Iron Complexes as Potential Carriers of Diffuse Interstellar Bands: The Photodissociation Spectrum of Fe⁺(H₂O) at Optical Wavelengths

The fullerene ion C₆₀⁺ is the only carrier of diffuse interstellar bands (DIBs) identified so far. Transition-metal compounds feature electronic transitions in the visible and near-infrared regions, making them potential DIB carriers. Since iron is the most abundant transition metal in the cosmos, we here test this idea with Fe⁺(H₂O). Laboratory spectra were obtained by photodissociation spectroscopy at 80 K. Spectra were modeled with the reflection principle. A high-resolution spectrum of the DIB standard star HD 183143 served as an observational reference. Two broad bands were observed from 4120 to 6800 Å. The 4120–4800 Å band has sharp features emerging from the background, which have the width of DIBs but do not match the band positions of the reference spectrum. Calculations show that the spectrum arises from a d–d transition at the iron center. While no match was found for Fe⁺(H₂O) with known DIBs, the observation of structured bands with line widths typical for DIBs shows that small molecules or molecular ions containing iron are promising candidates for DIB carriers.