ISOTOPIC EFFECT IN THERMAL REEMISSION OF HYDROGEN FROM GRAPHITE AT ELEVATED-TEMPERATURES

The thermal re-emission of protium and deuterium from graphite, implanted with 3 keV H-2+ or D2+ ion beams up to saturation at room temperature, due to isothermal annealing at temperatures of 450, 500, 550 and 600-degrees-C has been studied by means of the elastic recoil detection technique. It is found that the concentrations of retained protium and deuterium decrease rapidly in the beginning and hereafter very gradually with the increase of the annealing time and that the former decay is faster than the latter. The re-emission profiles have been analysed taking into account thermal detrapping (SIGMA(d)), retrapping (SIGMA(T)), local molecular recombination (K1) and hydrocarbon emission. It is determined that the activation energies of SIGMA(d) for H and D are both equal to 0.60 eV and the isotopic ratio of their pre-exponential factors is 1.4, and that the activation energies of the effective recombination rate constants (K1/C0)(SIGMA(d)/SIGMA(T)2 for H and D are also 1.2 eV (C0 is the trap density) and the isotopic ratio of their pre-exponential factors is about 2.5 on average. The isotopic effects are discussed in terms of elementary processes.