A simple Method to evaluate multiple ionization cross sections by ion impacto n atoms and small molecules

MARIEL GALASSI; R.D. RIVAROLA

San Carlos de Bariloche

Conferencia; 17th International Conference Physics of Highly Charged Ions (HCI 2014).; 2014

Centro Atomico Bariloche e Instituto de Fisica de Rosario

The study of multiple ionization of atoms and molecules by ion impact is of great interest in many subfields of physics, chemistry, astrophysics and radiobiology. In the last case, to understand the biological damage caused by protons and heavy ions (used as radiation particles in hadrontherapy), it is of crucial importance to know the physical processes that occur in the core of the ion track. During the irradiation of human tissues with fast ions, the biological molecules are ionized and excited. Mainly, in the region of maximum energy deposition (known as the Bragg?s peak), multi-electron processes occur (transfer-ionization, multiple ionization, multiple electron capture, etc). Experimental cross sections for multi-electron emission by heavy ion impact are scarce. Then, the development of theoretical models to calculate multiple-ionization cross sections (MICS) is necessary. In this work we calculate exclusive MICS of atoms and small molecules of biological interest in the framework of the Independent Particle Model (IPM), employing a binomial distribution in order to take into account contributions from different molecular target orbitals. The IPM requires the computation of single particle probabilities as a function of the impact parameter. As in previous works, we use two different methods to obtain these probabilities: the Exponential Model (MICS-EM) and the Continuum Distorted Wave - Eikonal Initial State (MICS-CDW-EIS) approximation [1-4]. Within the Exponential Model, it is assumed that the atomic and molecular orbitals present a spherical form (i.e. centered in the oxygen atom in the case of the water molecule). The single-electron emission probabilities for each shell are described by decreasing exponential functions with adjustable parameters to reproduce CDW-EIS net-ionization cross sections. For the case of proton impact on Ne and Ar, MICS-EM were tested to be in very good agreement with experimental data [1]. In the present work we extend the use of the MICS-EM to other systems, considering the contribution of post-collisional Auger electron emission. The dependence of multiple ionization cross sections with the energy and charge of the projectile is analyzed. When double ionization is considered it is shown that ejection from two different orbitals dominates the reaction, being largely preferable than emission of two outer-shell electrons. References [1] M.E. Galassi, R.D. Rivarola and P.D. Fainstein. Phy. Rev. A 75, 052708 (2007). [2] M.E. Galassi, R.D. Rivarola, P.D. Fainstein. NIM B 268 1637-1641 (2010). [3] B. Gervais, M. Beube, G. Olivera and M. Galassi. Rad. Phys. Chem. 75, 493-513 (2006). [4] R. D. Rivarola, M. E. Galassi, P. D. Fainstein, C. Champion. Book series "Advances in Quantum Chemistry: Theory of heavy ion collision physics in hadron therapy". Chapter nine. Editorial: Elsevier Inc. Copenhagen, Dinamarca. Editor: Dzevad Belkic. (2013).