H₃⁺ and D₃⁺ do not have a pure rotational spectrum because of their cyclic (triangular) symmetry, but H₃⁺ can been detected in the ISM by its rovibrational spectrum in the infrared region.
T. R. Geballe and T. Oka
used the United Kingdom InfraRed Telescope (UKIRT) for the
Detection of H₃⁺ in Interstellar Space
Nature 384, 334–335 (1996).
They detected the lowest para transitions R(1,1)⁺ and R(1,1)^– and the lowest ortho transition R(1,0) of the degenerate ν₂ band near 2700 cm^–1 in absorption in the dense molecular clouds RAFGL (or CRL) 2136 and W33A which both surround deeply embedded young stellar objects. CRL 2136 is also known as IRAS 18196-1331 or OH 17.6+0.2; W33A is also known as IRAS 18117-1753.

H₃⁺ was also observed toward many other dense molecular clouds. Moreover, shortly thereafter,
B. J. McCall, T. R. Geballe, K. H. Hinkle, and T. Oka
reported on the
Detection of H₃⁺ in the Diffuse Interstellar Medium Toward Cygnus OB2 No. 12
Science 279, 1910–1913 (1998).
UKIRT was again employed to detect the same three rovibrational transitions as above. The source is also known as VI Cygni 12 or IRC +40430 or IRAS 20308+4104. The authors found surprisingly high column densities of the molecular cation. Similar findings were made in other diffuse molecular clouds. An explanation was found by
B. J. McCall, A. J. Huneycutt, R. J. Saykally, T. R. Geballe, N. Djuric, G. H. Dunn, J. Semaniak, O. Novotny, A. Al-Khalili, A. Ehlerding, F. Hellberg, S. Kalhori, A. Neau, R. Thomas, F. Österdahl, and M. Larsson,
who derived
An Enhanced Cosmic-ray Flux towards ζ Persei Inferred from a Laboratory Study of the H₃⁺–e^– Recombination Rate
Nature 422, 500–502 (2003).

Hot H₃⁺ was detected in the Central Molecular Zone (CMZ), a region of radius ∼150 pc at the Galactic center both in diffuse molecular clouds via the metastable R(3,3)^l transition at 2829.9 cm^–1 by
M. Goto, B. J. McCall, T. R. Geballe, T. Usuda, N. Kobayashi, H. Terada, and T. Oka,
Absorption Line Survey of H₃⁺ toward the Galactic Center Sources I. GCS 3–2 and GC IRS3
Publ. Astron. Soc. Japan 44, 951–961 (2002)
using the Subaru IRCS; and also in dense molecular clouds via the metastable R(2,2)^l transition at 2762.1 cm^–1 in addition to R(3,3)^l by
M. Goto, T. Usuda, T. Nagata, T. R. Geballe, B. J. McCall, N. Indriolo, H. Suto, T. Henning, C. P. Morong, and T. Oka,
Absorption Line Survey of H₃⁺ toward the Galactic Center Sources. II. Eight Infrared Sources within 30 pc of the Galactic Center
Astrophys. J. 688, 306–319 (2008).
As it turned out, some velocity components toward GC IRS3 include dense molecular clouds because of the presence of this high density indicating transition, whereas it is absent in other velocity components. Subaru IRCS and UKIRT were employed in this study.

H₂D⁺ as well as HD₂⁺ have sizeable dipole moments because their centers of mass are different from the respective centers of charge.
R. Stark, F. F. S. van der Tak, and E. van Dishoek
used the JCMT 15 m radio telescope for the
Detection of Interstellar H₂D⁺ Emission
Astrophys. J. 521, L67–L70 (1999).
The 1_1,0 – 1_1,1 ortho transition at 372421.385 MHz was detected toward the young stellar object NGC 1333 IRAS 4A.
P. Caselli, F. F. S. van der Tak, C. Ceccarelli, and A. Bacmann
used the CSO 14.4 m dish to detect the same line and concluded
Abundant H₂D⁺ in the Pre-stellar Core L1544
Astron. Astrophys. 403, L37–L41 (2003).
Several further reports appeared.

There is a second rotational ground state transition of ortho-H₂D⁺, the 2_1,2 – 1_1,1 transition at 2363.243 GHz. It was detected in absorption with ISO by
J. Cernicharo, E. Polehampton, and J. R. Goicoechea:
Far-Infrared Detection of H₂D⁺ toward Sgr B2
Astrophys. J. 657, L21–L24 (2007).

S. Brünken, O. Sipilä, E. T. Chambers, J. Harju, P. Caselli, O. Asvany, C. E. Honingh, T. Kamiński, K. M. Menten, J. Stutzki, and S. Schlemmer,
used the GREAT instrument on board the SOFIA aircraft observatory to detect the 1_0,1 – 0_0,0 ground state transition of para-H₂D⁺ at 1370.085 GHz in absorption toward IRAS 16293-2422 and to deduce
H₂D⁺ observations give an age of at least one million years for a cloud core forming Sun-like stars
Nature 516, 219–221 (2014).
See also the MPIfR press release (in English; German version also available) or the Universität zu Köln press release (in German).

C. Vastel, T. G. Phillips, and H. Yoshida
used the CSO 10.4 m radio telescope for the
Detection of D₂H⁺ in the Dense Interstellar Medium
Astrophys. J. 606, L127–L130 (2004).
The 1_1,0 – 1_0,1 para transition at 691660.483 MHz was barely detected in emission toward the prestellar core IRAS 16293-2422E.
B. Parise, A. Belloche, F. Du, R. Güsten, and K. M. Menten
observed the same line with the APEX 12 m radio telescope in the H-MM1 prestellar core located in the Ophiuchus L1688 cloud and found
Extended Emission of HD₂⁺ in a Pre-stellar Core
Astron. Astrophys. 526, Art. No. A31 (2011).

J. Harju, O. Sipilä, S. Brünken, S. Schlemmer, P. Caselli, M. Juvela, K. M. Menten, J. Stutzki, O. Asvany, T. Kamiński, Y. Okada, and R. Higgins
used the GREAT instrument on board the SOFIA aircraft observatory to detect the 1_1,1 – 0_0,0 ground state transition of ortho-H₂D⁺ at 1476.606 GHz in absorption toward IRAS 16293-2422
Detection of Interstellar //Ortho//-D₂H⁺ with SOFIA
Astrophys. J. 840, Art. No. 63 (2017).

Contributor(s): H. S. P. Müller; 4, 2004; 01, 2014; 12, 2015; 05, 2017