We have calculated the potential energy curves and reaction pathways for extracting a hydrogen atom from dihydride and monohydride diamond C(100) surfaces under H beam irradiation, by the ab initio pseudopotential method. It was found that hydrogen adsorbed on the dihydride C(100) surface can be extracted by irradiating hydrogen with zero activation energy, and that hydrogen on the monohydride C(100) surface can be extracted by irradiating hydrogen with an activation energy of 0.20 eV and a kinetic energy of 0.22 eV. The value of the kinetic energy is consistent with the experimental conditions. An important finding is that the activation energy of 0.20 eV obtained with the aid of H beam irradiation is quite low compared with the H-2 associative desorption energy of 4.94 eV obtained in a previous study. The growth rate (GR) enhancement due to H beam irradiation observed in an experiment is thus successfully described using ab initio calculations.