  A binary neutron star merger produces a rapidly evolving transient known as a
kilonova (KN), which peaks a few days after merger. Modelling of KNe has often
been approached assuming local thermodynamic equilibrium (LTE) conditions in
the ejecta. We present the first analysis of non-local thermodynamic
equilibrium (NLTE) level populations, using the spectral synthesis code SUMO,
and compare these to LTE values. We investigate the importance of the radiation
field by conducting NLTE excitation calculations with and without radiative
transfer. Level populations, in particular higher lying ones, start to show
deviations from LTE a few days after merger. Excitation is lower in NLTE for
the majority of ions and states, and this tends to give lower expansion
opacities. While the difference is small for the first few days, it grows to
factors 2-10 after this. Our results are important both for demonstrating
validity of LTE expansion opacities for an initial phase (few days), while
highlighting the need for NLTE modelling during later phases. Considering also
NLTE ionisation, our results indicate that NLTE can give both higher or lower
opacities, depending on composition and wavelength, sometimes by orders of
magnitudes.
