| Charge exchange processes in ion surface scattering are important not only from a fundamental physical interest, but also in practical applications, i.e., surface analysis, astrophysics, plasma and fusion research. In the past, most studies on negative-ion formation have been confined to interactions between ions and metallic surfaces. Resonant charge transfer (RCT) is considered to be much more efficient both experimentally and theoretically. However, negative ion formation on a graphite surface, which is a largely used material in many fields, i.e., neutron moderator, structural and reflector material in nuclear reactor, is by far a less studied system. There are only a few investigations for hydrogen ions scattering, moreover, their results are different from each other.In this work, we study charge transfer in negative ion scattering from a HOPG surface at a scattering angle of 380. F-、C-、O- ions with energy from 12.5 up to 22.5 keV are produced by the cesium sputter negative-ion source. We measured the ion fraction as a function of incident energies and angles, and we found:The negative ion fraction increases monotonously with increasing incident energy, while the positive ion fraction almost keeps a constant and the neutral fraction decreases slowly as incident energy increases. The negative ion fraction firstly increases and then decreases as incident angle increases. The maximum value of the fraction generally corresponds to the specular scattering. On the contrary, the neutral fraction decreases and then increases with increasing incident angles. The positive ion fraction does not apparently vary.The negative ion fraction cannot be explained by classical Jellium model, so we have to take the incoming velocity effect into account. Negative ions are formed after the close collisions, and decay on outing trajectory. We found that the negative ion fraction obeys well exponential scaling. The positive ion is explained by the Molecular orbit (MO) promotion model qualitatively. |
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