Detection of chloronium and measurement of the35Cl/37Cl isotopic ratio atz= 0.89 toward PKS 1830–211

We report the first extragalactic detection of chloronium (H 2 Cl + ) in the z = 0.89 absorber in front of the lensed blazar PKS 1830-211.The ion is detected through its 1 11 -0 00 line along two independent lines of sight toward the North-East and South-West images of the blazar.The relative abundance of H 2 Cl + is significantly higher (by a factor ∼7) in the NE line of sight, which has a lower H 2 /H fraction, indicating that H 2 Cl + preferably traces the diffuse gas component.From the ratio of the H 35 2 Cl + and H 37 2 Cl + absorptions toward the SW image, we measure a 35 Cl/ 37 Cl isotopic ratio of 3.1 +0.3 -0.2 at z = 0.89, similar to that observed in the Galaxy and the solar system.