Molecular hydrogen in amorphous Si: NMR studies

The NMR signal, a Pake doublet, due to orientationally ordered molecular hydrogen has been observed in hydrogenated amorphous silicon and provides a quantitative determination of many of the properties of this system of quantum rotors in an a-Si matrix. The molecular H₂ content is found to be 0.25 at. % in as-deposited material and to reside in voids with a mean radius of order 25 Å. Both these parameters are increased significantly by annealing the sample. In both cases, the H₂ orientationally orders and solidifies at temperature significantly higher than in pure H₂, due to the electric field gradients and internal pressure exerted on the H₂ by the a-Si container.