Recent research has shown that there are a number of atoms and
atomic ions that can bind a positron. The number of atoms
known to be capable of binding a positron has expanded
enormously in recent years, with Li, He(3Se), Be, Na, Mg,
Ca, Cu, Zn, Sr, Ag and Cd all capable of binding a positron.
The structure of these systems is largely determined by the
competition between the positron and the nucleus to bind the
loosely bound valence electrons. Some systems, such as e+Li
and e+Na, can be best described as a Ps cluster orbiting a
charged Li+ or Na+ core, while others such as e+Be
consist of a positron orbiting a polarized Be atom. In
addition, a number of atoms (Li, C, O, F, Na, Cl, K, Cl, Cu,
Br) can bind positronium and a few systems capable of binding
two positrons have also been identified. These
positron-binding systems decay by electron-positron
annihilation with the annihilation rate for e+A systems
largely determined by the parent atom ionization potential.