Identification of H2CCC as a diffuse interstellar band carrier

We present strong evidence that the broad, diffuse interstellar bands (DIBs) at 4881 and 5450 angstrom are caused by the (BB1)-B-1 <- X(1)A(1) transition of H2CCC (l-C3H2). The large widths of the bands are due to the short lifetime of the (BB1)-B-1 electronic state. The bands are predicted from absorption measurements in a neon matrix and observed by cavity ring-down in the gas phase and show exact matches to the profiles and wavelengths of the two broad DIBs. The strength of the 5450 angstrom DIB leads to a l-C3H2 column density of similar to 5 x 10(14) cm(-2) toward HD 183143 and similar to 2 x 10(14) cm(-2) to HD 206267. Despite similar values of E(B-V), the 4881 and 5450 angstrom DIBs in HD 204827 are less than one-third their strength in HD 183143, while the column density of interstellar C-3 is unusually high for HD 204827 but undetectable for HD 183143. This can be understood if C-3 has been depleted by hydrogenation to species such as l-C3H2 toward HD 183143. There are also three rotationally resolved sets of triplets of l-C3H2 in the 6150-6330 angstrom region. Simulations, based on the derived spectroscopic constants and convolved with the expected instrumental and interstellar line broadening, show credible coincidences with sharp, weak DIBs for the two observable sets of triplets. The region of the third set is too obscured by the alpha-band of telluric O-2.