Eur. Phys. J. D 60, 45–49 (2010) DOI: 10.1140/epjd/e2010-00011-2 Regular Article T HE EUROPEAN P HYSICAL JOURNAL D Classical calculation of proton collisions with ethylene H. Getahun, L.F. Errea, C. Illescas, L. M´ endez a , and I. Rabad´ an Laboratorio Asociado al CIEMAT de F´ ısica At´omica y Molecular en Plasmas de Fusi´on, Departamento de Qu´ ımica, C-IX, Universidad Aut´onoma de Madrid, Cantoblanco, 28049-Madrid, Spain Received 11 November 2009 / Received in final form 21 December 2009 Published online 26 January 2010 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2010 Abstract. Single electron capture and single ionization total cross sections in collisions of proton with ethylene are calculated for an energy range 25 keV ≤ E ≤ 150 keV, using the classical trajectory Monte Carlo method. Multi-center model potentials are employed to represent the interaction of the active electron on each molecular orbital with the C2H + 4 core. The results are compared with experimental results for single electron capture. 1 Introduction In the last few years, several works have focused in under- standing the mechanism of the interaction of ionizing radi- ations with basic building block of cells and, in particular, in characterizing the mechanisms of radiation damage of DNA (see [1] and references therein). In this respect, it is known that DNA can be damaged through direct impact of the radiation agents with its molecular constituents, or by an indirect mechanism where radiation of the cell envi- ronment yields free radicals and low energy electrons that could trigger chemical reaction and hence, damage of the DNA strands. In general, ionization is the most important process in ion-molecule collisions at energies above 25 keV/amu. This process produces low-energy secondary electrons, which can lead to single and double strand breaks of DNA [2] through a mechanism of dissociative electron attachment. As a consequence, several calculations of ionization cross sections have been carried out for ion collisions with small biomolecules, which employed perturbative methods. In particular for proton collisions with H 2 O, the contin- uum distorted wave approximation was applied in refer- ences [3–5] and the Born approximation in reference [6]. In a recent work [7], electron capture cross sections have been also calculated for several ion-molecule collisions using the first Born approximation. Recently, Lekadir et al. [8] have reported classical overbarrier calculations of ioniza- tion and electron capture processes in ion collisions with DNA bases. Dal Cappello et al. [9] have carried out first- Born-coulomb-wave calculations of ionization in H + + cy- tosine collisions. The classical trajectory Monte Carlo method (CTMC) [10] allows to simultaneously evaluate cross sections for both reactions. Previous calculations on ion-atom colli- sions (see [11]) indicate that this method is in general a e-mail: l.mendez@uam.es appropriate at collision energies above the maximum of the electron capture total cross section, although the use of the microcanonical distribution [10] underestimates the ionization cross sections at collision energies below the maximum of the ionization total cross section. A first ap- plication of the CTMC treatment to H + +H 2 O collisions can be found in reference [12]. The main limitation of this work is the use of a one-centre effective potential to de- scribe the electron interaction with the molecular core. In this paper we study proton + ethylene collisions, as a prototype of collisions of ions with bio-molecules con- taining C=C bonds. We have calculated total cross sec- tions for single ionization (SI): H + +C 2 H 4 → H + +C 2 H + 4 + e - (1) and single electron capture (SEC): H + +C 2 H 4 → H+C 2 H + 4 . (2) While total cross section for reaction (2) was measured in the work of reference [13], no measurements of the SI cross sections have been reported. Suzuki et al. [14] have calculated SEC total cross section at low energies (E< 10 keV), using a molecular expansion. In the present work we have applied the method pro- posed in reference [15], where model potentials are em- ployed to describe the interaction of the active electron with the molecular core. The many-electron interpreta- tion of the one-electron probabilities is then carried out by means of an expression based on the independent event model [16]. Atomic units are employed unless otherwise stated. 2 Method Our treatment employs the eikonal-CTMC method [17] where the projectile follows a rectilinear trajectory with