Herschel/HIFI discovery of interstellar chloronium (H2Cl+)

We report the first detection of chloronium, H2Cl+, in the interstellar medium, using the HIFI instrument aboard the Herschel Space Observatory. The 2(12)-1(01) lines of ortho-(H2Cl+)-Cl-35 and ortho-(H2Cl+)-Cl-37 are detected in absorption towards NGC 6334I, and the 1(11)-0(00) transition of para-(H2Cl+)-Cl-35 is detected in absorption towards NGC 6334I and Sgr B2(S). The H2Cl+ column densities are compared to those of the chemically-related species HCl. The derived HCl/H2Cl+ column density ratios, similar to 1-10, are within the range predicted by models of diffuse and dense photon dominated regions (PDRs). However, the observed H2Cl+ column densities, in excess of 10(13) cm(-2), are significantly higher than the model predictions. Our observations demonstrate the outstanding spectroscopic capabilities of HIFI for detecting new interstellar molecules and providing key constraints for astrochemical models.