Luminescence spectra of NiO have been investigated under vacuum ultraviolet (VUV) and soft x-ray (XUV) excitation (DESY, Hamburg). Photoluminescence (PL) spectra show broad emission violet and green bands centered at about 3.2 and 2.6 eV, respectively. The PL excitation (PLE) spectral evolution and lifetime measurements reveal that the two mechanisms with short and long decay times, attributed to the d(e g )-d(e g ) and p(π )-d charge transfer (CT) transitions in the range 4-6 eV, respectively, are responsible for the observed emissions. The XUV excitation makes it possible to avoid the predominant role of the surface effects in luminescence and reveals a bulk violet luminescence with a puzzling well-isolated doublet of very narrow lines. These lines with close energies near 3.3 eV are attributed to recombination transitions in the self-trapped d-d CT excitons formed by the coupled Jahn-Teller Ni + and Ni 3+ centers. The conclusion is supported by a comparative analysis of the luminescence spectra for NiO and solid solution Ni x Zn 1−x O and by a comprehensive cluster model assignment of different p-d and d-d CT transitions and their relaxation channels. Our paper shows that the time-resolved luminescence measurements provide an instructive tool for the elucidation of the p-d and d-d CT excitations and their relaxation in 3d oxides.

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The investigation of luminescent and scintillation properties of 6 Li 2 0-MgO-SiO 2-Ce fibers is presented. The probable mechanisms of energy transfer to activator centers are discussed. The estimation of fibers absolute scintillation efficiency was implemented. The modeling of neutron registration efficiency in substitution of monocrystalline detector by a fiber one was carried out.

Abstract. Comparative analysis of photoluminescence (PL) and photoluminescence excitation (PLE) spectra of NiO poly-and nanocrystals in the spectral range 2-5.5 eV reveals two PLE bands peaked near 3.7 and 4.6 eV with a dramatic rise in the low-temperature PLE spectral weight of the 3.7 eV PLE band in the nanocrystalline NiO as compared with its polycrystalline counterpart. In frames of a cluster model approach we assign the 3.7 eV PLE band to the low-energy bulk-forbidden p-d (t1g(π)-eg) charge transfer (CT) transition which becomes the allowed one in the nanocrystalline state while the 4.6 eV PLE band is related to a bulk allowed d -d (eg-eg) CT transition scarcely susceptible to the nanocrystallization. The PLE spectroscopy of the nanocrystalline materials appears to be a novel informative technique for inspection of different CT transitions. IntroductionDespite several decades of studies there is still no literature consensus on the detailed electronic structure of the prototype 3d oxide NiO, in particular, the character of the low-energy charge transfer excitations. NiO has long been viewed as a prototype "Mott insulator" [1] with a gap formed by intersite d -d charge transfer (CT) transitions, however, this view was later replaced by that of a "CT insulator" [2] with gap formed by p-d CT transitions. At present we have no comprehensive assignment of intensive spectral features in NiO to different p-d or d -d CT transitions. The main reason for this is that the absorption coefficient rises steeply above 3.5 eV and reaches a giant value of 10 6 cm −1 at and above 4 eV [3] that complicates the detailed absorption and reflection measurements particularly near the fundamental edge. This difficulty can be avoided in part by making use of specific techniques such as electroreflectance measurements [4] or nonlinear absorption measurements [5] that are particularly effective for location of the forbidden transitions.Main goal of our paper is to show with NiO as an example that the photoluminescence excitation (PLE) spectroscopy of the nanocrystalline materials appears to be a novel informative technique for inspection of different charge transfer transitions. Indeed, on the one hand, nanocrystalline state gives rise to a noticeable modification of the optical response of materials, in particular, due to an enhanced role of the surface induced local structural distortions resulting in a shift and splitting of the energy levels, and, mainly, in a liberalization of the

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