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

## TL;DR

This paper studies the UV photodissociation of FeOH+ in space-like conditions, providing insights into its stability and electronic structure.

## Contribution

The study provides a detailed UV photodissociation spectrum of FeOH+ and compares it with quantum chemical calculations.

## Key findings

- The photodissociation threshold of FeOH+ is measured at 3.47 ± 0.02 eV.
- Experimental and theoretical results agree on electronic transitions between 3.45–4.25 eV.
- Photodissociation cross sections are significantly smaller than calculated absorption cross sections.

## Abstract

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.