  The recent observations show that comet C/2016 R2 (Pan-Starrs) has a unique
and peculiar composition when compared with several other comets observed at
2.8 au heliocentric distance. Assuming solar resonance fluorescence is the only
excitation source, the observed ionic emission intensity ratios are used to
constrain the corresponding neutral abundances in this comet. We developed a
physico-chemical model to study the ion density distribution in the inner coma
of this comet by accounting for photon and electron impact ionization of
neutrals, charge exchange and proton transfer reactions between ions and
neutrals, and electron-ion thermal recombination reactions. Our calculations
show that CO2+ and CO+ are the major ions in the inner coma, and close to the
surface of nucleus CH3OH+, CH3OH2+ and O2+ are also important ions. By
considering various excitation sources, we also studied the emission mechanisms
of different excited states of CO+, CO2+, N2+, and H2O+. We found that the
photon and electron impact ionization and excitation of corresponding neutrals
significantly contribute to the observed ionic emissions for radial distances
smaller than 300 km and at larger distances, solar resonance fluorescence is
the major excitation source. Our modelled ion emission intensity ratios are
consistent with the ground-based observations. Based on the modelled emission
processes, we suggest that the observed ion emission intensity ratios can be
used to derive the neutral composition in the cometary coma only when the ion
densities are significantly controlled by photon and photoelectron impact
ionization of neutrals rather than by the ion-neutral chemistry.
