  We demonstrate the use of hydrogen induced changes in the emission of
isoelectric Eu ions, in Mg-doped p-type GaN, as a powerful probe to study the
dynamics of hydrogen movement under electron beam irradiation. We identify,
experimentally, a two-step process in the dissociation of Mg-H complexes and
propose, based on density functional theory, that the presence of minority
carriers and resulting charge states of the hydrogen drives this process.
