COLOUR CENTRES IN NON-ALKALI HALIDES. F CENTERS IN IONIC CRYSTALS: ANALYSIS BY ABSORPTION SPECTRUM MOMENTS (*)

Résumé. — L'analyse par les moments du spectre d'absorption fournit, pour l'état fondamental du centre, le rayon moyen, l'énergie cinétique moyenne et la charge moyenne vue par l'électron. Cette méthode peut être appliquée aux cas des halogénures alcalins et halogénures d'argent; elle fournit également des indications sur la distribution de charge dans l'état fondamental du centre. Abstract. — Analysis by absorption spectrum moments gives the ground state mean radius, mean kinetic energy and mean charge density seen by the electron. This method can be applied to the cases of alkali and silver halides and provides information on the ground state charge distribution in the center. The moments of the absorption coefficient are numerically computed from the experimental absorption band and are compared with their theo-retical expressions. This allows us to determine characteristic parameters of the F-center ground state: mean radius, mean kinetic energy and mean charge density seen by the electron. This procedure can be used to study the ground state of the F center in alkali halides [1] and of a shallow center in silver halides [2]. In this last case, one has to assume that the experimental absorption bands [3] are due to absorption by electrons bound in an electrostatic potential. Let us recall the basic assumptions we need in our analysis by absorption spectrum moments