| Abstract
| - We report the first extragalactic detection of chloronium (H 2Cl +) 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 2Cl + is significantly higher (by a factor ~7) in the NE line of sight, which has a lower H 2/H fraction, indicating that H 2Cl + preferably traces the diffuse gas component. From the ratio of the H 235Cl + and H 237Cl + absorptions toward the SW image, we measure a 35Cl/ 37Cl isotopic ratio of 3.1 -0.2+0.3 at z = 0.89, similar to that observed in the Galaxy and the solar system.
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