The influence of exchange interaction between paramagnetic ions on the luminescence properties is investigated for Mn 2ϩ-Mn 2ϩ , Cr 3ϩ-Cr 3ϩ , Cr 3ϩ-Gd 3ϩ , and Mn 2ϩ-Gd 3ϩ pairs. Two effects are reported: shortening of the lifetime of the spin-forbidden emission and a shift of emission lines or bands to a longer wavelength. The shortening of the lifetime of the spin-forbidden emission is important in Mn 2ϩ doped phosphors. For application of Mn 2ϩ-doped phosphors in displays the long lifetime of the Mn 2ϩ emission is a problem. Shortening of the lifetime is possible by exchange coupling with Mn 2ϩ neighbors, or possibly other paramagnetic neighbors. The results reported here show that the lifetime shortening depends on the exchange-coupling parameter J. The strongest coupling is observed in Cr 3ϩ-Cr 3ϩ pairs and the lifetime shortening is strong ͑e.g., in LaAlO 3 :Cr 3ϩ J ϭ 63 cm Ϫ1 and the lifetime is about 30 times shorter for the pair emission͒. The coupling in Mn 2ϩ-Mn 2ϩ and Cr 3ϩ-Gd 3ϩ pairs is weak. Typically, values for J are around 1 cm Ϫ1 and the lifetime is reduced by a factor of 2-10 ͑e.g., J is Ϫ1.0 cm Ϫ1 for exchange coupling between Cr 3ϩ and Gd 3ϩ ; the lifetime of the spin-forbidden Cr 3ϩ emission is 14 ms in GdAlO 3 compared to 56 ms in LaAlO 3 ͒. The lifetime shortening for the Mn 2ϩ emission is not related to the observed red shift for the pair emission. For the Mn 2ϩ-Gd 3ϩ pair the exchange interaction is very weak and no significant lifetime shortening could be measured for Gd 3ϩ-Mn 2ϩ pairs ͑24 ms for the Mn 2ϩ emission in YF 3 vs. 22 ms in GdF 3 ͒.
The electron-phonon coupling for Cr3+ in different oxide host lattices
(MgO, α-Al2O3 and LaAlO3) was probed by performing temperature
dependent line broadening measurements for zero-phonon emission lines of single
ions and exchange coupled pairs. No differences in electron-phonon coupling
were found comparing the results for pairs and single ions in
α-Al2O3 (3NN and 4NN pairs) and LaAlO3 (NN pairs). In Cr3+ doped MgO
(Cr3+-VMg-Cr3+ pairs), it was found that the electron-phonon
coupling strength parameter ᾱ is larger for the pairs than for the
isolated Cr3+ ions. This is probably due to changes in the surroundings of
Cr3+ by local charge compensation by a magnesium vacancy, and not to
interaction between the Cr3+ ions.
ESR spectra of semiquinone radicals, formed as intermediary reduction products in reversibly reducible thiazine and oxazine dyestuffs, have been investigated in acid as well as in alkaline solution.
In acid solutions at relatively high concentration of the dye the spectra consist of four lines. In alkaline solutions at high concentration there are three lines. At lower concentrations most spectra could be resolved further into many components.
A tentative interpretation of the spectra is given.
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