A linear triplet isomer of HC has been detected by Fourier transform microwave spectroscopy in a 6N
supersonic molecular beam. A total of 85 hyperÐne components from six rotational transitions between 8
and 18 GHz were measured to an uncertainty of 5 kHz; a similar set of transitions were detected for the
15N isotopic species, produced using an isotopically enriched precursor gas sample. The spectroscopic
constants for both species, including the Ðne and hyperÐne coupling constants, were determined to very
high accuracy, and these allow calculation of the radio spectrum to a fraction of 1 km s~1 in equivalent
radial velocity. Triplet HC is a highly polar, low-lying isomer ; measurements show it to be about 10 6N
times more abundant than a ring-chain isomer recently detected with the same spectrometer.