Optical Formation of F-Aggregate Centers in KCl

and Cd have been irradiated with X-rays for various periods and bleached with F-light a t room temperature. The resulting decay of F-centres and changes in the Mand R-centre concentrations were studied. Coand Ba-doped crystals X-irradiated into the second stage of coloration have been found to exhibit, when compared to undoped crystals, (1) greater stability of F-, M-, and R-centres against F-light bleaching, and (2) higher F -M conversion efficiency. Cd-doped KCl showed a rapid bleaching of F-centres without any increase in the M-centre concentration. The role of impurities in the vacancy and electronic processes which control the bleaching and aggregation of F-centres is discussed.Undotierte KC1-Kristalle und KC1-Kristalle, die mit Co, Ba und Cd dotiert waren, wurden rnit Rontgenstrahlen unterschiedlich lange bestrahlt und mit F-Licht. bei Zimmertemperatur gebleicht. Der resultierende Zerfall von F-Zentren und die Anderungen der Mund R-Zentrenkonzentration wurden untersucht. Es wurde gefunden, daD irn VergIeich z u undotierten Kristallen Co-und Ba-dotierte Kristalle, die im zweiten Zustand der Verfarbung bestrahlt wiirden, (1) groDere Stabilitat der F-, Mund R-Zentren gegen F-Lichttilgung und (2) hohere F + M-Konversionuausbeute zeigen. Ca-dotiertes KCI zeigt eine starke Tilgung der F-Zentren ohne jeden Anstieg in der M-Zentrenkonzentrattion. Die R o b der Defekte bei den Leerstellen-und elektronischen Prozessen, die des Bleichen und die Aggregatbildung von F-Zentren kontrollieren, werden diskutiert.

Section: Introductionmentioning

confidence: 98%

Optical bleaching and F → M conversion in doped KCl crystals

Srinivasan¹,

Sastry²

1972

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

Nosenzo

Reguzzoni

Samoggia

1970

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

On the Formation of Aggregate Centres by Optical Bleaching of Electrolytically Coloured KCl Crystals

Pandey

Bose

1973

The decay of F centers and the growths of F3(R), F4(N) centres in electrolytically coloured KCl crystals under F light stimulation have been found to exhibit a two stage process; the fast exponential change is followed by a slow linear change. Appropriate mechanisms and rate determining processes have been suggested to explain the transformation of F centres to higher aggregate centres. It has been possible from the present measurements to estimate the oscillator strengths for the R1 and R2 centres. The dissolving action of X‐rays on F aggregate centres has been noticed which is attributed to the interaction of Cl°, produced by X‐rays, with the existing aggregate centres.