Remote observation of spectroscopic emissions is a potential tool for the
identification and quantification of various species in comets. CO Cameron band
(to trace \cod) and atomic oxygen emissions (to trace H$_2$O and/or CO$_2$, CO)
have been used to probe neutral composition in the cometary coma. Using a
coupled-chemistry emission model, various excitation processes controlling CO
Cameron band and different atomic oxygen and atomic carbon have been modelled
in comet 67P-Churyumov-Gerasimenko at 1.29~AU (perihelion) and at 3~AU
heliocentric distances, which is being explored by ESA's Rosetta mission. The
intensities of CO Cameron band, atomic oxygen and atomic carbon emission lines
as a function of projected distance are calculated for different CO and CO$_2$
volume mixing ratios relative to water. Contributions of different excitation
processes controlling these emissions are quantified. We assess how CO$_2$
and/or CO volume mixing ratios with respect to H$_2$O can be derived based on
the observed intensities of CO Cameron band, atomic oxygen, and atomic carbon
emission lines.The results presented in this work serve as base line
calculations to understand the behaviour of low out-gassing cometary coma and
compare them with the higher gas production rate cases (e.g. comet Halley).
Quantitative analysis of different excitation processes governing the
spectroscopic emissions is essential to study the chemistry of inner coma and
to derive neutral gas composition.