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abstract
A closely spaced group of bands near 5800 \AA in the emission spectrum of the nebula of the Red Rectangle were first observed in 1980. Others have speculated that carbon chain molecules could be responsible for these and other bands but quantitative assignments are still lacking. Phosphorescence from triatomic carbon, C$_{3}$, has been observed near that wavelength in a cryogenic matrix by several other groups. Recently, precise measurements have been made of the ground-state frequencies and overtones of C$_{3}$. These results and the particularities of the spectra have allowed us to assign definitively the Red Rectangle bands (RRBs) to phosphorescence from C$_{3}$. Several of these bands have been suggested to correspond to the enigmatic diffuse interstellar bands(DIBs). If the DIB/RRB correspondence is true, then several DIBs are due to very weak absorptions to an excited triplet state from vibrationally-excited, ground-state C$_{3}$ molecules.
