New Evidence for F′‐Type and Ionized F‐Aggregate Centres

Abstract

If the ionization probability of an optically excited defect is varied periodically by means of an external electric field of frequency v, a corresponding modulation in the absorption coefficient in the b m d s associated either with the neutral centre or with the ionized and F -t y p e centres can be achieved. In fact the results obtained on M-and R-centres in KC1and KBr crystals ah liquid nitrogen temperature give new evidence of the existence of absorption bands associated with M+-, R+-, M -, and R-centres. A new set of absorption bands has been observed amociated with ionized R-centres and ascribed to electronic transitions to different excited states. These bands are located a t 0.94, 0.76, 0.54, and 0.49 pm in KCI and a t 1.01 and 0.89 pm in KBr. Besides the bands due to ionized N,-centres, a n absorption band has been observed in the infrared, which has been tentatively attributed to N;-centres.Si l'on change pkrjodiquement la probabilitk de ionisation d'un dkfaut optiquement excite au moyen d'un champ klectrique externe ayant la frkquence v, on peut observer une modulation correspondante dans le coefficient de l'absorption des bandes associkes soit avec les centres neutres soit avec les centres ionisks e t les centres qui ont capturk un klectron. E n effet, les rksultats obtenus: L la tempkrature de l'azote liquide pour les centres M e t R avec des