A critical examination of the l-C₃H₂^- spectrum and the diffuse interstellar bands

It has recently been suggested by J. P. Maier's group that the origin band and three vibronic bands of the linear propadienylidene anion l-C ₃H₂^- match the diffuse interstellar bands (DIBs). We have examined the wavelength ranges in question using data from our ongoing DIB survey at the Apache Point Observatory. We find that the strongest DIB (λ6993) is not an acceptable wavelength match to the origin band of l-C₃H₂^-, based on high-resolution laboratory data. The nondetection of interstellar features corresponding to the K = 2 ← 1 and K = 0 ← 1 branches of para l-C₃H₂ ^- also argues against the assignment of λ6993 to the K = 1 ← 0 branch of ortho l-C₃H₂^-. Two of the three DIBs that have been attributed to vibronic bands do not correlate in intensity with λ6993, providing further evidence against the assignment of this set of DIBs to l-C₃H₂^-.