Hydrogen molecules in semiconductors

Molecular hydrogen, the simplest of all molecules, allows a direct insight into the fundamental properties of quantum mechanics. In the case of H2, the Pauli principle leads to two different species, para-H2 and ortho-H2. A conversion between these species is prohibited. Vibrational mode spectra reflect the fundamental properties and allow an unambiguous identification of the H2 molecules. Today, we have experimental evidence for the trapping of hydrogen molecules in the semiconductors Si, Ge and GaAs at the interstitial sites, within hydrogen-induced platelets, in voids and at impurities (interstitial oxygen in Si). Interstitial H2 is a nearly free rotor with a surprisingly simple behavior. We review on interstitial H2 in semiconductors and report on the unexpected preferential disappearance of the para-H2 or ortho-D2 species. The origin of the detected ortho-para conversion will be discussed