  We reveal the hydrogen isotope effect of three chemical reactions, i.e, the
reflection, the absorption and the penetration ratios, by classical molecular
dynamics simulation with a modified Brenner's reactive empirical bond order
(REBO) potential potential. We find that the reflection by pi-electron does not
depend on the mass of the incident isotope, but the peak of the reflection by
nuclear moves to higher side of incident energy. In addition to the reflection,
we also find that the absorption ratio in the positive z side of the graphene
becomes larger, as the mass of the incident isotope becomes larger. On the
other hand, the absorption ratio in the negative z side of the graphene becomes
smaller. Last, it is found that the penetration ratio does not depend on the
mass of the incident isotope because the graphene potential is not affected by
the mass.
