Marek Grinberg

Substitutional disorder and the optical spectroscopy of gallogermanate crystals

Results of optical absorption, photoluminescence, fluorescence lifetime and fluorescence-line-narrowing studies on several -doped crystals with the gallogermanate crystal structure are presented. Such crystals are unusual in that most fluoresce via both the broad-band transition and the sharper transition (R line), both of which are considerably broadened by the effects of substitutional disorder. An established adiabatic model is used to reproduce the selectively excited broad-band photoluminescence spectra and this shows that the substitutional disorder creates a distribution in the strength of both the octahedral crystal field and the electron - phonon coupling. The variations in electron - phonon coupling are attributed to variations in the strength of the non-octahedral crystal fields that determine the magnitudes of the coupling to non-symmetric phonon modes. This effect is shown to dominate the broadening of the band. The analysis of results of fluorescence-line-narrowing measurements on the R lines shows that these lines are also broadened by a distribution of both octahedral and non-octahedral crystal fields. In the case of crystals that contain a substitutionally disordered site in the close vicinity of the ion, the strong-field sites that give rise to R-line emission are shown to comprise an independent distribution and the broadening is dominated by the effects of the non-octahedral component of the crystal field. When the substitutionally disordered sites are further from the ion, the broad-band and R-line fluorescence derive from a single distribution of crystal fields. In this case the effect of the variation of the crystal field is again shown to be dominated by the non-octahedral contribution though the R-line fluorescence shows little evidence of this due to the strong-field sites being the extreme of the overall distribution.

The influence of substitutional disorder on non-radiative transitions in -doped gallogermanate crystals

The substitutionally disordered nature of gallogermanates creates a distribution of crystal-field strengths and symmetries for Cr 3+ dopant ions. This causes the Huang-Rhys factor of the 4 T 2 → 4 A 2 transition to vary from site to site, leading to a strongly broadened luminescence band. This luminescence is blue-shifted and strongly quenched with increasing temperature. Calculations show that this behaviour is due to large variations in the internal conversion rates for ions in different parts of the crystal-field distribution. It is shown that a confinement potential rather than a harmonic potential is required to account for the data.

Crystal field distribution and non-radiative transitions in Cr3+-doped gallogermanates

The large broadening of the fluorescence transitions in Cr3+-doped gallogermanates is attributed to variations in the distortions to the octahedron of nearest-neighbour oxygen ions. For the weak field sites that fluoresce via the 4T2 → 4A2 transition, this causes the Huang-Rhys factor to vary from site to site. Such a crystal field distribution results in strong broadening of the emission band, and also enhances non-radiative decay processes. The non-radiative decay is described in terms of an internal conversion process and modelling shows a confinement potential to reproduce the experimental data most accurately.

Ultrasensitive quadrature photopyroelectric quantum yield spectroscopy of Ti3+:Al2O3. Evidence for domination of de-excitation mechanism by inter-configurational nonradiative transitions in an unthermalized manifold

Abstract The lock-in quadrature channel of photopyroelectric spectroscopy (PPES) was used in a novel ultrasensitive non-contact configuration to measure Ti 3+ :Al 2 O 3 crystal nonradiative quantum yield bulk spectra, η (b) NR (λ), which were analyzed allowing for both inter- and intra-configurational decay pathways of the Ti 3+ ion in a two-dimensional harmonic oscillator adiabatic model. They were found to be dominated by inter-configurational de-excitation of the highly vibronically excited Ti 3+ ion above cross-over, in clear break with current theoretical practices in unthermalized systems.

Structure of the 2Eg-4A2 emission of the Cr3+: gahnite glass ceramics

The spectroscopic characteristics of chromium-doped gahnite glass ceramics have been evaluated. The material exhibits multi-site spectra, including a complicated emission spectrum and room temperature anti-Stokes emission. Careful analysis of the room and low temperature spectra, supported by luminescence decay measurements, proves the existence of a t least two high-field sites associated with single Cr3+ ions and pairs of Cr3+ ions. The R line of the highest field sites is accompanied by phonon side bands. Estimates of the associated phonon energies have also been made.

Spectroscopic evaluation of titanium-doped glass

We have evaluated a prototype glass doped with titanium from the point of view of its basic spectroscopic characteristics. Measurements of the luminescence lifetime at various wavelengths, luminescence spectra and time resolved luminescence spectra have been performed. Using the obtained characteristics we have attempted to interpret the de-excitation mechanism in the Ti:glass. For sufficiently high excitation energy the luminescence involves the following centers: Ti3+, emitting at 780 nm, Ti4+, emitting at 460 nm and an unidentified luminescence center emitting at approximately 560 nm.

Electron-lattice interaction of localized states in solids

In this contribution we analyze the electron-lattice coupling in a case of localized orbital triplet states using the emission, absorption and the excited state absorption (ESA) spectra of the d degree(s) complexes in CaWO4 and CaMoO4 crystals. We consider coupling to a fully symmetrical one-dimensional mode as well as to (epsilon) and (tau) 2 Jahn-Teller (J-T) modes. Our method allows us to estimate the contribution of the J-T effect and coupling to the full symmetry breathing mode to the overall electron-lattice coupling energy. We also estimate anharmonicity of the lattice vibrations related to the spin-orbit coupling between the components of the luminescing triplet.