Rotationally Resolved Predissociation Spectrum of the 1⁵Φ ← X⁵Δ Rovibronic Band of FeH

The elusive diatomic molecule FeH⁺ has long been hypothesized to exist in cool interstellar environments, yet its spectral signature has remained unidentified due to the lack of laboratory data. Its neutral counterpart FeH, on the other hand, is a well-known feature in the atmospheres of M-dwarfs, Sunspots, and hot Jupiter. Here we present the first rotationally resolved photodissociation spectrum of gas-phase FeH⁺, covering the energy range of 18550-18830 cm^-1 (5390.8-5310.7 Å). The rotational structure of the 1⁵Φ ← X⁵Δ electronic transition is resolved and conclusively assigned using state-of-the-art multireference and coupled cluster calculations and rovibrational spectra simulations. FeH⁺ spectral peaks in this specific band are broadened by predissociation, which arises from curve crossings of the bound 1⁵Φ potential curve with those of repulsive ⁷Π, ⁷Δ states. The photodissociation cross-section does not exceed 5 × 10^-20 cm², and comparison of the laboratory spectrum with observation data from HD 183143 does not reveal a match for this relatively weak band. Due to the uncertainty of the spin-rotation coupling constant, it is not possible to predict the precise positions of the rotational line spectrum. However, the high spin and orbital angular momentum quantum numbers of the electronic ground state place the onset of the spectrum above 1300 GHz, a frequency region that cannot be observed with ground-based telescopes.