LAUSR

The fundamental importance of semiconductor materials for modern microand nanoelectronics, photovoltaics, optoelectronics, microsensorics, together with a significant impact of hydrogen on their physical and chemical properties [1] as well as a key role of the hydrogen fuel cells for the future energy sources explain practical interest in the behavior of hydrogen in semiconductors. The interstitial H2 molecule is one of the most interesting hydrogen defect complexes occurring in these materials (e.g., see [2–8]), both from practical and academic viewpoints. The total nuclear spin of the molecule can take two values: I = 0 (para-hydrogen) and I = 1 (ortho-hydrogen). Without an external perturbation the transition time between the two states exceeds the age of the Universe [9]. A magnetic field caused by, for example, neighboring paramagnetic atoms or molecules [10, 11] renders the transitions between the ortho and para states allowed. For instance, an interaction of H2 with magnetic [12, 13], metallic [14], and dielectric [15] surfaces, as well as the with semiconductor crystal lattice [16, 17], results in the thermodynamic equilibrium of the H2 nuclear spins states. The conversion rate depends on the nature and strength of the interaction between hydrogen and matter. Physical mechanisms responsible for the ortho-para transitions of H2 were considered already in the early 20th century (e.g., see [10]). In the presence of inhomogeneous magnetic field sources (unpaired nuclear and/or electron spins) the main mechanism of the ortho-para conversion is a dipole-dipole interaction [10–13]. Without magnetic fields (metals, dielectrics) the ortho-para transition is accounted for by the exchange interaction between hydrogen and the host electrons [14, 15]. Despite the significant progress made over the past decade, there is still no clear understanding of the mechanisms responsible for the low-temperature ortho-para conversion of H2 in single-crystalline semiconductors [16– 20] (see [21] for a detailed discussion of the possible scenarios). The results of an analysis and ab initio calculations point to a dominant channel of the ortho-para conversion. A paramagnetic spin catalysis mechanism for the ortho-para conversion of the molecular hydrogen was suggested in [22]. It is realized by the introduction of unpaired electrons into the system activated, for example, at the H2 + O2 collision. In this case, the electron spin states of H2 become separated due to the mixing of the lowest triplet state 3 u b   with the activator ground state thus changing the symmetry of the electronic wavefunction which contributions localized in the vicinity of the hydrogen nuclei. As a result, an effective Fermi contact interaction between the electron and the proton takes place thereby binding ortho and para states of molecular hydrogen. Moreover, taking the H2 + O2 system as a benchmark it was shown that this mechanism is by two orders of magnitude stronger compared to the conventional inhomogeneous magnetic field model [22].