| The interaction between particle and surface has been comprehensively investigated by theory and experiment as an important research field,such as the formation of micro structure on surface,the energy loss and charge transfer mechanism in the scattering of particles on surface and so on.Some experiments observed abnormal high fraction of negative ions from the grazing scattering of atoms on the surface of insulating ionic crystals.Since the applications in the fields of ion implantation of fusion,laser cooling,surface modification,the measurement of neutral particles in space,the growth of neural tissue,the high conversion rate of negative ion obtained much attention.Considering the large energy defect between the affinity level of negative ion and the valence band of ionic crystal,the high conversion rate can not to be explained by resonant charge transfer.In previous investigation,the valence electron capture mainly occur at the anion site of surface.In this process,the affinity level of the formed negative ion will be downward shifted by Madelung potential of crystal and the energy defect will drastically decrease,in turn increase the electron capture probability.The experimental data of negative ion fraction as a function of incident velocity of F-LiF(100)/KI(100)/KCl(100)and O-KCl(100)/KI(100)systems at grazing angle have been investigated by the model which coupling the valence electron capture and Coulomb tunneling detachment model.While the explanation of the experimental data of negative ion fraction as a function of velocity of O-LiF(100)and H-LiF(100)systems are still absent.In this work,the quantitative explanation of these two systems have been proposed,the simulation result accords to the experimental data.For O-LiF(100)system,the negative ion conversion mainly includes the electron capture between projectile and Coulomb tunneling detachment for single collision.The calculation of electron capture probability considered the direct interaction between projectile,ML polarization and image potential contributions for the energy defect between the initial and final states.Since the binding energy of O~-is comparatively large,the formed negative ion can conserve the affinity electron in sequent collisions.For H-LiF(100)collision,the formed H~-ion will go across a surface state and has a probability to loss electron when leaving an active site besides valence electron capture and Coulomb tunneling detachment.After charge transfer of H~0-H~-in single binary collision,the formed H~-ion will possibly lose its affinity electron in sequent collisions with surface anions and leads to multiple charge transfer.The comparison of the probability between the electron loss by surface exciton state and image state is given in this work.The surface altitude at which the affinity level of H~-and the energy level of surface exciton state overlap is so large that the electron coupling strength at this altitude is negligible for electron transfer.On the other hand,the sub-eV order of the energy defect between the H~-ion affinity level and the image state induced by H~-image charge in the LiF crystal makes electron loss to this state through nearly resonant charge transfer possible. |
