  Recent experiments on hydrogenated amorphous silicon using infrared
absorption spectroscopy have indicated the presence of mono- and divacancy in
samples for concentration of up to 14\% hydrogen. Motivated by this
observation, we study the microstructure of hydrogen in two model networks of
hydrogen-rich amorphous silicon with particular emphasis on the nature of the
distribution (of hydrogen), the presence of defects, and the characteristic
features of the nuclear magnetic resonance spectra at low and high
concentration of hydrogen. Our study reveals the presence of vacancies, which
are the built-in features of the model networks. The study also confirms the
presence of various hydride configurations in the networks that include from
silicon monohydrides and dihydrides to open chain-like structures, which have
been observed in the infrared and nuclear magnetic resonance experiments. The
broad and the narrow line widths of the nuclear magnetic resonance spectra are
calculated from a knowledge of the distribution of spins (hydrogen) in the
networks.
