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Luminescence spectra of NiO have been investigated under vacuum ultraviolet(VUV) and soft X-ray (XUV) excitation. Photoluminescence (PL) spectra showbroad emission bands centered at about 2.3 and 3.2 eV. The PL excitation (PLE)spectral evolution and lifetime measurements reveal that two mechanisms withshort and long decay times, attributed to the d($e_g$)-d($e_g$) and p($\pi$)-dcharge transfer (CT) transitions in the range 4-6\,eV, respectively, areresponsible for the observed emissions, while the most intensive p($\sigma$)-dCT transition at 7\,eV appears to be a weak if any PL excitation mechanism. ThePLE spectra recorded in the 4-7\,eV range agree with the RIXS and reflectancedata. Making use of the XUV excitation allows us to avoid the predominant roleof the surface effects in luminescence and reveal bulk luminescence withpuzzling well isolated doublet of very narrow lines with close energies near3.3\,eV characteristic for recombination transitions in self-trapped\emph{d}-\emph{d} CT excitons formed by coupled Jahn-Teller Ni$^+$ andNi$^{3+}$ centers. This conclusion is supported both by a comparative analysisof the luminescence spectra for NiO and solid solutions Ni$_{x}$Zn$_{1-x}$O,and by a comprehensive cluster model assignement of different \emph{p}-\emph{d}and \emph{d}-\emph{d} CT transitions, their relaxation channels. To the best ofour knowledge it is the first observation of the self-trapping for\emph{d}-\emph{d} CT excitons. Our paper shows the time resolved luminescencemeasurements provide an instructive tool for elucidation of the\emph{p}-\emph{d} and \emph{d}-\emph{d} CT excitations and their relaxation in3d oxides.

Unusual X-ray excited luminescence spectra of NiO suggestive of a self-trapping of the d-d charge transfer exciton