Secondary ion emission dynamics model: A tool for nuclear track analysis P. Iza a , L.S. Farenzena a , T. Jalowy b , K.O. Groeneveld b , E.F. da Silveira a, * a Pontifı ´cia Universidade Cato ´ lica, Departamento de Fı ´sica, CP 38071, 22952-970 Rio de Janeiro, Brazil b Institut fu ¨ r Kernphysik der J. W. Goethe Universita ¨ t, D-60438 Frankfurt am Main, Germany Available online 4 January 2006 Abstract The initial velocity distribution of secondary ions is employed as a tool to analyze nuclear track formation processes occurring in the picosecond time range. The choice of the secondary ion for such analysis depends on the particular surface region and on the desorption time range of interest: (a) H + ions are emitted promptly from the impact site, (b) H  desorption occurs delayed and mostly from the impact periphery, (c) emitted molecular ions are accelerated away during tens of picoseconds exclusively from the impact periphery. The model is set up considering the track as two coaxial cylinders, the inner one positively charged and the outer one negatively charged. It takes into account effects due to the track charge image formed by a metallic substrate, the projectile angle of incidence, the variation of the electronic stopping power for projectiles out of the equilibrium-charge regime, and a positive and negative track neutralization whose rates are exponentially decreasing with time. Predictions of the model are presented for ion desorption of LiF bombarded by 1 MeV Ar ions. Ó 2005 Elsevier B.V. All rights reserved. PACS: 34.10.+x Keywords: Nuclear track potential model; Secondary ion emission; Angular distribution; Initial velocity distribution 1. Introduction For ion beams having velocities around or above the Bohr velocity, the projectile energy and momentum are mainly transferred to the target by ion–electron interac- tion. As a consequence, besides atomic and molecular elec- tronic excitations, electrons of a solid target are pushed away from the region surrounding the projectile trajectory: an inner positive track and an outer negative track are pro- duced. A small fraction of these electrons escape from the solid and a net positively charged region appears around the impact site. For over four decades, there have been a large number of publications on the nuclear tracks describ- ing them as the result of several processes such as ioniza- tion, d-electron energy deposition, secondary electron diffusion in solids, defect formation and induced chemical reactions e.g. [1–10]. The charge distribution of both positive and negative tracks depends on the angular and energy distributions of the secondary electrons, as well as on how these electrons are transported throughout the target under the influence of self-consistent local electric fields. For conducting tar- gets, the electron mobility is very high and the track neu- tralization occurs fast, in the femtosecond range. For insulating targets, the neutralization time may exceed tens of picoseconds, which is long enough to disturb sensibly the trajectories and kinetic energies of secondary ions (SI). Several techniques have been used to analyze the dif- ferent characteristics of the tracks, e.g. atomic force microscopy, sputtering yield and Auger emission measurements. In this article, it is discussed the suitability of the sec- ondary ion emission dynamics, particularly the initial 0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.11.080 * Corresponding author. Tel.: +55 21 31141272; fax: +55 21 3111040. E-mail address: enio@fis.puc-rio.br (E.F. da Silveira). www.elsevier.com/locate/nimb Nuclear Instruments and Methods in Physics Research B 245 (2006) 61–66 NIM B Beam Interactions with Materials & Atoms