Studies of Low-energy Positron-molecule Interactions using Trap-based Beams

Clifford M. Surko

L. D. Barnes

L. D. Barnes, Joan P. Marler, James Patrick Sullivan +1 more

An overview is presented of recent studies of the interaction of low-energy positrons with atoms and molecules using a trap-based positron beam. The beam is tunable from ~100 meV to many electron volts, with a typical energy resolution of 25 meV (FWHM). A variety of scattering processes are investigated using the properties of positron orbits in a spatially varying magnetic field. Recent scattering measurements are reviewed, and new data for electronic excitation, ionization, and positronium formation in noble gases are presented. In a separate set of experiments, positron annihilation below the threshold for positronium formation is studied in molecules as a function of positron energy. The data show large Feshbach resonances in the range of energies of the molecular vibrations. Within the framework of this model, these results provide the first experimental evidence that positrons bind to hydrocarbon molecules.

Clifford M. Surko

This is a personal account of the development of our buffer-gas positron trap and the new generation of cold beams that these traps enabled. Dick Drachman provided much appreciated advice to us from the time we started the project. The physics underlying trap operation is related to resonances (or apparent resonances) in positron-molecule interactions. Amusingly, experiments enabled by the trap allowed us to understand these processes. The positron-resonance "box score" to date is one resounding "yes," namely vibrational Feshbach resonances in positron annihilation on hydrocarbons; a "probably" for positron-impact electronic excitation of CO and NZ;an d a "maybe" for vibrational excitation of selected molecules. Two of these processes enabled the efficient operation of the trap, and one almost killed it in infancy. We conclude with a brief overview of further applications of the trapping technology discussed here, such as "massive" positron storage and beams with meV energy resolution.

Allen Paine Mills

Jonathan Peter Merrison

The low intensity and poor resolution of conventional slow positron beams is limiting the study of atomic and molecular collisions and prohibits more ambitious fundamental studies. The application of trapping promises huge enhancement in instantaneous beam intensity; ideal for studying short lived positron bound states. Cooling of trapped positrons gives vast improvement in energy and spatial resolution and gives access to a new low energy regime. This new technology is proving to be both challenging and exciting; the relative merits of several different trapping schemes will be discussed and recent progress presented.