Low-energy positron scattering from atoms and molecules using positron accumulation techniques

Positron scattering from atoms and molecules is studied at low values of in cident positron energy. The experiments use a cold magnetized positron beam formed in a positron accumulator. A discussion of positron scattering in a magnetic field is presented along with differential cross-sections (DCS) f or positron-atom collisions and positron-molecule total vibrational excitat ion cross-sections. Absolute values of the DCS for elastic scattering from argon and krypton are measured at energies ranging from 0.4 to 2.0 eV. The first low-energy positron-molecule vibrational excitation cross-sections ha ve been measured (i.e., for carbon tetrafluoride at energies ranging from 0 .2 to 1 eV). Using information gained from these experiments a second gener ation scattering apparatus is described, which was designed and built speci fically for scattering experiments using a magnetized positron beam. This a pparatus has a number of improvements, including an order of magnitude high er throughput, better energy separation between elastic and inelastic scatt ering events, and improved measurement of the absolute pressure of the test -gases. Analysis techniques for the data taken in these experiments using a retarding potential energy analyzer and possible extensions of these exper iments are also discussed. (C) 2000 Elsevier Science B.V. All rights reserv ed.