The ethynyl radical in its ²Σ ground vibrational state was detected with the NRAO 11 m telescope in the four stronger of the six hyperfine components of the N = 1 – 0 rotational transitions near 87.4 GHz by
K. D. Tucker, M. L. Kutner, and P. Thaddeus,
The Ethynyl Radical C₂H – A New Interstellar Molecule,
Astrophys. J. 193 L115–L119 (1974)
before it was actually detected in the laboratory. The identification was based on hyperfine parameters obtained from electron-spin resonance measurements of ethynyl isolated in argon matrices. The molecule is rather ubiquitous as it has been detected in several star-forming regions such as W3, Orion A, Sgr B2, Sgr A, of which several include photon-dominated regions (PDRs), most notably M17 SW. It has also been detected in the circumstellar envelope of the carbon-rich late-type star CW Leo, also known as IRC +10216.

A. Wootten, E. P. Bozyan, D. P. Garrett, R. B. Loren, and R. L. Snell reported on the
Detection of C₂H in Cold Dark Clouds,
Astrophys. J. 239 944–854 (1980).

The radical was also observed in the diffuse medium by:
L.-Å. Nyman,
Detection of CS and C₂H in Absorption,
Astron. Astrophys. 141 323–327 (1984).

C₂D was detected near 216.4 GHz with the 5 m MWO telescope toward Orion KL by:
F. Combes, F. Boulanger, P. J. Encrenaz, M. Gerin, M. Bogey, C. Demuynck, J. L. Destomb,
Detection of Interstellar CCD,
Astron. Astrophys. 147 L25–L26 (1985).

The detection of the ¹³C isotopic species was reported in:
A. H. Saleck, R. Simon, G. Winnewisser, J. G. A. Wouterloot,
Detection of Interstellar ¹³CCH and C¹³CH,
Can. J. Phys. 72 747–754 (1994).

Three out of four observable fine structure components of the N = 3 – 2 rotational transition of vibrationally excited C₂H were reportedly not overlapped:
E. D. Tenenbaum, J. L. Dodd, S. N. Milam, N. J. Woolf, and L. M. Ziurys,
Comparative Spectra of Oxygen-rich Versus Carbon-rich Circumstellar Shells: VY Canis Majoris and IRC +10216 at 215-285 GHz,
Astrophys. J. 720 L102–L107 (2010).
The features were detected in the outer part of the circumstellar envelope of CW Leo; hence, the detection must be facilitated by formation pumping or by IR pumping.

The A ²Π – X ²Σ⁺ first excited electronic transition of C₂H is distributed over at least 5 bands because of vibronic coupling of the A with various higher excited vibrational Π states of the ground electronic X state. Almost 30 lines belonging to two of the stronger bands were detected with the Mayall telescope near 4011 and 4108 cm^–1 in the outer region of the circumstellar envelope of CW Leo by
J. J. Keady and K. H. Hinkle,
C₂H in the 2 micron Infrared Spectrum of IRC +10216,
Astrophys. J. 331 539–546 (1988).
The rotational temperature of 12.5 ± 1.5 K suggests a peak abundance about 1200 stellar radii away from CW Leo, and the line intensities indicate a high fractional abundance with respect to H₂.

Contributor(s): H. S. P. Müller; 1, 2012