Assignment of 5069 Â diffuse interstellar band to HC 4H+: Disagreement with laboratory absorption band

Abstract

Krełowski etal. have reported a weak, diffuse interstellar band (DIB) at 5069 which appears to match in both mid-wavelength and width the A 2Πu-X 2Πg gas-phase origin absorption band of HC4H+. Here, we present laboratory rotational profiles at low temperatures which are then compared with the 5069 DIB using 0.1 and 0.3 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 DIB with HC4H+ unlikely. The magnetic dipole transition X 2Πg Ω = 1/2→X 2Πg Ω = 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 2Πu-X 2Πg 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. © 2011. The American Astronomical Society. All rights reserved.