The electronic absorption spectrum in the gas phase of the metastable 1thinsp;3B2state of acetylene has been observed. Itsrsquo; features, including rotational and vibronic structure as well as deuterium isotope effects, agree remarkably well withabinitiotheoretical predictions for the 1thinsp;3A2ndash;1thinsp;3B2electronic transition of the lowest triplet state in itscisgeometry. The metastable species was generated in the Hghyphen;photosensitized reactions of acetylene, ketene, and diazomethane. In the latter two cases it is probably formed by the reaction3CH2+3CH2rarr;C2H2(1thinsp;3B2)+H2.
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