Numerical study on electron transfer mechanism by collision of ions at graphite surface in highly efficient neutral beam generation

We investigated the neutralization mechanism of ions created by collisions with a graphite surface by numerical simulations using an efficient and stable simulator developed by us based on time-dependent density functional theory (TD-DFT) to clarify the mechanism responsible for generating neutral beams in a highly efficient neutral beam source developed by Samukawa et al (2001 Japan. J. Appl. Phys. 40 L779). The results from the simulations revealed that negative ions (Cl?) have higher neutralization efficiency than positive ions , which was consistent with previous experimental results. The origin of this difference was investigated in terms of the energy alignment between electronic states participating in the charge transfer process. We found that the electronic states of Cl? have similar energies with those of graphite, while those of and graphite have large differences in energies. This could be interpreted as resonant charge transfer occurring in the neutralization process of negative ions, while Auger charge transfer is dominant in that of positive ions. This interpretation was also strengthened by results where electron transfer probability to the excited states was much larger for collisions of graphite with than with Cl?. This suggested that the different mechanisms are the reason for the difference in neutralization efficiency between negative and positive ions.