  By the use of a point charge model based on the Judd-Ofelt transition theory,
the luminescence from Eu3+ ions embedded in Gd2O3 clusters is calculated and
compared to the experimental data. The main result of the numerical study is
that without invoking any other mechanisms such as crystal disorder, the pure
geometrical argument of the symmetry breaking induced by the particle surface
has influence on the energy level splitting. The modifications are also
predicted to be observable in realistic conditions where unavoidable size
dispersion has to be taken into account. The emission spectrum results from the
contribution of three distinct regions, a cluster core, a cluster shell and a
very surface, the latter being almost completely quenched in realistic
conditions. Eventually, by detailing the spectra of the ions embedded at
different positions in the cluster we get an estimate of about 0.5 nm for the
extent of the crystal field induced Stark effect. Due to the similarity between
Y2O3 and Gd2O3, these results apply also to Eu3+ doped Y2O3 nanoparticles.
