CO⁺ as a probe of the origin of CO in diffuse interstellar clouds

The chemistry of the diffuse interstellar medium is driven by combined influences cosmic rays, ultraviolet (UV) radiation, and turbulence. Previously detected at outer edges photodissociation regions (PDRs) formed from reaction C+ OH, CO+ main chemical precursor HCO+ CO in a thermal, cosmic-ray, UV-driven chemistry. Our aim was to test whether thermal cosmic-ray producing molecular gas through intermediate formation We searched for absorption with Atacama Large Millimeter Array (ALMA) toward two quasars known Galactic foreground gas, J1717-3342 J1744-3116, targeting strongest hyperfine components J=2-1 transition near 236 GHz. could not detect but obtained sensitive upper limits both targets. derived on column densities represent about 4% densities. corresponding limit abundance relative H2 <1.2 x 10^{-10}. non-detection confirms that mainly produced between oxygen carbon hydrides, CH2+ or CH3+ , induced suprathermal processes, while HOC+ result reactions OH H2O. required form molecules low extinction are consistent this scheme.