Renata Reisfeld

Judd-Ofelt parameters and chemical bonding

Abstract The three Judd-Ofelt intensity parameters for erbium(III) are given in 17 different environments. The Ω 2 parameter is strongly affected by covalent chemical bonding. In contrast, the Ω 6 parameter is related to the rigidity of the medium in which the ions are situated.

Organic fluorescent dyes trapped in silica and silica-titania thin films by the sol-gel method. Photophysical, film and cage properties

Abstract The sol-gel process is utilized to solve the notorious problem of incorporating an organic dye in an inorganic oxide thin film. Fluorescent thin films are prepared by this low temperature process with the aid of a surface active agent, with good homogeneity and reproducability. A variety of organic fluorescent molecules are embeded either in silica or in silica-titania films. The dye molecules are not leached out by water. Absorption and emission spectra, enhanced photostability, longer lifetimes and energy transfers between the trapped dye molecules are described and discussed in terms of the effects of molecule matrix-isolation on these properties. Potential uses of the special thin films are numerous, e.g., as laser or solar light guides.

Luminescence Quantum Efficiency of Gd and Tb in Borate Glasses and the Mechanism of Energy Transfer between Them

A study of the quantum efficiency of the luminescence of gadolinium and terbium in borate glasses and of the energy transfer between these ions is presented. From our measurements of fluorescence decay times, absorption, fluorescence, and excitation spectra, we calculated the fluorescence quantum yield of the 6P7/2 gadolinium and the 5D4 terbium levels on excitation to higher gadolinium and terbium levels, respectively. The energy transfer rates between gadolinium and terbium in borate glasses were calculated on the basis of the above measurements. These measurements were performed for gadolinium and terbium concentrations ranging from 0 to 4 wt%. Our results show that the energy transfer occurs mainly between the 6P7/2 gadolinium level and (5D0, 5H7) terbium levels and is due to a dipole‐dipole interaction. It is concluded that the energy transfer between gadolinium and terbium is a phonon assisted process.

Europium probe for estimation of site symmetry in glass films, glasses and crystals

The paper is devoted to the memory of Brian Wybourne the pioneer in theory of symmetry and spectra of trivalent rare earths. We evaluate here the influence of site symmetry of the surrounding molecules in glass matrices on europium ion used as a probe. A method by which systematic classification can be performed on descending site symmetry in a large number of solid hosts is based on the ratios of intensities of electric to magnetic dipole transition. Experimental work involves preparation of a number of glass matrices prepared by a sol–gel method, and incorporating trivalent europium. The fluorescence arising from the quintet D state is used to evaluate the site symmetry of the probe europium ion in the prepared matrices, crystals, conventionally prepared glasses, solutions, and complexes incorporated in sol–gel matrices. From the large amount of tabulated data one can see that for totally symmetric sites the factor I(D0–F2)/I(D0–F1) is lower than one, and increases to 10 for systems with very low symmetry sites. The absolute intensities of the transitions depend also on the amount of covalency of Eu with the surrounding ligands.

Spectroscopy and laser action of the “red perylimide dye” in various solvents

Abstract Optical properties of the red perylimide laser dye in various solvents are studied. The absorption spectrum exhibits two main bands, in the ranges 480–600 and 400–460 nm, due to the S0–S1 and S0–S2 transition. The fluorescence spectrum is a mirror image of the S0–S1 absorption (shift of ∼ 30–50 nm). The Stokes shift increases with solvent polarity. Such dye-solvent interactions are compared to theoretical predictions. The fluorescence quantum yields approaches unity in all the solvents studied. Laser tunability around 30 nm was obtained each time, covering the spectral range 580–640 nm. This interval is important for medical applications in photodynamic therapy and fluorescence diagnostics. The laser threshold energy varied from 0.35 mJ/pulse in cyclohexane to 1.87 mJ/pulse in methanol, and the slope efficiency from about 6.6% in methanol to 14% in xylenes. The laser output was stable for several hours of operation under an average pump energy of about 20 mJ/pulse at 1 Hz repetition rate, without flow.

Prospects of sol–gel technology towards luminescent materials

Abstract Sol–gel method for producing glasses or glass films at relatively low temperature allows incorporation of a large number of inorganic and organic additives during the process of glass formation. We outline some of the sophisticated products and devices obtained by the sol–gel method. Luminecence solar concentrators, tunable lasers in the visible part of the spectrum, active waveguides, semiconductor particles and sensors for environmental impurities.

Fluorescence of europium(III) in a flouride glass containing zirconium

Abstract Europium(III) incorporated in a new zirconium barium flouride glass shows 14 luminescent transitions (at 300 K) from 5 D J ( J = 0,1,2,3) to 7 F J ( J = 0,1,2,3,4) which are compared with Judd-Ofelt parameters for induced electric dipolar radiation, as well as for calculated magnetic dipolar transition probabilities.

Fluorescence of europium(III) trapped in silica gel-glass as a probe for cation binding and for changes in cage symmetry during gel dehydration

Abstract The fluorescence of Eu(III) is a sensitive probe for the gel-glass transformation. Silica gel-glass containing Eu(III) was prepared by hydrolysis of tetramethoxysilane. The total fluorescence of EU(III) from the 5 D o electronic state exhibits a gradual increase and a relative increase of intensity of the hypersensitive transition 5 D o → 7 F 2 as a function of time and temperature of dehydration of the gel. This behavior is explained by formation of a chemical bond between Eu(III) and the oxygens of the silicate group whereby Eu(III) acquires a symmetry similar to that in a regular glass obtained by melting the appropriate oxides. The degeneracy of the 7 F 1 and 7 F 2 states is removed in low-symmetry sites in the glass as reflected by the half-width of the emission bands.

Energy transfer from UO22+ to Sm3+ in phosphate glass☆

Abstract Energy transfer from UO22+ to Sm3+ is described. The transfer efficiencies are calculated from the decrease of donor luminescence and lifetimes and from the increase of the acceptor fluorescence. It is shown that the transfer is nonradiative. The energy transfer efficiencies are greater when the donor is excited at higher energy levels due to stronger overlap between electronic levels of donor UO22+ and acceptor Sm3+. From the comparison of energy transfer efficiencies from UO22+ to Sm3+ and Eu3+ it is deduced that the overlap between excitation levels of donor and acceptor is a sufficient condition for the transfer.

The spectroscopic behaviour of rhodamine 6G in polar and non-polar solvents and in thin glass and PMMA films

Abstract Absorption, fluorescence, quantum yields and lifetimes of rhodamine 6G in water, methanol, dichloroethane and chloroform were measured. From a comparison of these quantities with those for thin glass and plastic films conclusions are drawn about dimer formation and the stability of rhodamine 6G in these materials. The stability in the films is increased as compared with the solutions.