  The nebular spectra of Type Ia supernovae ($\gtrapprox$ 100 days after
explosion) consist mainly of emission lines from singly- and doubly-ionised
Fe-group nuclei. However, theoretical models for many scenarios predict that
non-thermal ionisation leads to multiply-ionised species whose recombination
photons ionise and deplete Fe$^{+}$ , resulting in negligible [Fe II] emission.
We investigate a method to determine the collisional excitation conditions from
[Fe II] line ratios independently from the ionisation state and find that it
cannot be applied to highly-ionised models due to the influence of
recombination cascades on Fe$^{+}$ level populations. When the ionisation state
is artificially lowered, the line ratios (and excitation conditions) are too
similar to distinguish between explosion scenarios. We investigate changes to
the treatment of non-thermal energy deposition as a way to reconcile
over-ionised theoretical models with observations and find that a simple work
function approximation provides closer agreement with the data for sub-Mch
models than a detailed Spencer-Fano treatment with widely-used cross section
data. To quantify the magnitude of additional heating processes that would be
required to sufficiently reduce ionisation from fast leptons, we artificially
boost the rate of energy loss to free electrons. We find that the equivalent of
as much as an eight times increase to the plasma loss rate would be needed to
reconcile the sub-Mch model with observed spectra. Future studies could
distinguish between reductions in the non-thermal ionisation rates and
increased recombination rates, such as by clumping.
