PHYSICAL REVIEW A 83, 052704 (2011) Classical treatment of ion-H 2 O collisions with a three-center model potential Clara Illescas, 1 L. F. Errea, 1 L. M´ endez, 1 B. Pons, 2 I. Rabad´ an, 1 and A. Riera 1 1 Laboratorio Asociado al CIEMAT de F´ ısica At ´ omica y Molecular en Plasmas de Fusi ´ on, Departamento de Qu´ ımica, m ´ odulo 13, Universidad Aut´ onoma de Madrid, Cantoblanco, E-28049 Madrid, Spain 2 CELIA, Universit´ e de Bordeaux I-CNRS-CEA, 351 Cours de la Lib´ eration, F-33405 Talence, France (Received 17 January 2011; published 18 May 2011) We present calculations of cross sections for one- and two-electron processes in collisions of H + , He 2+ , and C 6+ with water molecules in the framework of the Franck-Condon approximation. We employ an independent-electron method and a classical trajectory Monte Carlo approach. Anisotropy effects related to the structure of the target are explicitly incorporated by using a three-center model potential to describe the electron-H 2 O + interaction. We derive scaling laws with respect to the projectile charge. We also estimate cross sections for molecular fragmentation subsequent to electron removal. DOI: 10.1103/PhysRevA.83.052704 PACS number(s): 34.50.Gb, 34.70.+e, 82.39.Jn I. INTRODUCTION Ion-beam cancer therapy has been shown to be a valuable alternative to x- or γ -ray radiotherapy (see [1,2] for reviews). The use of this technique started in 1954 at the Lawrence Berkeley National Laboratory (United States) [3]; since then, several thousands of patients have been treated with proton beams in several installations and with carbon ion beams at Chiba (Japan) [4] and Darmstadt (Germany) [5]. Compared to conventional photon radiation, the use of ion beams has several advantages: it allows access to deeply seated tumors, and the lethal tumor dose is raised while the surrounding healthy tissue remains unaffected. Physicists and biologists can measure and/or compute the intensity, penetration depth, and lethal dose of the ion beams [6–9]. Nevertheless, the mechanisms responsible for the dissociation of the DNA chain, which subsequently lead to cell death, remain quite obscure. Some experiments have therefore considered, in the last few years, collisions of multicharged ions with DNA bases to shed light on the fragmentation processes (see, e.g., [10]). Further fundamental studies, especially on the theoretical side, are required since the underlying mechanisms are intricate. On the other hand, electron-DNA experiments [11] have shown that collisions of relatively slow electrons (with energy of about 10 eV) can lead to the breakdown of DNA through a mechanism that involves the formation of intermediate resonant states. Therefore, processes that lead to the production of electrons are also relevant to understanding biological damage and ion therapy. In this respect, electron emission in collisions of ions with water provides the most significant source of electrons in the interaction of ion beams with the cell. Although several experiments [12–14] have provided detailed information on ionizing proton-water collisions, data are scarce for multicharged ion impact. Furthermore, beyond purely ionizing reactions, all other processes, such as elastic scattering, excitation, and charge exchange in ion- H 2 O collisions, are amenable to target fragmentation (and subsequent biological effects). Therefore, these processes must also be explicitly considered to reliably simulate the passage of charged particles in biological (cell) environments [15,16]. Previous theoretical works aimed at filling in the collisional database of interest for radiation damage. Nevertheless, most of those works focused on H + + H 2 O collisions and employed perturbative methods [17], such as the continuum-distorted- wave–eikonal-initial-state (CDWEIS) [18–20] and first-order Born (FB) approximations [21], which are, in general, useful at impact energies E greater than 100 keV/amu. In a previous work [22], we employed the classical trajectory Monte Carlo (CTMC) method to evaluate single-ionization and single- capture cross sections in H + + H 2 O collisions at energies 25 keV <E< 5 MeV. We used the independent-particle method (IPM) [23–25], where the electrons are treated as independent particles that follow trajectories obtained by solving the Hamilton equations with a one-center (isotropic) model potential to describe the interaction between the active electron and the molecular core. Recently, L¨ udde et al. [26] applied the basis-set-generator method (BGM) to H + + H 2 O collisions, beyond the isotropic electron-core approximation; they have reported electron production and net capture cross sections in good agreement with experiment. With respect to water collisions with multicharged ions, He 2+ + H 2 O collisions have been considered in Refs. [27] and [28] in the framework of FB and classical models, respectively. Comparison with experimental data showed acceptable, but not very satisfactory, agreement. In spite of the interest in C 6+ + H 2 O collisions in ion-based cancer therapy, only CDWEIS [29] and FB [30] calculations of the total ionization cross section have been reported so far, together with a single experimental point at 6 MeV/amu [29]. In this work, we employ an improved CTMC model to cal- culate cross sections for single-ionization, single-capture, and two-electron processes (transfer ionization, double capture, and double ionization) in H + , He 2+ , and C 6+ + H 2 O collisions in the impact-energy range 20  E  10 000 keV/amu. We largely encompass the intermediate- and high-impact- energy regimes of interest for therapy applications. The basic assumptions of our treatment, together with preliminary results, were presented at the Radiation Damage (RADAM) Conference of 2008 (see [31]). As in most of the available calculations, our treatment is based on assuming that the electrons are independent, so that each electron moves in an effective field created by the nuclei and the remaining electrons. In practice, this involves the use of electron-core effective potentials. In contrast with previous calculations, 052704-1 1050-2947/2011/83(5)/052704(12) ©2011 American Physical Society