Redman, Michael James (1971) A study of coloration and bleaching phenomena in alkali halides and sodalite. PhD thesis, University of Warwick.

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Abstract

Experiments are described in this thesis on the optical bleaching at room temperature of F centres in freshly cleaved single crystal KBr, coloured by fast electron (~50 KV) irradiation and illuminated with light from a focused helium-neon laser. Using a specially constructed microspectrophotometer bleaching apparatus, which is described in Chapter Two, data was obtained on optical bleaching at illuminating power densities up to 10 KW.cm-2 in KBr having initial F centres concentrations of 10(18) - 10(19) centres cm.-3. The bleaching light intensity is some 10(6) times greater than that used in previous experiments, and the initial defect concentrations produced by electron irradiation are considerably higher than those found in the additively coloured specimens used in earlier experiments. Essentially, there have been no previous studies made on the optical bleaching of electron irradiated alkalil halide, and very little work has been done on the laser bleaching of this material coloured by any of the other methods possible.

It is found that KBr which has been coloured by electron irradiation at room temperature bleaches by the random aggregation of F centres, a process which also occurs in additively coloured material. Prolonged bleaching, to absorbed doses of >4x10(8) J.oa-3, does not result in a complete disappearance of absorption in the F band region, but leads to the formation of a residual coloration which is due to excited states of the M and R centres. A technique of optical modulation spectroscopy described in Chapter Five, showed that R+ centres are formed during illumination, which points to the importance of the F +o<∝ -> M+ and M+∝ -> R+ reactions in the aggregation process.

In marked contrast, a vacancy interstitial recombination mechanism occurs in KBr which has been coloured by electron irradiation at 90 K warmed in the dark to room temperature for bleaching. There is a direct proportionality between the height of the V band and the total number of vacancies present in F, M and R centre throughout the optical bleaching ot KBr coloured at low temperature. Prolonged bleaching leads to the complete disappearance of the F, M, R and V bands, and the optical absorption spectrum of the crystal is similar to that seen prior to coloration.

The atomic mechanisms responsible for bleaching under illumination at room temperature in KBr coloured additively, and by fast electron irradiation at room and liquid nitrogen temperatures, are discussed at length in Chapters Four and Five.

Chapter Six describes investigations into the coloration and bleaching properties of the sodalites using a dark trace cathode ray tube with an aluminised chlorosodalite powder screen. Coloration curves are given which show the increase in F centre concentration with electron dose during the reversible photochromic stage o£ coloration. Optical bleaching measurements are described for a number of bleaching sources, ranging from a low intensity microscope illuminator to a 1000 joule xenon flash tube. The fatigue observed in the sodalites is discussed, and its connection with changes in the cathodoluminescence spectrum noted.

The final chapter summarises optical bleaching phenomena in both electron irradiated KBr and sodalite, and makes comparisons, where appropriate, between the two materials. Some suggestions are a1so made for further experiments. In conclusion, a number of important technological applications of colour centre phenomena are described and discussed.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics
Library of Congress Subject Headings (LCSH):	Optical brighteners, Alkali metal halide crystals
Official Date:	1971
Dates:	Date Event 1971 Submitted
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Tubbs, M. R.
Sponsors:	Science Research Council (Great Britain) ; Ferranti Ltd.
Extent:	145 leaves : illustrations (black and white), charts
Language:	eng

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