Effect of spatial redistribution of valence holes on the formation of a defect halo of swift heavy-ion tracks in LiF

Abstract

In-situ spectroscopy is applied to analyze the spatial distribution of point defects ($F$-color centers) formed in the nanometric vicinities of the trajectories of different swift heavy ions (from C to U) decelerated in LiF crystals in the electronic stopping regime. The formation of these defects results from the decay of self-trapped excitons appearing due to self-trapping and further relaxation of valence holes generated during beam induced ionization processes. A Monte Carlo model is applied to estimate the initial radial distribution of valence holes in the ion tracks by the time the ionization cascades are finished. It is demonstrated that there exists a large difference between this and the radial distribution of point defects detected in the experiments. This difference is described by fast and long-range diffusion of valence holes before their self-trapping. The effect illustrates the fundamental importance of the relaxation kinetics of the valence band on structure transformations in tracks. The average diffusion coefficient of holes ${D}_{h}$ and their conversion efficiencies into color centers \ensuremath{\beta} in relaxing tracks of different ions in LiF are estimated (${D}_{h}$ = 0.016--1.4 cm${}^{2}$/s, \ensuremath{\beta} = 0.01--0.04).