  Infrared emission lines arising from transitions between fine structure
levels of heavy elements are expected to produce kilonova nebular emission. For
the kilonova in GW170817, strong emission at 4.5 ${\rm \mu m}$ at late times
was detected by the Spitzer Space Telescope but no source was detected at 3.6
${\rm \mu m}$. This peculiar spectrum indicates that there exist strong line
emitters around 4.5 ${\rm \mu m}$ and the absence of strong lines around 3.6
${\rm \mu m}$. To model the spectrum we prepare a line list based on the
selection rules in LS coupling from the experimentally calibrated energy levels
in the NIST database. This method enables to generate the synthetic spectra
with accurate line wavelengths. We find that the spectrum is sensitive to the
abundance pattern whether or not the first r-process peak elements are
included. In both cases, the synthetic spectra can match the observed data,
leading to two possible interpretations. If the first peak elements are
abundant a Se III line dominates the flux. If otherwise, W III with Os III, Rh
III, and Ce IV can be the main sources. Observing nebular spectra for the
future kilonovae in a wider wavelength range can provide more conclusive
elemental identification.
