Secondary ion emission dynamics model: A tool for nuclear track analysis

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.