FH Center Research Articles

Enhancement in gamma-ray induced decomposition of sodium and potassium nitrates by borate and cyanide additives

Gamma — radiolytic decomposition of sodium and potassium nitrates and its admixtures with respective cyanide and borate additives has been studied over a wide absorbed dose range from 675 to 500 kGy. The decomposition of nitrate increases with the nature and concentration of the additive in the admixture. The enhancement is more significant at >80 mol% of the additive.G(NO2−) values, calculated on the basis of electron fraction of the nitrate salt, decrease with increasing concentration of the nitrate. ESR spectral studies suggest the formation of radical species such as BO4 and BO32− etc, in borates whereas in case of cyanide additive FH centres are produced. The radical species and colour centres so produced may then transfer their energy to nitrate and cause enhancement in decomposition. A comparison with other oxyanion additives shows thatG(NO2−) values decrease in the order PO43−>B4O72−>SO42−>CO32−. Similarly, the nature of the cation also affects the decomposition.

Influence of OH- impurities on the relaxation of F centers studied with picosecond optical pulses

Abstract The relaxation of the two configurations of the FH(OH−) center in KBr is studied with a time-resolved induced transparency technique. Multiple decay components are observed. The slowest of these is interpreted as conversion in the ground state between the red and blue configuration of the FH center, on a longer than nanosecond time scale. A contribution of the order of 100 ps might be related to non-radiative electronic relaxation, but cannot be identified with certainty. An unresolved transient, faster than 10 ps, might be related to a cross-over process.

FH(O2−) Centers in Calcium Fluoride Crystals

AbstractThe spatial structure of FH(O2−) centers in CaF2 is studied by optical dichroism and ESR methods. The dichroism of absorption is induced by light in both FH bands in additively and X‐ray colored samples. The results are correlated with the C2 crystal axes of FH defects. The ESR spectrum of FH centers is rather similar to that of pure F centers. Both dichroism and ESR results prove that an oxygen ion occupies a (110) anion site around the F center.

Spin coupling and memory effects in the optical cycle of FH centers involving electron-vibrational energy transfer

Spin-coupling and spin memory effects of FH(OH−) and FH(CN-) centers have been studied in various hosts at 2 K. The spin-memory loss ϵ, i.e. the probability of spin-flip during a complete electronic excitation and relaxation cycle, has been determined. The main result is a strong increase of the spin-memory loss ϵ for FH(CN-) in CsCl and CsBr compared to F centers, while ϵ remains nearly unchanged (only a slight decrease) for FH(OH-) centers in KCl and KBr. First attempts will be made to explain these spin-memory loss results in correlation to the electron relaxation and electron-vibrational energy transfer processes occuring in the same optical cycle.

Thermoluminescence and Optical Absorption of NaBr:S- Crystals

Thermoluminescence and optical absorption in NaBr:S- crystals have been studied in the temperature range between 80 and 300 K. The kinetics of formation and decomposition of the so-called FH center, which consists of an F center associated with a neighboring sulfur ion, has been investigated in relation to thermoluminescence. It has been found that the thermoluminescence glow peak, which has an emission band with a peak at 815 nm (1.52 eV), appears at 230±3 K and is strongly concerned with recombination of the electron of the FH center. Such a model has been supported by observing the change in the optical absorption due to the S- centers and the FH centers.

Molecular Model Parameters for the Saddle‐Point Configuration of F, FA, FB, and FH Centres in the Alkali Halides

AbstractA molecular model for the saddle‐point configuration of F‐type centres, which previously led to the correct prediction of type II emission for FA centres in RbF:Li and RbF:Na, based on an empirical parameter Z, is extended to other impurity F centres in the alkali halides. CsF:Li and CsF:Na are predicted to have type II FA centres, and a number of triangular FB centres in the NaCl structure are also predicted to be type II, particularly in materials with small host anions combined with Li+ or Na+ impurities. Materials having small Z values, presumed to favour type II behaviour, are also identified, for FH centres with large host anions and H−; or F−; impurities, and for FA and triangular FB centres in the CsCl structure containing Li+ or Na+ impurities, where the FB centre which has impurities on adjacent corners of a unit‐cube face is strongly favoured.

Ion‐Size Effects on the Energy Levels of F, FA, and FH Centers in Cesium Halides

AbstractThe energy levels of F centers in cesium halides are calculated variationally using the point‐ion lattice approximation with different sets of trial wavefunctions for the ground and the excited states. The effect of the finite size of the lattice ions on these energy levels is then evaluated self‐consistently within the framework of a local model pseudopotential approach. This method is then used to obtain the changes in the ion‐size effects induced by a substitutional monovalent cation impurity in a nearest neighbour position — the FA center — or a substitutional monovalent anion impurity in the second neighbor position — the FH center. In each case the ion‐size effects are calculated for all the trial functions considered. The calculated transition energies are compared with the observed positions of the corresponding optical absorption bands. It is concluded that the results of optical measurements on all the three centers can be accounted for, at least qualitatively, on the basis of a reasonably simple theory.