Diatomic FeH+ has almost all it needs to be a carrier of diffuse interstellar bands

The carriers of the diffuse interstellar bands (DIBs) in the interstellar medium are mostly unidentified, with C60+ as a notable exception. We here present electronic photodissociation spectra of the diatomic iron hydride cation FeH+, which underline that diatomic molecules can cause absorption bands that resemble DIBs. The spectrum is dominated by a broad absorption centered around 310 nm, which results from allowed transitions into the repulsive section of quintet state potential curves. However, we also find weak absorptions to bound states with band widths in the range of 10-20 Å. Multireference calculations including spin-orbit coupling indicate the presence of a dense manifold of states, which arises from spin-orbit splitting of a group of repulsive septet states connecting to the Fe+(6D) + H ground state asymptote, and bound triplet states connecting to excited state asymptotes. Since we observe the Fe+ fragment in the mass spectrum, the molecule undergoes predissociation following excitation to these bound states, most likely via repulsive septet states, but multiple curve crossings involving triplet and quintet states may also lead to dissociation. Predissociation is known to cause substantial line broadening, which is a mechanism that leads to a diffuse absorption band in a diatomic spectrum. Several bands are observed close to known DIBs and have matching widths, but do not show the required match in the position, and their intensity is very weak. Given the complex photophysics of this diatomic molecular ion, further experiments are needed to fully rule out FeH+ as a DIB carrier.