Cluster ion emission from LiF induced by MeV Nq+ projectiles and 252Cf fission fragments

Abstract

Ion cluster desorption yields from LiF were measured at PUC-Rio with ≈0.1 MeV/u Nq+ (q = 2,4,5,6) ion beams by means of a time-of-fight (TOF) mass spectrometer. A 252Cf source mounted in the irradiation chamber allows immediate comparison of cluster emissions induced by ≈65 MeV fission fragments (FF). Emission of (LiF)nLi+ clusters are observed for both the N beams and the 252Cf fission fragments. The observed cluster size n varies from 1 to 6 for Nq+ projectiles and from 1 to ≈40 for the 252Cf-FF. The size dependence of the Y(n) distributions suggests two cluster formation regimes: (i) recombination process in the outgoing gas phase after impact and (ii) emission of pre-formed clusters from the periphery of the impact site. The corresponding distribution of ejected negative cluster ions (LiF)nF− closely resembles that of the positive secondary (LiF)nLi+ ions. The desorption yields of positive ions scale as Y(n) ∼ q5. A calculation with the CASP code shows that this corresponds to a cubic scaling ∼Se3 with the electronic stopping power Se, as predicted by collective shock wave models for sputtering and models involving multiple excitons (Frenkel pair sputtering). We discuss possible interpretations of the functional dependence of the evolution of the cluster emission yield Y(n) with cluster size n, fitted by a number of statistical distributions.