The dependence of the dispersion parameter α and of the time dependent diffusion parameter DH(tL) (estimated over a fixed diffusion length L) on annealing temperature Ta was investigated with elastic-recoil detection analysis (ERDA) in an undoped a-Si:H sample. We found that the α dark value increases with Ta as already observed by other groups, and that the illuminated α value also increases. But we observe that DH(tL), measured under illumination, deviates from an Arrhenius law, while DH(tL) measured in the dark follows an Arrhenius behavior. The hydrogen diffusion mechanisms under intense illumination are discussed in the framework of existing models.