Anomalous Positive Ion Formation in Negative Ion Collisions with HOPG

• Published in: Journal of physical chemistry. C 2016-08, Vol.120 (33), p.18538-18547
• Main Authors: Liu, Pinyang, Chen, Lin, Qiu, Shunli, Xiong, Feifei, Jiang, Haoyu, Lu, Jianjie, Liu, Yuefeng, Li, Guopeng, Liu, Yiran, Ren, Fei, Xiao, Yunqing, Gao, Lei, Ding, Bin, Guo, Yanling, Chen, Ximeng
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.6b03680

Charge transfer on graphite, a typical substrate and one of the fusion first wall materials, is of great importance in the plasma–wall interaction, thin-film growth, and surface catalysis. We present an experimental study of 8.5–22.5 keV energy carbon, oxygen, and fluorine negative ions scattering from a highly oriented pyrolytic graphite (HOPG) surface at a scattering angle of 8°. It is found that the positive ion fraction decreases monotonically with the increase of both incident velocity and angle. In particular, these dependences are very different from those presented in previous studies. A molecular dynamics simulation reveals that, around the critical condition for planar surface channeling, a number of projectiles may penetrate into the subsurface and become energetic atoms when they emerge from the surface. Hence, an exponential scaling related to the penetration probability has been proposed to well describe the velocity and angle dependences of positive ion fractions.