Electronic Spectrum of Propadienylidene (H2C=C=C:) and Its Relevance to the Diffuse Interstellar Bands

The electronic spectrum of singlet propadienylidene (H 2 C=C=C:) matrix isolated in neon at 6 K was measured. Propadienylidene is the only member of the homologous series of neutral carbon chain molecules H 2 C(=C) n to be observed by electronic spectroscopy. The spectrum displays three electronic transitions: two in the visible ( 1 A 2 ← 1 A 1 and 1 B 1 ← 1 A 1 ) and one in the UV ( 1 A 1 ← 1 A 1 ). The longest wavelength transition ( 1 A 2 ← 1 A 1 ) is symmetry forbidden but is weakly seen in the spectrum via vibronic coupling with the 1 B 1 state. The other two transitions exhibit well-resolved vibrational progressions. The high density of absorptions in the visible region confirms earlier suggestions that the H 2 C(=C) n family of cumulene carbenes merits careful consideration as potential carriers of diffuse interstellar bands.