INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS B: ATOMIC, MOLECULAR AND OPTICAL PHYSICS J. Phys. B: At. Mol. Opt. Phys. 34 (2001) 1–13 PII: S0953-4075(01)21572-7 Positronium formation in positron–simple metal clusters collisions B/121572/P PROC Digital by Design Printed: 28/9/2001 CRC data Issue no: Total pages: First page: Last page: File name: b1 .tex Date required: O A Foj ´ on 1 , R D Rivarola 1 , J Hanssen 2 and P A Hervieux 2 1 Instituto de F´ ısica Rosario, CONICET-UNR and Escuela de Ciencias Exactas y Naturales, Facultad de Ciencias Exactas, Ingenier´ ıa y Agrimensura, Av. Pellegrini 250, 2000 Rosario, Argentina 2 Laboratoire de Physique Mol´ eculaire et des Collisions, Institut de Physique Rue Arago, Technopˆ ole 2000, 57078 Metz Cedex 3, France Received 31 January 2001, in final form 24 July 2001 Published Online at stacks.iop.org/JPhysB/34 Abstract Positronium formation through electron capture from ground-state closed-shell sodium clusters is studied theoretically at intermediate impact energies. The charge transfer process is described in the independent electron model by a continuum distorted-wave approximation. In this approximation, distortions in the final state are introduced by using two Coulomb wavefunctions associated with the positron- and electron-residual target interactions. The cluster is described within the framework of the spherical background jellium model and the Kohn–Sham formalism with a local-density approximation that includes exchange, correlation, and a self-interaction correction. Using a partial-wave technique, differential and total cross sections for the isoelectronic neutral and charged closed-shell sodium clusters Na 20 , Na - 19 and Na + 21 are computed and compared with the predictions of the Coulomb–Born type approximation which neglects the distortions in the final state. 1. Introduction Collisions of positrons impacting on atoms (ions) have been the subject of active research over the last few years, partly motivated by the measurements obtained with the presently available high-intensity positron beams. There are interesting reviews on the issue (see, for instance, [1, 2]). A continuum distorted-wave final-state (CDW-FS) approximation has been developed to study positron–hydrogenic atom (ion) [3–5] and positron–alkali atom collisions [6]. A four-body CDW-FS (CDW-FS-4b) model has been developed recently to study the collision positron–alkali-earth metal [7] and positron–metastable helium [8]. In CDW-FS, the final state of the collision is distorted by two Coulomb wavefunctions associated with the positron- and electron residual target interactions. In this way, higher-order terms are included to describe the collision process. 0953-4075/01/000001+13$30.00 © 2001 IOP Publishing Ltd Printed in the UK 1