Assignment of 5069 Å diffuse interstellar band to HC4H+ : disagreement with laboratory absorption band

Krelowski et al. have reported a weak, diffuse interstellar band (DIB) at 5069 angstrom which appears to match in both mid-wavelength and width the A(2)Pi(u)-X-2 Pi(g) gas-phase origin absorption band of HC4H+. Here, we present laboratory rotational profiles at low temperatures which are then compared with the 5069 angstrom DIB using similar to 0.1 and 0.3 angstrom line widths based on a realistic line-of-sight interstellar velocity dispersion. Neither the band shape nor the wavelength of the maximum absorption match, which makes the association of the 5069 angstrom DIB with HC4H+ unlikely. The magnetic dipole transition X-2 Pi(g)Omega = 1/2 -> X-2 Pi(g)Omega = 3/2 within the ground electronic state which competes with collisional excitation is also considered. In addition, we present the laboratory gas-phase spectrum of the A(2)Pi(u)-X-2 Pi(g) transition of HC4H+ measured at 25 K in an ion trap and identify further absorption bands at shorter wavelengths for comparison with future DIB data.