  Interstitial magnesium acts as a moderately deep double donor in silicon, and
is relatively easily introduced by diffusion. Unlike the case of the chalcogen
double donors, the binding energies of the even-parity valley-orbit excited
states 1sT$_2$ and 1sE have remained elusive. Here we report on temperature
dependence absorption measurements focusing on the neutral charge species. Our
results demonstrate thermal activation from the ground state 1sA to the
valley-orbit states, as observed by transitions from the thermally populated
levels to the odd-parity states 2p$_0$ and 2p$_{\pm}$
