Evidence for Diacetylene Cation as the Carrier of a Diffuse Interstellar Band

High-quality spectra acquired at three different observatories point to the presence of a new diffuse interstellar band (DIB) at 5069 Å. The spectral profile of this DIB matches published laboratory measurements of the diacetylene cation A 2 Π u – X 2 Π g (0–0) low-temperature gas-phase optical absorption. HC 4 H + is approximately 60–80 times less abundant than CH along the analyzed lines of sight. Only an upper limit could presently be inferred from the search for an analogous band of the triacetylene cation HC 6 H + , expected at 6001.1 Å, which implies the HC 6 H + to HC 4 H + ratio of less than ∼1/3.