When I first saw it I expected it to be atomic in origin because it looks so narrow (Think NaD or K 7699 interstellar lines) but I found no matches. Apparently it is probably (I should say possibly, nothing seems very clear from the literature I have found) formed by a high molecular weight PAH molecule but exactly which one is not clear. It is fascinating that something that is seen in so many spectra and has been known about for many decades is still a mystery
Cheers
Robin