Emission and excitation spectra were studied, at temperatures ranging from 18 to 300K, for samples that contained unperturbed M-centers. Various coloration techniques and aging effects were tested in order to obtain pure M-center emission bands. On the basis of the data, new trends and correlations were proposed between the half-width of the emission bands, the effective modes and the Huang-Rhys factors for M and F centers. The emission band values for M centers were roughly half of those for F centers. The Huang-Rhys factors for M-center emission were very similar for various crystals. The effective modes and bands for M- and F-center emissions were proportional to the transverse optical frequency values.

L.Bosi, D.Gallo: Radiation Effects and Defects in Solids, 1995, 134, 461-3