Hydroxymethylium, also known os protonated formaldehyd, was detected toward Sgr B2(M) and (N), Orion KL, W51, and possibly in NGC 7538 and DR21(OH). It was not found toward several dark clouds. The account was reported by
M. Ohishi, S. Ishikawa, T. Amano, H. Oka, W. M. Irvine, J. E. Dickens, L. M. Ziurys, and A. J. Apponi,
Detection of a New Interstellar Molecular Ion, H₂COH⁺ (Protonated Formaldehyde)
Astrophys. J. 471, L61–L64 (1996).
Altogether, six transitions were observed between 31 and 174 GHz using the Nobeyama 45 m dish and the Kitt Peak 12 m dish.

A. Bacmann, E. García-García, and A. Faure,
reported on the
Detection of Protonated Formaldehyde in the Prestellar Core L1689B
Astron. Astrophys. 588, Art. No. L8 (2016).
The authors used the IRAM 30 m telescope to search for four transitions between 102 and 169  GHz. Three lower energy transitions were detected, the higher lying transition near 102 GHz was within the noise level, in agreement with a rotational temperature of about 10 K.

Contributor(s): H. S. P. Müller; 10, 2006; 04, 2016