The luminescence properties of dehydrated silica-and alumina-supported vanadium and chromium oxide catalysts at low loadings at 4.2 K are reported and discussed. It is shown that one luminescent oxc-vanadium (-chromium) species is present on silica surfaces and more than one species on alumina surfaces. The Cr/AIz03 sample does not show luminescence. The monomeric tetrahedral species show vibrational structure in their emission spectra, and their decay times amount to 10-30 ms. These luminescence properties are different from those of similar complexes in the solid state. The discrepancy is ascribed to a different electronic structure of the surface species which localizes the excited state in a short metal-oxygen bond.
The luminescence properties of the Gd3~ion in LiYF 4 and the energy migration among the Gd 3 ions via the 6P 7 2 level and the 617 2 level in the isostructural compound LiGdF4 are reported. The isolated Gd 3~ions in this structure show efficient emission from the 6j level. The non-radiative 6j -* 6P relaxation can be ascribed by a 5-phonon relaxation process, involving 490 cm phonons which are available from the Aṽibrational mode. The decay of the Gd3 ± emission in Li(Y, Gd)F 4 Th 3± can be described by a theory of energy migration developed by Burshtein et a!. Parameters following from these fits indicate that the energy migration among the Gd3 ± ions via the 6 2 level and via the 617 2 level is governed by exchange interaction. Parameters for the Gd3+ Gd3+ and Gd3 + Th3 + transfer rates in this structure are given.
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