Prior to the launch of Herschel, HCl was the only chlorine-containing molecule detected in the interstellar medium (ISM). However, chemical models have identified chloronium, H_2Cl^+, as a relatively abundant species that is potentially detectable. H_2Cl^+ was predicted to be most abundant in the environments where the ultraviolet radiation is strong: in diffuse clouds, or near the surfaces of dense clouds illuminated by nearby O and B stars. The HIFI instrument on Herschel provided the first detection of H_2Cl^+ in the ISM. The 2_(12)-1_(01) lines of ortho- H_2^(35)Cl^+ and ortho- H_2^(37)Cl^+ were detected in absorption toward NGC 6334I, and the 1_(11)-0_(00) transition of para- H_2^(35)Cl^+ was detected in absorption toward NGC 6334I and Sgr B2(S). The derived HCl/H_2Cl^+ column density ratios, ∼1-10, are within the range predicted by models of diffuse and dense Photon Dominated Regions (PDRs). However, the observed H_2Cl^+ column densities, in excess of 10^(13) cm^(−2), are significantly higher than the model predictions. These observations demonstrate the outstanding spectroscopic capabilities of HIFI for detecting new interstellar molecules and providing key constraints for astrochemical models
