Negative-ion Conversion Of Atoms And Ions At Ionic-crystal Surfaces Under Grazing Geometry

Interaction of atoms(ions)with ionic-crystal surfaces has attracted a considerable attention over the past years,for understanding various fundamental processes that are important in catalysis,development of gas sensors,problems of adhesion etc.the dynamics of electron transfer plays an important role in chemisorption and reactions at surfaces.These information can be obtained from charge state analysis of scattered particles by atoms or ions grazing scattering off various ionic-crystal surfaces.Most of experiments have shown that although the existence of large bandgaps which is unfavourable for resonant electron transfer,negative-ion conversion efficient is still unexpectly high.This result implies that the dynamics of electron transfer on ionic-crystal surfaces cannot be understood by simple models that extensively used for the case of metal surfaces.Negative ions formation is closely related to an effecient electron capture,while a simultaneously loss process cannot be neglected.The existing theoretical treatment(ab initio calculations,coupled-cluster and simple model calculations)can only reproduce the low-velocity part of the experimental data(including the velocity thresholds for negative-ion formation)quite well.This result from the neglect of the possible electron-loss processes that can lead to the destruction of the formed negative ion.In this work,taking into account both key effects—ML polarization and image interactions in electron capture and considering into the Coulomb barrier tunneling of the affinity electron on the formed negative-ion during their interaction with surface anion sites,a simple general approach that can reproduce the complete velocity range of negative-ion formation in grazing scattering from any AB-type ionic crystal surface is obtained.The negative-ion formation in grazing scattering of atoms from insulating ionic-crystal surface was studied by three combinations of O-KCl(100),O-KI(100)and F-KCl(100).Moreover,detailed influences of electron affinity and lattice constant were also analysied.A good agreement between the theoretical calculation and experimental results was obtained.For negative ions incident within the limited velocity range of v=0-0.1 a.u.,where the Coulomb barrier tunneling loss was non-effective,however,there exists a significant experimental negative-ion loss process,a Landau-Zener quasi-molecule diabatic potential energy level crossing model was introduced,which successful explained the experimental results.