Clustering of molecular hydrogen in fused silica

Molecular hydrogen H2 in glass optical fibers has been detected calorimetrically at the 50 ppm level by monitoring the ortho-para conversion of H2 at low temperatures (T<1 K). The number of H2 molecules is determined and information about the local configuration can be inferred. It is concluded that the majority of the H2 is located in strings or very small clusters of two or three molecules each, rather than being uniformly distributed as isolated molecules.