J. Hodges

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

The electronic spectrum of singlet propadienylidene (H2C=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 H2C(=C)n to be observed by electronic spectroscopy. The spectrum displays three electronic transitions: two in the visible ( 1A2 ← 1A1 and 1B1 ← 1A1) and one in the UV ( 1A1 ← 1A1). The longest wavelength transition ( 1A2 ← 1A1) is symmetry forbidden but is weakly seen in the spectrum via vibronic coupling with the 1B1 state. The other two transitions exhibit well-resolved vibrational progressions. The high density of absorptions in the visible region confirms earlier suggestions that the H2C(=C)n family of cumulene carbenes merits careful consideration as potential carriers of diffuse interstellar bands.