Two groups reported almost simultaneously the detection of the J_K^v_t = 1₁^– – 2₁⁺ at 307192.41 MHz:
J. M. Hollis, E. B. Churchwell, E. Herbst, and F. C. de Lucia,
An Interstellar Line Coincident with the P(2,1) Transition of Hydronium (H₃O⁺)
Nature 322, 524–626 (1986);
with the NRAO 12 m telescope towards Orion KL; and
A. Wootten, F. Boulanger, M. Bogey, F. Combes, P. J. Encrenaz, M. Gerin, and L. Ziurys,
A Search for Interstellar H₃O⁺
Astron. Astrophys. 166, L15–L18 (1986); with the NRAO 12 m telescope towards OMC-1 (essentially identical to Orion KL) and Sgr B2(OH).
Note: see below for the Orion KL case !

Later, additional transitions have been reported, e.g. the 3₂⁺ – 2₂^– transition at 364797.427 MHz:
A. Wootten, J. G. Mangum, B. E. Turner, M. Bogey, F. Boulanger, F. Combes, P. J. Encrenaz, and M. Gerin,
Detection of Interstellar H₃O⁺ – A Confirming Line
Astrophys. J. 380, L79–L83 (1991),
towards OMC-1, Sgr B2(OH) and (N).

The 3₀⁺ – 2₀^– transition at 396272.412 MHz along with the previously mentioned transitions have been detected in several hot core sources:
T. G. Phlips, E. F. van Dishoek, J. Keene,
Interstellar H₃O⁺ and its Relation to the O₂ and H₂O Abundances
Astrophys. J. 399, 533–550 (1992).
The sources, in which the cation reportedly was detected newly, are W3 IRS5 (aka W3 M), W3(OH), W51 M, and G34.3 +0.15.

A line detected at 4.3121 (5) THz with KOA was tentativel assigned to the 4₃^– – 3₃⁺ transition:
R. Timmermann, T. Nikola, A. Poglitsch, N. Geis, G. J. Stacey, and C. H. Townes,
Possible Discovery of the 70 Micron H₃O⁺ 4₃^– – 3₃⁺ Transition in Orion BN–IRc2
Astrophys. J. 463, L109–L112 (1996);
three additional transitions were reported, one of which was overlapped with a line of H₂¹⁸O:
J. R. Goicoechea and J. Cernicharo,
Far-Infrared Detection of H₃O⁺ in Sagittarius B2,
Astrophys. J. 554, L213–L216 (2001).

Recent Herschel/HIFI observations failed to detect any H₃O⁺ in Orion KL near 984.7, 1655.8, and 1657.25 GHz. Thus, previous reports on observations of this molecular ion in Orion KL could no be confirmed. It is possible that emission lines observed at lower frequencies (near 307.2, 364.8, and 396.3 GHz; see above) are caused by other species:
H. Gupta, P. Rimmer, J. C. Pearson, et al.,
Detection of OH⁺ and H₂O⁺ towards Orion KL
Astron. Astrophys. 521, (2010) Art. No. L47.

Contributor(s): H. S. P. Müller; 01, 2008; 05, 2011