Electronic sputtering of thin lithium fluoride films induced by swift heavy ions

The sputtering of secondary ions from thin lithium fluoride films of different thickness (2 nm ≤ z ≤ 106 nm) irradiated by swift heavy ions (Ni and Ge: ~10 MeV/u) was studied under ultrahigh-vacuum conditions by a time-of-flight imaging technique (XY-TOF-SIMS). Most of the positive ions emitted are (LiF)nLi+ clusters. Cluster structure and stability (magic numbers) are analyzed. A pronounced dependence of sputtered particle yields on the film thickness z was observed. The onset of large cluster emission occurs at a thickness of about z = 20 nm. For thinner films, z = 2 and z = 4 nm, only small clusters (n < 6) are ejected. A threshold for the emission of large clusters was also observed as a function of the electronic stopping power Se at around 6 keV nm−1. This leads to the important conclusion that both a sufficient amount of material (z-dependence) and of deposited energy (Se-dependence) are needed for large ionic cluster emission to occur.