title: Excited-state positronium formation from helium, argon, and xenon.
abstract: The cross sections for the formation of positronium in the 2P state in collisions of positrons with He, Ar, and Xe atoms have been determined by measuring coincidences between the remnant ion and the Lyman-alpha photon from positronium. The maximum fractional contributions of these to the total Ps formation cross sections increase from approximately 0.06+/-0.01 in He to 0.12+/-0.04 in Ar and 0.26+/-0.09 in Xe. In the case of He, good agreement is found with a coupled-state calculation; for Ar and Xe, measurements are compared with a distorted-wave Born approximation.
authors.affiliation: UCL Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom.
authors.forename: D J
authors.initials: DJ
authors.lastname: Murtagh
authors.forename: D A
authors.initials: DA
authors.lastname: Cooke
authors.forename: G
authors.initials: G
authors.lastname: Laricchia