Cyclic C₄ ⁺ as the Carrier of the Diffuse Interstellar Band at 503.9 nm?

The diffuse interstellar bands (DIBs) have remained a mystery in astronomy since their discovery over a century ago. The only currently known carrier is C 60 + , responsible for five DIBs, while more than 550 are yet to be interpreted. The spectra of short carbon chain cations ( C n + ) , which are considered one of the most promising classes of species for the role of carriers of DIBs, are successfully recorded using He-tagging spectroscopy. The comparison of laboratory spectra with observations demonstrates a close match of two absorption bands of C 4 + with the broad DIB at 503.9 nm. This defines a high abundance of these ions in the interstellar medium (ISM), which should exceed those of other similar-sized carbon chain cations. It is anticipated that all other short carbon chain cations will exhibit linear geometry and, as a consequence, will have a long vibrational progression. However, the distinctive cyclic geometry of C 4 + is postulated to underpin the elevated abundance of these ions in the ISM, as well as the distinctive spectrum of this ion, which displays a single, strong, relatively narrow absorption band that exceeds in intensity all other absorption bands in the visible range.