We report a new measurement of the positronium (Ps) 2 3 S 1 → 2 3 P 0 interval. Slow Ps atoms, optically excited to the radiatively metastable 2 3 S 1 level, flew through a microwave radiation field tuned to drive the transition to the short-lived 2 3 P 0 level, which was detected via the time spectrum of subsequent ground state Ps annihilation radiation. After accounting for Zeeman shifts we obtain a transition frequency ν 0 ¼ 18501.02 AE 0.61 MHz, which is not in agreement with the theoretical value of ν 0 ¼ 18498.25 AE 0.08 MHz.

We present a range of cross section measurements for the low-energy scattering of positrons from pyridine, for incident positron energies of less than 20 eV, as well as the independent atom model with the screening corrected additivity rule including interference effects calculation, of positron scattering from pyridine, with dipole rotational excitations accounted for using the Born approximation. Comparisons are made between the experimental measurements and theoretical calculations. For the positronium formation cross section, we also compare with results from a recent empirical model. In general, quite good agreement is seen between the calculations and measurements although some discrepancies remain which may require further investigation. It is hoped that the present study will stimulate development of ab initio level theoretical methods to be applied to this important scattering system.

We report new measurements of the positronium (Ps) 2 3 S1 → 2 3 PJ fine-structure intervals, ν J (J = 0, 1, 2). In the experiments, Ps atoms, optically excited to the radiatively metastable 2 3 S1 level, flew through microwave radiation fields tuned to drive transitions to the short-lived 2 3 PJ levels, which were detected via the time spectrum of subsequent ground-state Ps annihilation radiation. Both the ν 1 and ν 2 line shapes were found to be asymmetric, which, in the absence of a complete line-shape model, prevents accurate determination of these fine-structure intervals. Conversely, the ν 0 line shape did not exhibit any significant asymmetry; the observed interval, however, was found to disagree with QED theory by 4.2 standard deviations.

The direct single-ionization cross section for Ar by positron impact has been measured in the region above the first ionization threshold. These measurements are compared to semiclassical calculations which give rise to a power law variation of the cross section in the threshold region. The experimental results appear to be in disagreement with extensions to the Wannier theory applied to positron impact ionization, with a smaller exponent than that calculated by most previous works. In fact, in this work, we see no difference in threshold behavior between the positron and electron cases. Possible reasons for this discrepancy are discussed.

Experimental determinations of the absolute differential positronium-formation cross sections near 0 • for Ne, Ar, Kr, and Xe are presented and compared with theory. The degree of forward collimation, expressed by the ratios of the differential-to-integral positronium-formation cross sections, is also computed and compared with theories and other targets. Trends among targets and structures at low energies emerge when considered as a function of the reduced total energy.