SURFACE EVOLUTION OF NICKEL UNDER HE AND H ION IRRADIATION BY MEANS OF KELVIN PROBE

摘要

Polycrystalline Ni samples were irradiated by He and H ion beams of 1 MeV, 2x10~(16) ions/m~2/s under a pressure of 1x10~(-4) Pa, and a He ion beam of 500 eV, 2x10~(17) ions/m~2/s under a working pressure of about 1 x 10~(-2) Pa. The work function (WF) change, extremely sensitive to various surface changes, was recorded using a Kelvin probe via measuring the contact potential difference between the probe and the sample. The results indicate that the irradiation of 500 eV He ions result in WF decrease at low fluence range, and then WF increase till saturation with increasing the fluence, while 1 MeV ions only induce WF decrease, then saturation. A surface model of loosely bound adsorbed layer plus native oxide layer on metals is presented to explain the observed phenomena. The nuclear stopping is responsible for the results in the case of 500 eV He~+ irradiation that is powerful enough to sputter away the whole overlayer from the bulk surface. In MeV case, the electronic stopping plays a decisive role, which allows merely the topmost adsorbed layer to be removed partially by He and H ions of 1 MeV. Due to desorption of the topmost adsorbed layer, the WF decreases as a result of decrease of the surface dipoles towards the bulk. If the oxide layer is removed subsequently due to sputtering, the WF will increase.