CLASSICAL TRAJECTORIES STUDIES OF DIET FROM CALCIUM-FLUORIDE

We investigate desorption of positive ions resulting from the repulsiv e environment created by core-hole Auger decay from relaxed CaF2 surfa ces. The molecular dynamics simulations in the lamina geometry (with t wo-dimensional ion-lattice summation) is used. For both (011) and (111 ) surfaces the simulation with changed charge without providing additi onal kinetic energy does not lead to the ejection of F+ ion due to the lattice rearrangement and trapping of the ion. We also assume that th e positive ion gains a substantial amount of kinetic energy at the ons et of simulations, crudely mimicking ion-stimulated desorption. For th e (011) surface the results are extremely sensitive to the size of the considered system, in sharp contrast to the ejection of positive ions from alkali halides. For a 384 ion system, ejection occurs if the kin etic energy, equal to 0.25 eV or more, is delivered to the F+ ion at t he start of the simulation. For a 768 ion system ejection occurs only for the initial kinetic energy of 4 eV. This result is probably caused by inadequate classical potential and lack of full convergence of the two-dimensional Ewald summation scheme for a highly disordered system . For the (111) surface with 1536 ions in the cell, ejection occurs fo r an initial kinetic energy of 0.4 eV.