Fine Structure in the Optical Spectra of Divalent Europium in the Alkaline Earth Sulfates

Abstract

Fine structure has been observed in the fluorescence and excitation spectra of Eu +2 in CaSO4 and in the fluorescence spectrum of Eu +~ in BaN~g(SO4)2. The fine structure in the fluorescence of Eu +2 in CaSO4 is identified as a zero phonon line with many sharp phonon replicas. The fine structure in its excitation spectrum is due to the closely spaced energy levels in the excited state produced by a strong exchange interaction splitting of the 4f65d states. The fine structure in the fluorescence of Eu +2 in BaMg (SO4)2 is due to the normal splitting of the 4ff excited state and the dominance of 4ff ~ 4ff transitions.The alkaline earth sulfate compounds activated with Eu 2+ generally exhibit broad, featureless fluorescence and excitation spectra. Their decay times are relatively short (~0.1 ~ec) and the resulting fluorescence is located in the near ultraviolet. These are familiar characteristics that are common to a great many other Eu +2 activated phosphors. The fast decay has been attributed to the fluorescence being an "allowed" f6d ~ f7 transition. The broad featureless fluorescence envelopes are accounted for by a large Stokes' shift due to the different coupling to the lattice modes of the excited f6d states relative to the U ground state.In recent years, it has become apparent that in certain hosts Eu +2 can exhibit much more interesting optical spectra characterized by sharp fluorescence and/ or absorption lines. The presence of such sharp line spectra has caused some detailed experimental measurements and theoretical analyses to be made on these systems. There are four types of such spectra which have been reported to date: (type 1) sharp structure in the f6d--> ff transitions in both absorption and fluorescence due to local vibrational modes, (type 2) sharp structure in the ffd --> ff fluorescence and absorption transitions due to selective coupling to certain lattice modes and/or to structure in the phonon density of states of the lattice modes, (type 3) sharp structure in absorption due to a splitting of the f6 levels by interaction with the single 5d electron and the crystal field, and (type 4) sharp structure in fluorescence due to fT-> f7 transitions in the special case where the lowest lying excited state is an f7 state rather than an f6d.Type 1 spectra are exhibited by Eu +2 in the alkali halides (1) where the Eu +2 is substituting for a monovalent ion and an adjacent compensating vacancy forms. The high energy and low damping of the local vibrational modes about the vacancy give sharp structure to both the optical fluorescence and absorption spectra. Types 2 and 3 spectra have been reported for Eu +2 in calcium fluoride (2, 3) where a zero phonon line accompanied by sharp vibrational sidebands have been observed in fluorescence while some sharp lines can be seen in absorption. Type 4 spectra have been reported for Eu +2 only in the alkaline earth aluminum fluorides (4, 5) and in potassium and sodium magnesium fluoride (6).We have examined Eu 2+ in the alkaline earth sulfates. We have observed shar...

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A ligand field theory-based methodology for the characterization of the Eu 2+ [Xe]4f 6 5d 1 excited states in solid state compounds

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