OH⁻ Dissociation and U₂ Decomposition in Hydroxide‐Doped KCl and KBr Crystals

Abstract

We report quantitative results concerning the dissociation of OH-, supporting the conception given first by Kerkhoff. U, centres created by this reaction are thernially stable below about 100 "K. It is shown that U, centres can be destroyed by optical bleaching. A t 4.5 "K, U and H centres are created simultaneously; this is described as h v ( n z ) HP,, (U,) -+ H; (U) + halogen; ,(H) .LVc suggest for the underlying mechanism that the first process after U, excitation is a noncubic relaxation of the lattice (static Jahn-Teller effect) followed by a (110) replacement sequence as described by Williams to form a H-centre-vacancy pair, the vacancy finally being occupied by the Hion. -In KCI a reverse reaction can be induced either by thermal activation or by bleaching with H light. -Oscillator strengths of the U,, H, and OHbands are calculated from the results using the Smakula equation.Die quantitativen Ergebnisse dieser Arbeit uber die Dissoziation von OH--Ionen stutzcn eine erstmalig von Kerkhoff angegebene Reaktionsgleichung. Die dabei erzeugten U,-Zcntren sind unterhalb etwa 100 OK thermisch stabil. Es wird gezeigt, daB U,-Zentren durch optisches Bleichen zerstort werden konnen. Bei 4,5 " K entstehen dabei paarweise U-und H-Zentren. d. h.Als Erzeugungsmechanismus schlagen wir vor : Nach der U,-Anregung erfolgt zunachst eine nichtkubische Relaxation des Gitters (statischer Jahn-Teller-Effekt). Anschliefiend fuhrt ein von Williams beschriebener ProzeB iiber eine (1 10)-Verschiebungssequenz zur Erzeugung eines Paares H-Zentrum und Anionliicke. Das H--Ion stabilisiert sich schliel3lich in der Liicke als U-Zentrum. -I n KCl kann eine Riickreaktion entweder durch Zufuhrung thermischer Energie oder durch optisches Bleichen mit H-Licht ausgelost werden. ~ Unter Zugrundelegung der Smakula-Gleichung werden aus den Ergebnissen die Oszillatorenstarken der U2-, H-iind OH --Bande errechnet.

ESR Detection of H Centre Generation by Photodestruction of U2 Centres in KCl

“…. It was reported by Kurz (1,2) that by irradiation into the UV absorption band of U2 centres in KC1 at 4.5 OK it i s possible to destroy the U band and to create two absorption bands, which a r e identical with those of be observed in the ESR spectrum (Fig. 1).…”

“…. It was reported by Kurz (1,2) that by irradiation into the UV absorption band of U2 centres in KC1 at 4.5 OK it i s possible to destroy the U band and to create two absorption bands, which a r e identical with those of U centres (H-ions on C1-positions, H i ) and H centres (hole centre, C1-on C1-Cii, s). The photodestruction of U centres was described by Kurz a s Subsequently the crystal was irradiated in the U2 band (peak 5.27 eV) with filtered light (filter W-R-250, Schott) for 14 h , resulting in a 30% bleaching of the U2 centres.…”

Generation of dichroic h centres in KCl by polarized optical bleaching of interstitial atomic hydrogen

“…HP centres were produced by low-temperature photodecomposition of OHcentres [lo, 111 using a deuterium lamp and a 204 nm interference filter for selective irradiation in order to avoid the simultaneous production of further centres [7]. The Czochralski grown KC1 crystals were doped in the melt with about 2 x ppm OHand 2 to 10 x 10-2 ppm Brfor HP centres in a mixed configuration with one Brand three C1-as nearest halogen neighbours [12].…”

Coloration of KCl: OH^- by ultraviolet light

“…Approximately 6 x 10 hydrogen molecules must be present 16 in the cryst l along with 0-centers and isolated anion vacancies.…”