3 mm spectral line survey of two lines of sight toward two typical cloud
  complexes in the Galactic Center
  We present the results of two Mopra 3-mm spectral line surveys of the Lines
of Sight (LOS) toward the Galactic Center (GC) molecular complexes Sgr B2
(LOS+0.693) and Sgr A (LOS-0.11). The spectra covered the frequency ranges of
~77-93 GHz and ~105-113 GHz. We have detected 38 molecular species and 25
isotopologues. The isotopic ratios derived from column density ratios are
consistent with the canonical values, indicating that chemical isotopic
fractionation and/or selective photodissociation can be considered negligible
(<10%) for the GC physical conditions. The derived abundance and rotational
temperatures are very similar for both LOSs, indicating very similar chemical
and excitation conditions for the molecular gas in the GC. The excitation
conditions are also very similar to those found for the nucleus of the
starburst galaxy NGC 253. We report for the first time the detection of HCO and
HOC+ emission in LOS+0.693. Our comparison of the abundance ratios between CS,
HCO, HOC+ and HCO+ found in the two LOSs with those in typical Galactic
photodissociation regions (PDRs) and starbursts galaxies does not show any
clear trend to distinguish between UV and X-ray induced chemistries. We propose
that the CS/HOC+ ratio could be used as a tracer of the PDR components in the
molecular clouds in the nuclei of galaxies.
