D. Przybyla

POSITRON-MOLECULE SCATTERING EXPERIMENTS

Abstract We have extended our measurements of the relative elastic differential cross sections (DCSs) and positronium formation cross sections (QPss) for positron scattering to include molecules of varying complexity and symmetry. Diffraction effects observed in our earlier e+–Ar and e+–CH4 DCS measurements [Phys. Rev. A 55 (1997) 4244] are shown to have disappeared for a linear tri-atomic molecule, CO2. In contrast to the structureless, bell-shaped Ps-formation curves of H and H2 [Phys. Rev. A 55 (1997) 361], the Ps-formation cross section curves for CH4, CO2, and SF6 show structure which may be associated with electron transfer of inner orbital electrons of the molecule.

Positron differential quasielastic scattering by molecules

Abstract An overview of relative differential cross section (DCS) measurements in the range 30–135° for quasielastic scattering (elastic scattering plus rotational and vibrational excitations) of 4–200 eV positrons by N 2 , O 2 , CO, CO 2 , N 2 O and CH 4 , along with recent measurements for C 2 H 2 and S 6 is reported. For N 2 , O 2 , CO and CH 4 well-defined structure is found in the DCSs below the positronium (Ps) formation thresholds, which becomes washed out as the energy is increased above the Ps formation threshold. N 2 O and CO 2 show the least evidence of structure below (and above) the Ps threshold, while SF 6 exhibits noticeable structure not only below, but also above the Ps threshold. These observations indicate that the Ps formation channel may have an effect on the elastic scattering channel, as does the degree of symmetry of the target system, which is consistent with earlier elastic DCS measurements for the inert gas atoms.

Measurements of positron–molecule differential scattering cross-sections

Abstract Relative differential cross-sections (DCSs) have been measured for the quasi-elastic scattering (elastic scattering plus rotational and vibrational excitations) of positrons from CH 4 , N 2 , O 2 , CO, CO 2 , N 2 O, C 2 H 2 , and SF 6 molecules in a crossed-beam experiment. The energy range extends from below the positronium (Ps) formation threshold of each molecule to well above the Ps formation threshold and the angular range extends from 30° to 135°. For CH 4 , N 2 , CO, O 2 , and SF 6 the DCS exhibits structure (a clear minimum and maximum) at low energies that tends to wash out as the energy is increased above the Ps formation threshold. In contrast, for N 2 O, CO 2 , and C 2 H 2 we find no pronounced structure below the Ps formation threshold. These comparisons suggest that the degree of spherical symmetry of the target may be correlated with the presence of the structure.

Evidence for resonance and absorption effects in positron-atom differential elastic scattering

This review focuses on recent positron-atom elastic differential cross section (DCS) measurements which reveal evidence that interference effects may be occurring between the elastic and inelastic scattering channels. These effects appear as (1) absorption effects which are observable in relative elastic DCSs by the “washing out” of structure in the DCS versus angle as the positron energy is increased above the positronium formation threshold, and (2) a new type of resonance-like structures at intermediate energies (well above the normal inelastic thresholds) that have been observed in absolute elastic DCS measurements when plotted at a fixed scattering angle versus energy. A discussion is provided of possibly relevant information on positron-atom scattering that also indicates that coupling is occurring between various scattering channels.

Search for resonance‐like structure in the total scattering of positrons by argon and krypton

Total scattering cross sections (QT’s) have been measured for positrons scattered by argon and krypton using a beam transmission technique. We are specifically searching for resonance‐like structure in the QT’s.

Measurements of total cross sections for positron-K and-Rb scattering

A modification of the apparatus that we have been using to measure absolute total cross sections (QT′s) for positron and electron scattering by alkali metal atoms has enabled us to more easily obtainQT′s at low energies (<10 eV). Our positron-K and-RbQT measurements now extend from 1 to 98 eV and we have made the correspondingQT measurements for electrons in the same system using the same technique (a beam transmission technique). Our measured positron-K and-RbQT values show a peak in the vicinity of 6 eV and decrease substantially as the positron energy is reduced below 6 eV, in sharp contrast to the results obtained in recent 5-state close-coupling approximation calculations. At higher energies (above 10 eV), we have found that the same calculations are in quite good agreement with our measured positron-K and-RbQT′s. Our measurements of the correspondingQT′s for electrons below 10 eV do not show a peak, but rather continue to increase as the energy is decreased.