The Role of Neutral Hydrogen in Setting the Abundances of Molecular Species in the Milky Way’s Diffuse Interstellar Medium. I. Observational Constraints from ALMA and NOEMA

We have complemented existing observations of H i absorption with new observations of HCO + , C 2 H, HCN, and HNC absorption from the Atacama Large Millimeter/submillimeter Array and the Northern Extended Millimeter Array in the directions of 20 background radio continuum sources with 4° ≤ ∣ b ∣ ≤ 81° to constrain the atomic gas conditions that are suitable for the formation of diffuse molecular gas. We find that these molecular species form along sightlines where A V ≳ 0.25, consistent with the threshold for the H i -to-H 2 transition at solar metallicity. Moreover, we find that molecular gas is associated only with structures that have an H i optical depth >0.1, a spin temperature