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Physical Measurement Laboratory

Observed Interstellar Molecular Microwave Transitions, Table 2

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Table 2. The 17 other species observed in circumstellar and interstellar sources at IR and UV wavelengths, and 8 additional species found only in comets, which are not included in Table 4.

Empirical
Formula		 Name Isotopic
Species		 Spectral
Region		 CA
Number		 Reference
Interstellar and Circumstellar Species
CH+ Methyliumylidene CH+ IR [24361-82-8] Cer97, Dou41
CH3 Methyl CH3 IR [2229-07-7] Feu00
CH4 Methane CH4 IR [74-82-8] Lac91
CO2 Carbon dioxide CO2 IR [124-38-9] Jus96
C2 Carbon molecule C2 UV [12070-15-4] Cha80
C2H2 Ethyne (acetylene) HCCH IR [74-86-2] Lac89
C2H4 Ethylene H2CCH2 IR [74-85-11] Cer01a
C4H2 1,3-Butadiyne HCCCCH IR [460-12-8] Cer01a
C5 Pentacarbon molecule C5 IR [12595-82-3] Ber89
C6H2 1,3,5-Hexatriyne HCCCCCCH IR [3161-99-7] Cer01a
C6H6 Benzene C6H6 IR [71-43-2] Cer01
HN Imidogen HN UV [13774-92-0] Mey91, Swi41
HS Mercapto (thiohydroxyl) SH IR [13940-21-1] Yam00
H2 Hydrogen H2 UV [1333-74-0] Car70
H3+ Hydrogen ion H3+ IR [28132-48-1] Geb96
H4Si Silane SiH4 IR [7803-62-5] Kea93
N2 Nitrogen N2 UV [7727-37-9] Kna04
Species Observed Only in Comets.
CN+ Cyanogen ion CN+ UV [12539-57-0] Val92
CN2 Cyano imidogen NCN UV [1884-64-6] Val92
CO2+ Carbon dioxide ion CO2+ UV [12181-61-2] Swi50
C2H6 Ethane CH3CH3 IR [74-84-0] Wea99
HO+ Oxoniumylidene OH+ UV [12259-29-9] Val92
H2O+ Oxoniumyl H2O+ UV [56583-62-1] Weh74
N2+ Nitrogen ion N2+ UV [13966-04-6] Lut93
S2 Sulfur S2 UV [23550-45-0] A'H83
Analytical chemistry, Atomic / molecular / quantum and Reference data

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Created June 13, 2018, Updated August 13, 2025
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