On the Detection of Pentadiynylidyne, C₅H, in Space

The pentadiynylidyne radical in its ²Π_1/2 lower spin component has been detected in six lines of three rotational transitions toward IRC +10216 by
J. Cernicharo, C. Kahane, J. Gómez-González; and M. Guélin,
Tentative Detection of the C₅H Radical,
Astron. Astrophys. 164 L1–L4 (1986)
before it was actually detected in the laboratory.

Confirmation came also through the observation of four lines pertaining to the ²Π_3/2 upper spin component in the 3 mm range for which the λ-doubling is just not resolvable:
J. Cernicharo, C. Kahane, J. Gómez-González; and M. Guélin,
Detection of the ²Π_3/2 State of C₅H,
Astron. Astrophys. 167 L5–L7 (1986).

Subsequently, it was also detected toward TMC-1 in one transition with resolved ¹H hyperfine splitting as well as λ-doubling:
J. Cernicharo, M. Guélin, and C. M. Walmsley,
Detection of the ²Π_3/2 State of C₅H,
Astron. Astrophys. 172 L5–L6 (1987).