Experimental studies of the interaction of low energy positrons with atoms and molecules

Abstract

This paper presents recent data on positron annihilation below and above the threshold for positronium formation in atoms and molecules. Use of a trap-based beam allows experimentation down to 50 meV and up to tens of electron volts with an energy resolution of 25 meV (FWHM). Above positronium formation, results are presented for absolute cross-section measurements in argon. Below positronium formation, absolute Zeff spectrum in argon, xenon and the alkane molecules are presented. Data are compared to other experimental and theoretical results where available. The effect of fluorination of hydrocarbons on Zeff below the threshold for Ps formation as well as other outstanding questions are discussed.