J.O. Rubio

Optical absorption and luminescence spectra of Eu-impurity precipitated in the lattice of NaBr and NaI crystals

Abstract The absorption, excitation, and luminescence spectra associated with the precipitated phases of divalent europium in NaBr and NaI single crystals are discussed. In both systems, three different fluorescence bands were observed in crystals which were stored for five years at room temperature without any previous heat-treatment. By analyzing the emission spectrum of the well-aged crystals as a function of the annealing temperature, it was found that the thermal dissolution of the precipitated phases of Eu2+ in these two crystals takes place in the temperature range 80–500°C. The optical absorption spectrum consists, in all cases, of two broad bands in the UV range with a crystalline field splitting 10 Dq which strongly depends on the aging and the thermal treatments given to the samples.

Thermally stimulated depolarization of Eu2+-cation vacancy dipoles in alkali halide single crystals☆

Abstract Ionic thermocurrent (ITC) measurements have been performed on eight alkali halide single crystals doped with divalent europium. In all cases, the observed ITC peaks were fitted with a mono-energetic model without to appeal to any dipole-dipole interaction. Values for the reorientation parameters have been calculated. The relationship TM1nτ−1 α E previously found for I–V complexes in alkali halides has been found to be very well obeyed for the experimental data obtained in this investigation. It is also reported that the logarithm of the experimentally determined energies for free dipole reorientation shows a linear dependence on the interaction distance between the Eu2+ ion and the surrounding halogen ions in the distorted cubic site occupied by this impurity in the alkali halides.

Effects of X-irradiation in europium-doped NaCl

The first stage of F- and M-centre colouring has been investigated at room temperature in quenched samples of europium-doped NaCl as a function of the impurity concentration in the range 30-500 PPM. Attention has been paid to determine whether a correlation exists between the change in the concentration of Eu2+-cation vacancy dipoles and the colour centre formation in order to get a deeper insight into the nature of the interstitial halogen traps. The effects of x irradiation are (a) to reduce the number of (I-V) dipoles, (b) to increase the number of trapped electrons, (c) to convert the doubly valent europium ions to the monovalent, as well as to the tripositive state, and (d) to increase the rate of aggregation of dipoles. The number of dipoles destroyed by irradiation was found to be equal to or to exceed the sum of the number of europium ions which had changed their valence state and the number of F centres produced. The results presented in this paper suggest that the (I-V) dipoles act as the dominant traps for the radiation-induced interstitials. It was also ascertained that the M-centre density is proportional to the square of the F-centre density, the proportionality constant being clearly dependent on the inverse of the impurity concentration.

X-ray induced precipitation of Eu2+ in the alkali halides

Abstract The stability of the europium second phase precipitates in NaCl, NaBr, KC1, and KBr under room temperature X-irradiation has been analyzed using the photoluminescence technique. The irradiation tends to destroy the pre-existing metastable precipitates creating complexes of a much simpler nature. In parallel with this destruction an enhanced europium precipitation into the stable dihalide phase EuX2 (X=Cl, Br) was found to occur.

Calcium precipitation in NaCl monitored by the Eu2+ -fluorescence

Abstract Taking into account that the optical spectroscopy of the Eu 2+ ion is quite sensitive to the crystalline environment in which this ion is located, in the present investigation the fluorescence of a small concentration (8 ppmm) of divalent europium incorporated into a NaCl crystal which was also doped with ≈4500 ppmm of Ca 2+ has been systematically investigated as a function of different thermal treatments in order to study the calcium-precipitation processes in the host NaCl. The data presented in this paper strongly suggest that the annealing of quenched samples at 200°C produces the incorporation of Eu 2+ into the stable dihalide phase CaCl 2 , as well as into the metastable precipitated CaCl 2 -like plate zones parallel to the {111} planes of the NaCl matrix. This fact is associated with the increase in intensity of two overlapping emission bands peaking at 430 and 432 nm. On the other hand, the aging of quenched samples at room temperature produces the growth of three emission bands peaking at 400, 414, and 447 nm. The former two emission bands have been associated with Eu 2+ embedded into calcium precipitates, the structures of these precipitates being different from that of CaCl 2 , while the band at 447 nm has been ascribed to europium ions incorporated into the metastable precipitated CaCl 2 -like plate zones parallel to the {310} planes of the NaCl matrix. Some of the characteristics of the different calcium second phase precipitates have been obtained by measuring the crystal field splitting (10 Dq) of the 4f 6 5d configuration of the divalent europium ions when they were located inside them. Values for the 10 Dq splitting were determined from the excitation spectrum of each of the emission bands associated with the different calcium-precipitated phases.

