  We have detected strong dichroism in the Ni $L_{2,3}$ x-ray absorption
spectra of monolayer NiO films. The dichroic signal appears to be very similar
to the magnetic linear dichroism observed for thicker antiferromagnetic NiO
films. A detailed experimental and theoretical analysis reveals, however, that
the dichroism is caused by crystal field effects in the monolayer films, which
is a non trivial effect because the high spin Ni $3d^{8}$ ground state is not
split by low symmetry crystal fields. We present a practical experimental
method for identifying the independent magnetic and crystal field contributions
to the linear dichroic signal in spectra of NiO films with arbitrary
thicknesses and lattice strains. Our findings are also directly relevant for
high spin $3d^{5}$ and $3d^{3}$ systems such as LaFeO$_{3}$, Fe$_{2}$O$_{3}$,
VO, LaCrO$_{3}$, Cr$_{2}$O$_{3}$, and Mn$^{4+}$ manganate thin films.
