  Magnesium hydride has great potential as a solid hydrogen (H) storage
material because of its high H storage capacity of 7.6 wt%. However, its slow
hydrogenation and dehydrogenation kinetics and the high temperature of 300
$^\circ$C required for decomposition are major obstacles to small-scale
applications such as automobiles. The local electronic structure of
interstitial H in MgH$_2$ is an important fundamental knowledge in solving this
problem, which has been studied mainly based on density functional theory
(DFT). However, few experimental studies have been performed to assess the
results of DFT calculations. We have therefore introduced muon (Mu) as pseudo-H
into MgH$_2$ and investigated the corresponding interstitial H states by
analyzing their electronic and dynamical properties in detail. As a result, we
observed multiple Mu states similar to those observed in wide-gap oxides, and
found that their electronic states can be attributed to relaxed-excited states
associated with donor/acceptor levels predicted by the recently proposed
"ambipolarity model". This provides an indirect support for the DFT
calculations on which the model is based via the donor/acceptor levels. An
important implication of the muon results for improved hydrogen kinetics is
that dehydrogenation, serving as a $reduction$ for hydrides, stabilizes the
interstitial H$^-$ state.
