Nuclear Instruments and Methods in Physics Research B9 (1985) 397-399 North-Holland, Amsterdam 397 zyxwvutsr ABSOLUTE CROSS SECTIONS FOR MULTI-ELECTRON PROCESSES IN LOW ENERGY A1.4 +-Ar COLLISIONS: COMPARISON WITH THEORY A. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA BAtiNY ‘), G. ASTNER”, H. CEDERQUIST *), H. DANARED *), S. HULDT ‘), P. HVELPLUND 4), A. JOHNSON ‘), H. KNUDSEN 4), L. LILJEBY *) and K.-G. RENSFELT *) Ii Institute oj Theoretical Physics, Unioersity of Uppsala, S-752 38 Uppsala, Sweden ‘I Research Institute of Physics, S 104 05 Stockholm, Sweden ‘) Department of Physics, University of Lund, S-223 62 Lund, Sweden 4J Institute of Physics, University oj Aarhw, DK - 8000 Aarhus, Denmark Absolute cross sections have been measured for a variety of multi-electron processes in low-energy collisions of multiply charged argon recoil ions with neutral argon. The cross sections are compared with theoretical estimates based on an extension of the classical barrier model. Comparison is also made with the statistical theory of Miiller et al. 1. Introduction Multiply charged ions having low kinetic energy may be produced in laboratory and astrophysical plasmas by, e.g., electron impact ionization of photoionization. Their presence constitutes an important factor influenc- ing the non-equilibrium thermodynamic properties of the plasma [l]. For modelling and diagnostic purposes it is of some importance to know absolute cross sections of the various multi-electron processes that result when multiply charged ions collide with many-electron atoms. These collisions are characterised by the large amount of potential energy which resides in the multiply charged ion. During the collision this energy may be released and used to eject electrons or emit photons. In order to keep track of the multi-electron processes that occur, coincidence registration of the final charge states of both projectile and target is essential [2]. We have measured absolute cross sections for a large number of reactions Ar4++ X -+ Ar’q-k’++ Xtk+“)++ ne- where X = Ne, Ar, Kr. The ions were produced in a recoil ion source at the Research Institute of Physics [3], using a beam of 110 MeV C4+ as hammer, and had charge states q ranging from 1 to 10. Projectile recoil ions were accelerated to 1.8q keV and made to collide a gas target. Charge state analysis and coincidence reg- istration of projectiles and target ions were performed by time-of-flight techniques [4]. In this contribution we will compare our experimental results for Arq+-Ar (q = 4-8) with two theoretical models. Complete results together with full details of the experimental technique and data treatment will be given elsewhere. While single-electron processes have received much 0168-583X/85/$03.30 Q Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) attention during the last decade and several useful mod- els have been proposed (besides more or less sophisti- cated close-coupling calculations), the situation is differ- ent for multi-electron processes [5]. Here there is an acute need even for a model that gives only rough quantitative results. We here use an extension of the classical barrier model (for single-electron capture [6,7]) to multi-electron processes along the lines proposed in [5]. This extension gives absolute cross sections for production of target ions of given charge state. The model does not differentiate between direct electron capture and transfer ionization when more than one electron leaves the target. This is done in the statistical theories of Miiller et al. [8] and Aberg et al. [9], but these predict only the fractions of target ions in a given charge state. To produce absolute values the theories have to be normalised to absolute cross sections for capture of a given number of electrons. We will here make a comparison with the normalised fractions of ref. [8] only, but a more detailed comparison with statistical theories will be given together with the publication of the complete results. 2. Theory We first consider the classical barrier model as ap- plied to an electron being transferred from a core of charge z = 1 to an ion of charge q having a quasicon- tinuum of unoccupied energy levels. As explained, e.g. in ref. [lo], the transfer is supposed to take place when the potential barrier between the ionic attractive wells is so low that the first-order Stark-shifted binding energy of the electron equals the top of the barrier, i.e. when I. COLLISION PROCESSES