The hexatriynyl radical in its ²Π_3/2 lower spin component has been detected toward TMC-1 by
H. Suzuki, M. Ohishi, N. Kaifu, S.-I. Ishikawa, and T. Kasuga,
Detection of the Interstellar C₆H Radical,
Publ. Astron. Soc. Japan 38 911–917 (1986) and
Detection of C₆H in the ²Π_1/2 State toward IRC +10216 by
S. Saito, K. Kawaguchi, H. Suzuki, M. Ohishi, N. Kaifu, and S.-I. Ishikawa,
Publ. Astron. Soc. Japan 39 193–199 (1987)
before it was actually detected in the laboratory.

Transitions of the lowest excited v₁₁ = 1 vibrational state have been identified very recently:
J. Cernicharo, M. Guélin, M. Agúndez, M. C. McCarthy, and P. Thaddeus,
Detection of C₅N^– and Vibrationally Excited C₆H in IRC +10216
Astrophys. J. 688 L83–L86 (2008).
The Renner-Teller interaction causes this state to split into three components: ²Σ^–, ²Δ with spin-orbit splitting, and ²Σ⁺. The lowest ²Σ^– state as well as both spin-components of the ²Δ state were detected. The ²Σ⁺ was not seen; it is probably overlapped by the lower ²Σ^– state and possibly too weak to be recognized.

Contributor(s): H. S. P. Müller; 11, 2006; 10, 2008