  Alice ultraviolet spectrometer onboard Rosetta space mission observed several
spectroscopic emissions emanated from volatile species of comet
67P/Churyumov-Gerasimenko (hear after 67P/C-G) during its entire escorting
phase. We have developed a photochemical model for comet 67P/C-G to study the
atomic hydrogen (HI 1216, 1025, & 973 Ang), oxygen (OI 1152, 1304, & 1356 Ang),
and carbon (CI 1561 & 1657 Ang) line emissions by accounting for major
production pathways. The developed model has been used to calculate the
emission intensities of these lines as a function of nucleocentric projected
distance and also along with the nadir view by varying the input parameters,
viz., neutral abundances and cross sections. We have quantified the percentage
contributions of photon and electron impact dissociative excitation processes
to the total intensity of the emission lines, which has important relevance for
the analysis of Alice observed spectra. It is found that in comet 67P/C-G,
which is having a neutral gas production rate of about 10$^{27}$ s$^{-1}$ when
it was at 1.56 AU from the Sun, photodissociative excitation processes are more
significant compared to electron impact reactions in determining the atomic
emission intensities. Based on our model calculations, we suggest that the
observed atomic hydrogen, oxygen, and carbon emission intensities can be used
to derive H$_2$O, O$_2$, and CO, abundances, respectively, rather than electron
density in the coma of 67P/C-G, when the comet has a gas production rate of
$\ge$ 10$^{27}$ s$^{-1}$.
