  Luminescence spectra of NiO have been investigated under vacuum ultraviolet
(VUV) and soft X-ray (XUV) excitation. Photoluminescence (PL) spectra show
broad emission bands centered at about 2.3 and 3.2 eV. The PL excitation (PLE)
spectral evolution and lifetime measurements reveal that two mechanisms with
short and long decay times, attributed to the d($e_g$)-d($e_g$) and p($\pi$)-d
charge transfer (CT) transitions in the range 4-6\,eV, respectively, are
responsible for the observed emissions, while the most intensive p($\sigma$)-d
CT transition at 7\,eV appears to be a weak if any PL excitation mechanism. The
PLE spectra recorded in the 4-7\,eV range agree with the RIXS and reflectance
data. Making use of the XUV excitation allows us to avoid the predominant role
of the surface effects in luminescence and reveal bulk luminescence with
puzzling well isolated doublet of very narrow lines with close energies near
3.3\,eV characteristic for recombination transitions in self-trapped
\emph{d}-\emph{d} CT excitons formed by coupled Jahn-Teller Ni$^+$ and
Ni$^{3+}$ centers. This conclusion is supported both by a comparative analysis
of 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 of
our knowledge it is the first observation of the self-trapping for
\emph{d}-\emph{d} CT excitons. Our paper shows the time resolved luminescence
measurements provide an instructive tool for elucidation of the
\emph{p}-\emph{d} and \emph{d}-\emph{d} CT excitations and their relaxation in
3d oxides.
