June 2013


Acetylene (C2H2) has no dipole moment and is thus not amenable to detection via pure rotational spectroscopy. The first detection of non-terrestrial acetylene was in the atmosphere of Jupiter, as reported by Ridgway in 1974 via the ν5 bending mode (see below). Due to symmetry, there is a second, degenerate bending mode, ν4. The initial observations of C2H2 were made with the McMath solar telescope at Kitt Peak. The detection was confirmed later in 1974 by Combes et al.. Acetylene has been detected in all of the other gas giants as well: in Saturn in 1979 by Moos & Clarke, in Uranus in 1984 by Encrenaz et al. and in Neptune in 1986 by Caldwell et al. Acetylene has also been observed in the atmosphere of Titan and in comet Hyakutake.

Detections outside the solar system followed the initial detections on Jupiter and elsewhere. It was discovered in the circumstellar shell of IRC +10 216 by Ridgway et al. in 1978 using the Mayall 4-meter telescope at Kitt Peak. This detection used the combination band of ν5 and the ν1 (stretching) mode (see below). The first interstellar detection was reported in 1989 by Lacy et al.

  ν1  : ν5

Acetylene is well known in terrestrial chemistry, where it used for things like welding due to its high flame temperature. Much of it is also used in chemical synthesis, such as in formation of acetylenic polymers.

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