Fluorescence of MgO:Cr3+Ions in Noncubic Sites

In addition to the $R$ line at 6981 Å, the fluorescence spectrum of MgO:${\mathrm{Cr}}^{3+}$ contains a number of sharp lines in the region between 6981 and 7040 Å. The strongest of these are identified, by their splitting under axial stress, with ions in sites of tetragonal symmetry. Such tetragonal symmetry at the ion site can be produced by an adjacent magnesium-ion vacancy. The vacancy also produces some hemihedral field at the chromium ion, and so the transitions from these chromium ions need not be purely magnetic-dipole. They are found to have also some electric-dipole character. Lines from pairs of chromium ions, with a small exchange interaction, are also identified.