The UV Photodissociation Spectrum of FeOH⁺: Electronic Insight into the Simplest Iron Hydroxide Complexes

FeOH⁺ has been proposed to exist in cold interstellar environments due to the high cosmic abundance of hydrogen, oxygen, and iron. In this study, we report the UV photodissociation spectrum of FeOH⁺ in the photon energy range 2.5-5.9 eV, complemented by high-level quantum chemical calculations. Previous studies focused on determining the bond dissociation energy of FeOH⁺ through measuring the threshold energy for its photodissociation into Fe⁺ and OH. Our observed photodissociation threshold of 3.47 ± 0.02 eV is an upper limit for the Fe⁺-OH bond dissociation energy and agrees within error limits with recent collision-induced dissociation data. In addition, the spectra provide insight into electronically excited states and quantitative photodissociation cross sections. The experimental band positions agree very well with theoretical calculations on the EOM-CCSD/aug-cc-pVTZ level in the energy range of 3.45-4.25 eV, while the measured photodissociation cross sections <3 × 10^-19 cm² are an order of magnitude smaller than the calculated absorption cross sections. The overall broad electronic transitions of FeOH⁺ in this region exhibit a weak substructure with an energy spacing in the range of calculated frequencies of the Fe-O stretching mode in various electronic states, which hints at vibrational progressions. After a gap of about 0.75 eV, both experiment and theory exhibit a second broad absorption feature above 5.0 eV. At these high energies, experimental photodissociation and calculated absorption cross sections show better agreement. The measured photodissociation cross sections allow estimation of the lifetime of FeOH⁺ in interstellar environments with respect to photodissociation, which is key data for the integration of the molecule in models of interstellar chemistry.