Iron L 3 -edge energy shifts for the full range of possible 3d occupations within the same oxidation state of iron halides

Oxidation states are integer in number but dn configurations of transition metal centers vary continuously in polar bonds. We quantify the shifts of the iron L3 excitation energy, within the same formal oxidation state, in a systematic L-edge X-ray absorption spectroscopy study of diatomic gas-phase iron(II) halide cations, [FeX]+,where X = F, Cl, Br, I. These shifts correlate with the electronegativity of the halogen, and are attributed exclusively to a fractional increase in population of 3d-derived orbitals along the series as supported by charge transfer multiplet simulations and density functional theory calculations. We extract an excitation energy shift of 420 meV ± 60 meV spanning the full range of possible 3d occupations between the most ionic bond in [FeF]+ and covalently bonded [FeI]+.

1 Sample preparation

Magnetron sputter source
Fe + cations also used as a precursor for FeCl + production are generated by argon sputtering of an iron target. For FeCl + production, iron ions are exposed to CH 2 Cl 2 gas in a collision cell at a pressure of ≈ 10 −4 mbar. FeCl + is subsequently isolated using a quadrupole mass filter.

Data treatment
All data sets were acquired at the UE52PGM Ion Trap setup [1] with a bandwidth of 80 meV (step size 0.03 eV) for the high resolution spectra and 200 meV (step size 0.08 eV) for the overview spectra.
For consistency across different samples only partial ion yield of Fe 2+ product ions are considered. We, however, also checked the partial ion yield of all other photo ions to be proportional to Fe 2+ . Hence, the presented data is proportional to the total ion yield and therefore also to the X-ray absorption.

Energy calibration
In order to account for photon energy drift over time, the energy in all scans was sequentially referenced after acquisition to one FeCl + scan of high signal-to-noise ratio. The latter was acquired right after energy calibration during commissioning of the beamline.

Averaging procedure and error estimation
To improve the signal-to-noise ratio, multiple scans of the same sample were averaged after single scan referencing for energy calibration, using linear interpolation. The total uncertainty ∆E total was obtained through error propagation taking into account the uncertainties of beamline (∆E 1 ), sample (∆E 2 ) and reference (∆E 3 ) scan reproducibility, and of the sequential referencing during analysis (∆E 4 ).

Charge transfer multiplet calculations
The calculated spectra were shifted to fit the energy position of the experimental data by the amounts shown in table S5.  Table S5: Parameters of the charge transfer multiplet calculations (using CTM4XAS code [2]) shown in the main text of the paper in fig. 1 and the resulting 3d occupation of FeX + .

Turbomole density functional theory (DFT)
The Turbomole DFT calculation were done with the B3LYP functional and the def2-TZVP basis set, with the iron 3d orbital population derived by natural population analysis, after a geometry optimization was performed. [3] System Fe 3d(tot.)/e FeF + 6.34 FeCl + 6.53 FeBr + 6.57 FeI + 6.73 Table 4: Natural populations of iron 3d derived from Turbomole DFT calculations.

Overview spectra
In addition to the high resolution spectra of the iron L 3 -edge, we also measured overview spectra of the iron L 2,3 -edge (see figure S1).