2010ApJ...714L..64K

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 ²Π_u-X ²Π_g(0-0) low-temperature gas-phase optical absorption. HC₄H⁺ 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 HC₆H⁺, expected at 6001.1 Å, which implies the HC₆H⁺ to HC₄H⁺ ratio of less than ∼1/3.