M-centre luminescence in NaF and LiF: Towards a comprehensive interpretation of the M-centre emission properties in alkali halides

Emission and excitation spectra were accurately detected as a function of the temperature (in the range 18-300 K) in NaF and LiF samples containing unperturbed M-centres: different coloration techniques and ageing effects were tested in order to obtain the pure M-emission band. On the basis of collected data new trends and correlations are proposed between the halfwidth of the emission bands (H), the effective modes (omega), and the Huang-Rhys factors (S) for M and F centres: the H(M) values are roughly half the H(F) ones, M-emission S values are very similar in different crystals, omega and H values for the M- and F-emissions are proportional to the transverse optical frequency omega(T) values.