Excited states of modified oxygen-deficient centers and Si quantum dots in Gd-implanted silica glasses: emission dynamics and lifetime distributions.

The emission centers and excited state characteristics of silica glasses implanted with Gd ions were studied by time-resolved pulsed cathodoluminescence. It was found that in the process of ion implantation, two types of new emission centers associated with Gd ions as well as Si quantum dots are formed in glassy silica. The distributions of excited states over the lifetime were found for both new centers and Si quantum dots. The nature of dispersion of the emission decay time was discussed in terms of structural disorder in the matrix. Thermal annealing and an increase in the ion fluence lead to the stimulation of the formation of Gd-related new centers and Si quantum dots. The micromechanisms for the formation of new Gd-related centers and two types of Si quantum dots were proposed on the basis of two scenarios for the introduction of Gd ions into the SiO2 network: insertion of Gd into interstitial voids near oxygen-deficient centers and Gd → Si substitution with subsequent expulsion of Si atoms to the interstitial voids. New emission oxygen-deficient centers and quantum dots created by ion-beam technology in silica glasses are of interest for the development of new functional materials for photonics, and micro- and opto-electronics.

A luminescence spectroscopy and theoretical study of 4f-5d transitions of Ce3+ ions in SrAlF5 crystals.

This research is focused on the 4f-5d transitions in Ce(3+) centers doped into tetragonal ß-SrAlF(5) single crystals belonging to the I4(1)/a space group. The presence of four non-equivalent Sr(2+) sites in this compound leads to the appearance of three spectroscopically non-equivalent Ce(3+) luminescence centers, which can be well distinguished using a time-resolved laser spectroscopy technique. All 4f-5d transitions have slightly varying excitation and emission energies with characteristic probabilities resulting in several decay times that can be determined experimentally. One of these centers experiences strong perturbation due to a defect nearby, probably the O(2-) impurity ion substituting for the F(-) ion and acting as a charge compensator as well. Identification of these photoluminescence centers is performed using crystal field calculations. The crystal field parameters are calculated for two identified centers using the structural data for SrAlF(5); diagonalization of the crystal field Hamiltonian results in obtaining the splitting of the Ce(3+) 5d states. This method allows 'regular' unperturbed Ce(3+) centers with selected Sr(2+) sites to be assigned.

Luminescence properties of undoped LiBaAlF6 single crystals.

This paper presents the results of the study of electronic excitations in undoped LiBaAlF(6) single crystals by means of luminescence spectroscopy and complimentary optical methods. The intrinsic emission at 4.2 eV due to self-trapped excitons was identified. The fast nanosecond defect-related luminescence was revealed at 3.0 eV. Both emissions degrade under electron beam irradiation, the most probable reason of which is defect creation introducing an additional non-radiative relaxation channel prohibiting energy transfer to luminescence centers. These defects can be recovered and luminescence intensity restored at higher temperatures (>200 K). The permanent damage by electron beam irradiation results only in overall growth of the absorption coefficient in the whole 1.5-6.5 eV spectral region studied. The analysis of thermally stimulated luminescence glow curves in the temperature range of 5-410 K revealed two shallow charge carrier traps with the activation energies of 0.22 and 0.33 eV, respectively. The luminescence of an impurity peaked at 2.5 eV was found and tentatively assigned to an oxygen-related emission center.

Transformation of the excitation energy in aniondefective corundum.

Some regular features of the excitation energy transformation in anion-defective crystals of alpha-Al2O3 were analysed by the sub-nanosecond spectroscopic method. The samples were excited using synchrotron radiation and high-density pulsed electron beams. It was shown that much of the energy of the ionising radiation absorbed by the crystals was liberated as the luminescence of F- and F-centres immediately at the moment of irradiation. Luminescence of F+-centres dominated in the nanosecond range, while luminescence of F-centres prevailed in the millisecond range. The excitation energy transformation processes and their kinetics were governed by the temperature, the concentration ratio of intrinsic defects in alpha-Al2O3 crystals, and the excitation density.