1996A&A...305..950G

We have used the SEST 15m telescope to make a spectral survey of molecular absorption lines at frequencies of 82-113 GHz. The background source used was the Sgr B2 `M' HII region complex, and the line of sight intercepted six distinct foreground regions with different velocities. A total of 17 transitions was detected, from 11 different molecules and their variants. N₂H⁺ and CH₃OH were detected for the first time in absorption, and SO was confirmed along this line of sight. The major result of this survey is a unique, uniformly calibrated dataset of all the species known to produce molecular absorption at millimetre wavelengths. We have estimated the abundances of the 11 species (HCO⁺, HCN, HNC, CN, CCH, C₃H₂, CS, SiO, N₂H⁺, CH₃OH and SO) in each of the foreground regions. These estimates are believed to be very reliable, as data from isotopic lines were used to reduce the effects of excitation variations in the more optically thick transitions, and the abundances for the most chemically stable species are found to vary by only 30%, close to the minimum errors. Overall, the chemistry of the absorption clouds is very similar to that found in the dark cloud TMC1, with most abundances agreeing within factors of 2. The exceptions are C₃H₂, CS, and SiO, where the average abundances relative to TMC1 are 0.1, 4 and 500, respectively. The chemistry is also similar to that found in low-A_V absorption clouds seen towards extragalactic continuum sources. However, the absorption clouds have HCN/HNC ratios intermediate between those of low A_V clouds and TMC1. These similarities and differences are discussed in the context of chemical models. The high SiO abundance appears to be a result of temperature dependence of the formation mechanism, and the largest abundances are found in clouds which are known to be warmer than average. The CS and C₃H₂ abundances are not temperature-dependent, but instead show a trend of decreasing with increasing gas shielding. This may provide evidence for the recently proposed two-phase chemistry in cool molecular clouds, where abundances fluctuate with the C/CO ratio.