Chloronium has been detected in absorption with the Herschel satellite toward the star-forming regions NGC 6334I and Sagittarius B2(S):
D. C. Lis, J. C. Pearson, D. A. Neufeld, et al.
//Herschel///HIFI Discovery of Interstellar Chloronium (H₂Cl⁺)
Astron. Astrophys. 521, (2010) Art. No. L9.
Three transitions have been detected toward NGC 6334I which show broadening by Cl hyperfine splitting and no recognizable absorption caused by spiral arm clouds. The 2₁₂ –1₀₁ transition of ortho-H₂Cl⁺ has been observed near 781.6 and 780.05 GHz for the ³⁵Cl and ³⁷Cl isotopologs, respectively. The 1₁₁ –0₀₀ transition of para-H₂³⁵Cl⁺ was observed near 485.4 GHz.
The absorption features near 485.4 and 780.05 GHz were both affected by emission features from dimethyl ether. A preliminary model for this molecule permitted accounting for these emmsion features in the analyses. This is a clear example that features by so-called “weed” species can affect “flowers” even in the case of Herschel observations. The recent report on the rotational spectroscopy of dimethyl ether by C. P. Endres et al., Astron. Astrophys. 504, (2009) 635, were instrumental.
The 1₁₁ –0₀₀ transition of para-H₂³⁵Cl⁺, observed toward Sgr B2(S), displays absorption features of spiral arm clouds.

More recently, H₂Cl⁺ has turned out to be rather widespread. The 1₁₁ –0₀₀ transition of the para species was observed with the Herschel satellite for both ³⁵Cl and ³⁷Cl isotopologs in absorption toward the strong contiunuum sources W31C and Sgr A (+50 km/s cloud). The transition was seen in emission in Orion S and in the photon-dominated region Orion Bar by:
D. A. Neufeld, E. Roueff, R. L. Snell, et al.
//Herschel// Observations of Interstellar Chloronium
Astrophys. J. 748, (2012) Art. No. 37.

Contributor(s): H. S. P. Müller; 10, 2010; 3, 2012