Positron impact ionization of molecular nitrogen R.I. Campeanu a, * , V. Chis b , L. Nagy b , A.D. Stauffer a a Department of Physics and Astronomy, York University, 4700 Keele Street, Toronto, ON Canada M3J IP3 b Faculty of Physics, Babes ß-Bolyai University, str. Kog alniceanu nr. 1, 3400 Cluj, Romania Abstract We have carried out distorted wave calculations of positron ionization of molecular nitrogen in order to compare with recent experimental measurements. In this work, the nitrogen molecule was represented by a Gaussian wave function. We find that our CPE model gives the better agreement with the measurements in spite of its simplicity. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Positron impact ionization of molecules was recently studied both experimentally and theoret- ically. Experimental total ionization cross sections were measured for H 2 [1,2], N 2 [3], O 2 [4], CO [5], CO 2 [6] and for organic molecules [7]. The theoretical studies have been limited so far to molecular hydrogen. Distorted wave calcula- tions have used a one-center formalism [8] or two- center molecular wavefunctions [9,10]. The paper by Campeanu et al. [10] used a Gaussian repre- sentation of the molecule which can be employed for more complex molecules. In this paper we will use the method of [10] for molecular nitrogen. 2. Theory The triple differential cross section for the ion- ization of a homonuclear molecule by positron impact may be written as d 3 r d ^ k f d ^ k e dE e ¼ X r ð2pÞ 4 E i jf r j 2 ; ð1Þ where E i is the energy of the projectile, E e the en- ergy of the ejected electron, while ^ k e and ^ k f stand for the direction of the momenta of the ejected electron and scattered positron, respectively. The summation over r is done over all occupied molecular orbitals. The amplitude can be written as f r ¼h/ f ðr 1 Þ/ e ðr 2 ÞjV ðr 12 Þj/ i ðr 1 Þ/ r ðr 2 Þi; ð2Þ where / i and / f stand for the wavefunction of the incident and scattered positron, respectively, / e is the wavefunction of the ejected electron, while / r describes the initial state (orbital) of the active electron. In order for Eq. (1) to be valid, the ejected electron wave function must be orthogo- nalized to the target wave function. In the above amplitude r 1 is the position vector of the positron, while r 2 stands for the position vectors of the ac- tive electron. We are assuming in this model that the electron orbitals in the residual molecular ion are the same as in the target ion during the time of the collision. * Corresponding author. E-mail address: campeanu@yorku.ca (R.I. Campeanu). 0168-583X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.03.025 Nuclear Instruments and Methods in Physics Research B 221 (2004) 21–23 www.elsevier.com/locate/nimb