We present a detailed atomic-scale analysis of the postdeposition treatment of hydrogenated amorphous silicon (a-Si:H) thin films with H 2 plasmas. The exposure of a-Si:H films to H atoms from a H 2 plasma was studied through molecular-dynamics (MD) simulations of repeated impingement of H atoms with incident energies ranging from 0.04 to 5.0 eV. Structural and chemical characterizations of the H-exposed a-Si:H films was carried out through a detailed analysis of the evolution of the films' Si-Si pair correlation function, Si-Si-Si-Si dihedral angle distribution, structural order parameter, Si-H bond length distributions, as well as film surface composition. The structural evolution of the a-Si:H films upon exposure to H atoms showed that the films crystallize to form nanocrystalline silicon at temperatures over the range of 500-773 K, i.e., much lower than those required for crystallization due to thermal annealing. The MD simulations revealed that during H exposure of a-Si: H the reactions that occur include surface H adsorption, surface H abstraction, etching of surface silicon hydrides, dangling-bond-mediated dissociation of surface hydrides, surface H sputtering/desorption, diffusion of H into the a-Si:H film, and insertion of H into strained Si-Si bonds.