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The ortho-para conversion of interstitial H2 in single-crystalline natural Si (natSi) and 29Si (enrichment 96.2%) is studied by Raman scattering. The conversion process was found to be practically independent of the isotope composition of Si. The characteristic ortho-to-para conversion time at 77 K was found to be 220 ± 35 and 200 ± 35 h for 29Si and natSi, respectively, whereas at room temperature the back conversion occurs with a characteristic time of 8.8 ± 1.5 and 10.5 ± 1.5 h for 29Si and natSi, respectively. These values agree very well with the transition rates found in previous IR absorption studies by Peng et al. for natSi [Phys. Rev. B 80, 125207 (2009)]. Our findings imply that an interaction of H2 with the nuclear spin of nearby 29Si has marginal, if any, effects on the ortho-para conversion rate of molecular hydrogen in silicon.

Hydrogen donors in ZnO and rutile TiO2 are probed by means of photoconductivity and IR absorption. It is shown that the O–H bonds giving rise to the local vibrational modes (LVMs) of interstitial hydrogen at 3611 and 3290 cm−1 in the case of ZnO and TiO2, respectively, also occur in the photoconductivity spectra as Fano resonances. The effects of isotope substitution, concentration, sample thickness, influence of other donors present in both oxides are considered. Based on the shape and frequency of these resonances, it is concluded that the apparent ionization energy of interstitial hydrogen in rutile TiO2 is less than 300 meV. By a direct comparison, we also demonstrate that photoconductive detection of LVMs of defects in thin semiconductor films is superior to the standard IR absorption.