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 C<sub>2</sub>H – 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 C<sub>2</sub>H 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 C<sub>2</sub>H 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 <sup>13</sup>CCH and C<sup>13</sup>CH,
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,
C<sub>2</sub>H 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

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