Ab initio molecular treatment of charge transfer processes induced by collision of C 2+ ions with the OH radical: A linear approach E. Bene a , Á. Vibók b , G.J. Halász c , M.C. Bacchus-Montabonel d, * a Institute of Nuclear Research of the Hungarian Academy of Sciences, P.O. Box 51, H-4001 Debrecen, Hungary b Department of Theoretical Physics, University of Debrecen, P.O. Box 5, H-4010 Debrecen, Hungary c Department of Information Technology, University of Debrecen, P.O. Box 12, H-4010 Debrecen, Hungary d Laboratoire de Spectrométrie Ionique et Moléculaire, CNRS et Université de Lyon, 43 Bd. du 11 Novembre 1918, 69622 Villeurbanne Cedex, France article info Article history: Received 18 January 2008 In final form 26 February 2008 Available online 2 March 2008 abstract The charge transfer of C 2+ ions in collision with the OH radical has been studied theoretically by means of ab initio quantum chemistry molecular methods followed by a semiclassical dynamical treatment. In this first step, only the linear approach has been investigated, taking account of the influence of the variation of the distance r OH of the target during the process. The total and partial cross-sections have been deter- mined, and an evaluation of the cross-sections on the different vibration levels has been performed. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Interaction of ionizing radiation with biological tissue can in- duce severe damage to DNA [1], in particular single and double strand breaks which appear to be the main underlying mechanisms both for cancer disease and for controlled cell killing used in radio- therapy. As shown previously [2], important damage is due to the secondary particles – i.e., low energy electrons, radicals or singly and multiply charged ions – generated along the track after inter- action of the ionizing radiation with the biological medium. Increasing interest has thus been devoted to the possible reactions involving these secondary particles, in particular recent experi- mental [3–6] and theoretical [7,8] studies have been undertaken on reactions involving ions. Up to now, direct processes where the ions are interacting directly with biomolecules have been mainly considered, but, from a biophysical point of view [9], very important damage is relied to indirect processes corresponding to the action on the medium – solvent, generally water, environ- ment, etc. In particular, the action of ions on water molecules pro- duces OH radicals which can then interact strongly with the biological medium. In such low energy collisions between an ion and a biomolecu- lar target, different processes may be considered: excitation and fragmentation of the molecule, ionization of the gaseous target, and also possible charge transfer from the multicharged ion to- wards the biomolecule. From the experimental point of view, exci- tation and fragmentation processes have been mainly investigated, but charge transfer has been shown to be a complementary process [7]. In a previous study on the collision of C q+ ions on the RNA base uracil [7,8], direct processes have been investigated. Such reactions have been shown to be strongly anisotropic [8] with electron delo- calization from the target to the colliding ion [7,8]. However, for such systems, possible variations of the geometry of the biomolec- ular target molecule were not taken into account in the theoretical treatment. We have thus undertaken the study of the C 2+ + OH ? C + + OH + reaction, with both the idea of modelling the action of ions on OH radicals created in the medium, in order to have a look to some possible indirect processes and also improve our theoret- ical treatment by taking into account the influence of the geometry of the molecular target in such collision. We could thus have an evaluation of the cross-sections with regard to the different vibra- tion levels of the molecule. In such a process, the orientation of the target towards the ion projectile is of course determinant [8,10]. In a first step, we have investigated the collision C 2+ + OH in the linear approach in order to focus our attention on the influence of the variation of the geometry of the target on the process. This work will be completed by the consideration of the orientation of the target towards the projectile. Ab initio molecular calculations of the potentials and couplings followed by a semiclassical dynamics in the [3–30] keV collision energy range are performed throughout this study. 2. Molecular calculations The geometry of the C 2+ + OH system may be described by the internal Jacobi coordinates {R, r, h} with the origin at the centre- of-mass of the OH molecule, as defined in Fig. 1. In this linear ap- proach, the collision of the C 2+ ion towards the oxygen atom of the OH target has been investigated, which corresponds to an angle h = 180°. Spin–orbit coupling being negligible in the energy range of interest, we can assume the electron spin to be conserved in 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.02.094 * Corresponding author. Fax: +33 4 72 43 15 07. E-mail address: bacchus@lasim.univ-lyon1.fr (M.C. Bacchus-Montabonel). Chemical Physics Letters 455 (2008) 159–163 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett