Non-covalent interactions
A hydrogen bond (H-bond), is a specific type of interaction that involves dipole-dipole attraction between a partially positive hydrogen atom and a highly electronegative, partially negative oxygen, nitrogen, sulfur, or fluorine atom (not covalently bound to said hydrogen atom). It is not a covalent bond, but instead is classified as a strong non-covalent interaction. It is responsible for why water is a liquid at room temperature and not a gas (given water's low molecular weight). Most commonly, the strength of hydrogen bonds lies between 0 - 4 kcal/mol, but can sometimes be as strong as 40 kcal/mol In solvents such as chloroform or carbontetrachloride one observes e.g. for the interaction between amides additive values of about 5 kJ/mol. According to L. Pauling the strength of a hydrogen bond is essentially determined by the electrostatic charges. Measurements of thousands complexes in chloroform or carbontetrachloride have led to additive free energy increments for all kind of donor-acceptor combinations.
Staebler–Wronski effect
Historically, the most favored model has been the hydrogen bond switching model. It proposes that an electron-hole pair formed by the incident light may recombine near a weak Si–Si bond, releasing energy sufficient to break the bond. A neighbouring H atom then forms a new bond with one of the Si atoms, leaving a dangling bond. These dangling bonds can trap electron-hole pairs, thus reducing the current that can pass through. However, new experimental evidence is casting doubt on this model. More recently, the "H collision model" proposed that two spatially separated recombination events cause emission of mobile hydrogen from Si–H bonds to form two dangling bonds, with a metastable paired H state binding the hydrogen atoms at a distant site.