Neutralization Of Li~+ And Na~+ Ions Scattering From A Pd(111) Surface

Charge exchange processes between ions and surfaces are important in fundamental research and technological applications,such as surface catalysis,plasma etching, thin-film growth, adsorption desorption and surface analysis. Comparing with noble ions, alkali-metal ions have high detection efficiency and can be used as probe for surface analysis. The neutralization probability is one of the key factors for concentration analysis, and determines its accuracy. The neutralization of alkali-metal ions is not only important for the choice of experimental conditions of low energy ion scattering, but also for high accuracy and sensitivity of this surface analysis technology. Moreover, Pd(111) is a high efficient catalyst, and the key material for microelectronics and space fields. At present, we have not found the experimental report of neutralization of alkali-metal ions on Pd(111).In this thesis, we studied the charge transfer process of low energy alkali-metal ions(Na+ and Li+) scattering on a Pd(111) surface. We measured the energy and exit angle dependences of neutral fractions by a position-sensitive detector with the scattering angle of 53°. We find that the energy-dependent neutral fractions for Li and Na are high and decrease monotonically with the increase of incident energies. For the exit angle dependence, the neutral fraction first decreases and then increases with the increase of exit angles. These characteristics cannot be explained by the jellium free electron model.In this work, we carefully analyzed the physical mechanism of neutralization based on the Brako-Newns(BN) model calculations. We find that:(1) taking into account the reduced work function by adsorbates, the calculated results are in agreement with the experimental data;(2) for exit angle dependence, the parallel velocity effect affects the final charge state distribution, and plays an important role in grazing exit angles.(3)the L-gap of Pd(111) suface contribute something to the high neutralization fraction of low-velocity area.