Reports on the Detection of H₃O⁺ in the ISM

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 and Sgr B2.

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 and Sgr B2;
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);
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).