The hexatriynyl radical in its 2Π3/2 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 911917 (1986) and
Detection of C6H in the 2Π1/2 State toward IRC +10216 by
S. Saito, K. Kawaguchi, H. Suzuki, M. Ohishi, N. Kaifu, and S.-I. Ishikawa,
Publ. Astron. Soc. Japan 39 193199 (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 L83L86 (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.