Molecular hydrogen in $a$-Si: H

The past few years have seen a number of studies involving the presence of molecular hydrogen trapped in $a$-Si: H. The main techniques used have been nuclear magnetic resonance, calorimetry, and infrared absorption. The results suggest that ${\mathrm{H}}_2$ is trapped in a number of samples in microbubbles (≈ 10 Å) under high pressure (≈ 2 kbar) at room temperature, solidifies around 25 K, and undergoes an orientational phase transition around 15 K. The data are reviewed here, and the effects that such ${\mathrm{H}}_2$ microbubbles may have on the properties of $a$-Si: H are discussed.