Axial energy distributions of Li + and F − desorbed from LiF surfaces by fast ion impact

Abstract

Initial axial kinetic energy ( E z ) distributions of Li + and F − secondary ions desorbed from LiF surfaces, bombarded by low current (∼500 particles/s) N 3+ beams in the 0.075–7.5 MeV primary ion energy ( E PI) range, were measured by using the time-of-flight technique. In this energy range, the electronic energy loss, S e, increases from 25 to 160 eV/Å, while the nuclear stopping power, S n, decreases from 8.0 to 0.25 eV/Å. The observed F − initial axial kinetic energy distribution can be described by the linear collision cascade theory and no other contribution was found. The F − total yield decreases proportionally to S n. The Li + distribution presents a remarkable deviation from the cascade prediction, indicating the existence of additional mechanisms related to the electronic energy-loss process. For the range of E PI studied, these mechanisms produce a Li + energy distribution with a Maxwell-Boltzman-like shape, which vanishes above E z ∼ 10 eV and presents a maximum at E z ∼ 1.2 eV. The Li + yield is nonlinear with the electronic energy loss, S e. A simple desorption model, based on the spatial distribution of the energy deposited by the projectile and on the effective energy-loss concept, is presented. This spatial distribution of the deposited energy is due to secondary electron cascades and is connected with a ESD-like mechanism on the surface.