THERMAL RELEASE OF SIMULTANEOUSLY IMPLANTED H+ AND D+ FROM GRAPHITE

Thermal releases of simultaneously implanted H+/D+ (for energy varying from 0.9 to 10 keV per H or D) from graphite have shown the extent of H-D mixing in the form of mixed-isotope hydrogen and methane molecule emissions to be dependent on the D+-H+ range separation, indicating that both hydrogen and methane molecules are formed at the end of ion range in the absence of an energetic incident ion beam. Due to the wall contribution to the measured HD signals, one cannot totally rule out the possibility of some atom diffusion in the implantation layer during the thermal release process. The amount of methane emitted during thermal desorption was found to decrease with increasing implantation energy and to be approximately inversely proportional to the ion straggling. The dependence of methane emission during thermal desorption of implanted H+ on the H+ implantation flux density clearly demonstrates that methane molecules are formed during H+ implantation rather than during postbombardment thermal release. An apparent fluence threshold was found in the methane emission and was shown to be directly proportional to ion straggling. Below this fluence threshold, no methane molecules were emitted.