2004A&A...428..117L

We used the Plateau de Bure Interferometer to search for λ3mm absorption lines of HOC⁺ from local diffuse and translucent clouds occulting compact extragalactic mm-wave continuum sources. We detected HOC⁺ in three directions with column densities only 70-120 times below that of the HCO⁺ isomer, a factor 5-50 higher than typically found in dense dark gas but comparable to recent observations of dense photon-dominated regions. The observed amounts of HOC⁺N(HOC⁺)/N(H₂)=3-6x10^–11 can be made in quiescent diffuse gas at thermal gas-kinetic rates if the H₂O/OH ratio is of order unity, in mild violation of extant observational limits.