Dose rate dependence of first stage coloration of nacl doped with divalent impurities

Abstract The amount of first stage coloration in NaCl doped with Ca2+, Sr2+ Cd2+, or Eu2+ has been investigated as a function of the X-irradiation dose rate. It was ascertained that in all cases the amount of first stage coloration is proportional to the square root of X-irradiation dose rate in agreement with the models recently developed by Comins and Carragher and Aguilar et al for F-center produc tion in the alkali halides doped with divalent impurities.

Influence of the precipitated phases of Pb++ and Sr++ on the f-coloring behavior of NaCl

Abstract The F-coloring efficiencies of NaCl:Pb ++ and NaCl:Sr ++ have been measured during stage I as a function of lead and strontium precipitation into the metastable and stable phases, the optical absorption spectrum of the doped crystals was analyzed after each annealing treatment. On the other hand, strontium precipitation was monitored using Eu ++ as an optical probe taking advantage of the fact that this ion can be incorporated into the Sr-precipitates producing emission bands characteristic of each type of precipitate in which it is embedded. The results indicate that the metastable and stable second phase precipitates of these ions contribute in a quite different manner to the coloring processes.

Energy transfer between europium and lead close pairs in monocrystalline NaCl

Abstract The optical properties of lead and europium doubly doped NaCl single crystals have been analyzed. It is found that energy transfer not only occurs from the Pb2+P1 level to the 4f6 5d (t2g) state of the Eu2+ ions, but also from the 4f6 5d (eg) state of Eu2+ to the 3P1 level of the lead ions. This transfer is observed in samples which have been quenched from high temperatures, as well as for low impurity concentration. The results obtained suggested that the energy transfer occurs between Pb-Eu close pairs. In order to get a better insight into this point, electron paramagnetic resonance measurements were also performed in parallel with the optical ones. The analysis of the Eu-EPR spectra shows that the site symmetry normally occupied by the Eu2+ ions in NaCl is strongly modified by the presence of perturbing neighboring lead ions.

Divalent manganese in an axial symmetry site of additively coloured CaO single crystals

Electron paramagnetic resonance data dealing with the first observation of a Mn2+ axial center in CaO are reported. The tetragonal EPR spectrum was found in additively coloured crystals at 100K the best-fit parameters to the spin-Hamiltonian were found to be : g/sub ///=2.0015 (5), gperpendicular to =2.0020 (5), b20=+132.6 (4)*10-4 cm-1, b40=+3.5 (3)*10-4 cm-1, b44=+11.7 (8)*10-4 cm-1, and A=-67.2 (5)*10-4 cm-1. The hyperfine coupling constant is anomalously small and points towards a considerable covalent bonding. The analysis of the data presented in this paper strongly suggests that the axial spectrum is due to Mn2+ ions perturbed by an adjacent F centre. Assuming this model, the reduction in the value of the hyperfine interaction relative to that of the free ion was estimated in terms of the approach of Simanek and Sroubek which puts the main emphasis of such a reduction on the covalent effects of the 4s orbitals of the Mn2+ ion and the result compares reasonably well with the experimental determination.

Effect of temperature and ionizing radiation on the Eu-Mn pair in NaCl

In the present paper photoluminescence measurements have been carried out in single crystals of NaCl doubly doped with Eu2+ and Mn2+ ions in order to determine the influence of annealing temperature and ionizing radiation on the Eu-Mn pairs which are formed preferentially in this material. From the results obtained it appears that the NaCl:Eu:Mn system may be employed not only as a good device for optical convertion of U.V. light into visible, being sure that its efficiency is not affected by the aggregation-precipitation processes of the manganese ions occurring in the matrix, but also as an efficient photo-luminescence dosimeter.