S.J. Ward

Hidden crossing theory applied to positronium formation

Abstract The hidden crossing theory that employs the hyperspherical representation has been applied to Ps-formation in positron–hydrogen collisions in the Ore gap. S-, P- and D-wave cross-sections have been computed. For S-wave scattering, there occurs almost complete destructive interference between the two amplitudes that correspond to different paths leading to Ps-formation. However, for the D-wave there occurs almost complete constructive interference.

The effect of the polarization potential on low-energy atomic processes

Abstract We have illustrated the importance of explicitly taking into account the polarization potential in the final outgoing channel to describe the threshold behavior of positronium formation in positron–hydrogen collisions, positronium formation in positron–helium collisions and the photodetachment of lithium negative ion into e − −Li(2p).

The hidden crossing theory applied to positronium formation

We have applied the hidden crossing theory to compute S- and P-wave positronium formation cross sections for positron-hydrogen collisions in the Ore gap. This calculation, which is the first application of the hidden crossing theory to positron collisions, provides a physical explanation of why the S-wave cross section is so small.

Recent development in the theory of fast positron-atom collisions

Abstract We will review some recent high energy theories of positronium formation in positron-hydrogen collisions, and compare these theories to the recent experimental measurements. We will also present high energy theories of positron-impact ionization of hydrogen. A comparison of these theories with the two sets of experimental measurements clearly shows that positron-impact ionization of hydrogen is not completely understood. In particular, the role of positronium formation to the continuum in positron-hydrogen ionization is presently not known. Structures arising from the interference between two double-binary collision processes in the triply differential cross section of positron-impact ionization of hydrogen will be discussed.