatment of the complicated interactions between positrons and interstellar dust grains. Fully relevant data were not always available, but we were nonetheless able to reach satisfactory understanding of positron annihilation on grains, both qualitatively and quantitatively. All factors of the problem have been considered, including the grain size distribution and composition, the electric charge of the grains, the backscattering, positronium formation and ejection from the grain, the pick-off annihilation inside them and the partial destruction of dust in the hot regions of the ISM. New reaction rates and widths of the line resulting from the annihilation inside and outside of the grain have been obtained. The final results of our calculations (reaction rates and spectra) showed that dust is only important in the hot phase of the ISM, where it dominates all other processes. Combining the new calculations, we have constructed annihilation spectra for each phase of the ISM, considering various grain contents, as well as an overall combined spectrum for the ISM as a whole.

/pdf/the-lives-and-deaths-of-positrons-in-the-interstellar-medium-o90ntzgbwx.pdf

The lives and deaths of positrons in the interstellar medium

Ionization cross sections for positrons impacting on atomic hydrogen have been measured for kinetic energies in the range 15-700 eV This has been done in a crossed-beam geometry where a magnetically guided positron beam intersects a hydrogen gas jet emanating from a radio frequency discharge tube Electron impact ionization cross sections were also measured with the same apparatus thus facilitating comparison with, and normalization to, published results The positron-atomic hydrogen results are found to be significantly lower than those obtained by Spicher et al (1990)

Positron impact ionization of atomic hydrogen

We reexamine in detail the various processes undergone by positrons in the ISM from their birth to their annihilation using the most recent results of positron interaction cross sections with H, H2 and He. The positrons' lives are divided into two phases: the 'in-flight' phase and the thermal phase. The first phase is treated with a Monte Carlo simulation that allows us to determine the fraction of positrons that form positronium and annihilate as well as the characteristics of the annihilation emission as a function of the medium conditions. The second phase is treated with a binary reaction rate approach, with cross sections adopted from experimental measurement or theoretical calculations. An extensive search and update of the knowledge of positron processes was thus undertaken. New reaction rates and line widths have been obtained. We investigate the treatment of the complicated interactions between positrons and interstellar dust grains. New reaction rates and widths of the line resulting from the annihilation inside and outside of the grain have been obtained. The final results of our calculations showed that dust is only important in the hot phase of the ISM, where it dominates all other processes. Combining the new calculations, we have constructed annihilation spectra for each phase of the ISM, considering various grain contents, as well as an overall combined spectrum for the ISM as a whole.

/pdf/the-lives-and-deaths-of-positrons-in-the-interstellar-medium-5f2y1hrbtz.pdf

In a crossed-beam experiment the angle-integrated ionization cross section for both positron and electron impact on atomic hydrogen (excluding positronium formation) has been measured in the energy region of 17.5 to 600 eV. Below 450 eV the positron cross section is larger than the electron one, at 50 eV by about a factor of 2. Absolute cross sections are obtained by comparing the electron data with literature values. At energies above 30 eV all theoretical predictions lie below our measured cross sections for positron-impact ionization.

Ionization of atomic hydrogen by positron impact.

In a crossed‐beam experiment the impact‐ionization cross section, integrated over all angles and energy partitions but excluding positronium formation, has been measured for positrons as well as electrons in the energy region of 30 to 600 eV. Below 400 eV the positron cross section is larger than the electron one, at 40 eV by about a factor of 2. Absolute cross sections are obtained by comparing the electron data with literature values. Our cross sections for positron‐impact ionization lie above all theoretical predictions; within the errors they barely agree with the distorted‐wave polarized‐orbital calculations of Ghosh et al. for the model in which the scattered positron is described by a plane wave.

Positron-impact ionization of atomic hydrogen

We have built a crossed beam apparatus to measure the relative ionisation cross sections of atomic hydrogen by positron impact. A schematic diagram is given in Fig. 1.

/pdf/ionisation-of-atomic-hydrogen-by-positron-impact-v1s9l9g0ag.pdf

Björn Olsson

Papers

Ionization of atomic hydrogen by positron impact.

Positron-impact ionization of atomic hydrogen

Ionisation of Atomic Hydrogen by Positron Impact