Positive Ion Formation In Negative Ion Grazing Scattering On HOPG

Charge transfer is not only one of the important topics of surface physics and chemistry,but also of great importance in practical applications,such as low-energy ion spectroscopy and secondary ion mass spectroscopy in the surface analysis,particle detection in astrophysics,and plasma-wall interaction in fusion research.At present,one of the most direct way to probe positive or negative ions is the measurement of charge state distribution of scattered particles using the ion-surface scattering technique.Most research on charge transfer has been confined to interactions between ions and metallic surfaces so far,However,charge transfer on a graphite surface,one of plasma facing material and divertor plate in tokamaks,is a less studied system,and most of them up to now have been confined to proton projectiles.Those studies vary from each other,and some key problems have still to be resolved.The charge transfer progress of multielectron projectiles scattering on HOPG has grown strongly since such researches can be used to examine many-body interaction.On the other hand,negative(positive)ion formation involved in charge transfer processes is one of the important topics,howevr,the positive-ion formation has not been the subject of similar attention as compared to a lot of research for negativeion formation.In this work,we performed the measurement on charge transfer procosses of 8.5-22.5 keV C-?O-?F-ions scattering on HOPG with a scattering angle of 8°.The scattered particles are detected by a one-dimensional positive sensitive microchannel plate(PSMCP)detector and possess three different charge states,i.e.negative,neutral and positive.We focused on the variation of positive ion fraction with incident energy and angle,and found that: positive ion fraction decreases monotonically with increasing incident energy and angle,the energy dependence is significantly different from previous studies,and cannot be understood by conventional ionization mechanisms.A molecular dynamics simulation reveals that,under a condition of the incident angle close to the critical angle of planar surface channeling,a fraction of projectiles may penetrate into subsurface and become energetic atoms to exit from the surface.We introduce a conception of “penetration probability” related to the incident perpendicular velocity to describe this phononminon.Therefore positive ion was ascribed to the rest projectiles,to form in close collisions and then survived on the outgoing trajectory.As a result,the anomalous energy dependent positive-ion fraction was well explained by the penetration probability.