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Abstract Atacama Large Millimeter/submillimeter Array (ALMA) data toward QSO J1851+0035 (l= 33.°498,b= +0.°194) were used to study absorption lines by Galactic molecular gas. We detected 17 species (CO,¹³CO, C¹⁸O, HCO⁺, H¹³CO⁺, HCO, H₂CO, C₂H,c-C₃H,c-C₃H₂, CN, HCN, HNC, CS, SO, SiO, and C) and set upper limits to 18 species as reference values for chemical models. About 20 independent velocity components at 4.7–10.9 kpc from the Galactic center were identified. Their column density and excitation temperature estimated from the absorption study, as well as the CO intensity distributions obtained from the FUGIN survey, indicate that the components withτ≲1 correspond to diffuse clouds or cloud outer edges. Simultaneous multiple-Gaussian fitting of COJ= 1–0 andJ= 2–1 absorption lines shows that these are composed of narrow- and broad-line components. The kinetic temperature empirically expected from the high HCN/HNC isomer ratio (≳4) reaches ≳40 K and the corresponding thermal width accounts for the line widths of the narrow-line components. CN-bearing molecules and hydrocarbons have tight and linear correlations within the groups. The CO/HCO⁺abundance ratio showed a dispersion as large as 3 orders of magnitude with a smaller ratio in a smallerN(HCO⁺) (or lowerA_V) range. Some of the velocity components are detected in single-dish CO emission and ALMA HCO⁺absorption but without corresponding ALMA CO absorption. This may be explained by the mixture of clumpy CO emitters not resolved with the ∼1 pc single-dish beam surrounded by extended components with a very low CO/HCO⁺abundance ratio (i.e., CO-poor gas). 

Abstract We present emission maps ( $1.\prime 5\times 1.\prime 5$scale, corresponding to 0.18 pc) of the DCN (J= 2 − 1) and DCO⁺(J= 2 − 1) lines in the 2 mm band toward the Orion KL region obtained with the 2 mm receiver system named B4R installed on the Large Millimeter Telescope. The DCN emission shows a peak at the Orion KL hot core position, whereas no DCO⁺emission has been detected there. The DCO⁺emission shows enhancement at the west side of the hot core, which is well shielded from the UV radiation from OB massive stars in the Trapezium cluster. We have derived the abundance ratio of DCN/DCO⁺at three representative positions where both species have been detected. The gas components withV_LSR≈ 7.5–8.7 km s⁻¹are associated with low abundance ratios of ∼4–6, whereas much higher abundance ratios (∼22–30) are derived for the gas components withV_LSR≈ 9.2–11.6 km s⁻¹. We have compared the observed abundance ratio to our chemical models and found that the observed differences in the DCN/DCO⁺abundance ratios are explained by different densities.