Hexapentaenylidene in its 1A1 ground electronic
state has been detected with the 70 m DSN antenna in emission in
two rotational transitions toward the dark cloud TMC-1 by
W. D. Langer, T. Velusamy, T. B. H. Kuiper, R. Peng, M. C. McCarthy, M. J. Travers, A. Kovács, C. A. Gottlieb, and P. Thaddeus,
First Astronomical Detection of the Cumulene Carbon Chain Molecule H2C6 in TMC-1,
Astrophys. J. 480, L63L66 (1997)
The ortho transitions 71,7 61,6 and 81,8 71,7 near 18.8 and 21.5 GHZ, respectively, were detected. The lighter homolog C4H2 is about 25 times more abundant.