Doublet Separations #

Fe 2p = 13.1 eV

Fe 3p = 0.8 eV

Common Overlaps for Fe 2p #

Sn 3p – Th 4d – Pt 4s – Cs 3d – Tl 4p In 3p – Po 4p – Ag 3s – Co LMM (Al Ka X-rays)

Auger Energies #

Note – these are listed in KINETIC ENERGY

Fe LMM ≈ 695 eV

Common Binding Energies – Fe 2p #

Theory and Background #

Iron Metal #

Iron metal exhibits a typical asymmetric peak shape with a doublet consistent with a p-type orbital. The 2p_3/2 peak is centred around 706.7 eV and the doublet separation is 13.1 eV.

Low spin Iron compounds #

Like Iron metal, low spin Fe compounds do not undergo multiplet splitting and hence exhibit a single photoemission peak. Unfortunately, due to the Fe²⁺ centre requiring a rather large ligand field strength to split the crystal field, such low spin states are not often found amongst the commonly analysed iron compounds such as oxides.

High spin Iron compounds #

High spin iron compounds are where things become tricky. Immediately obvious is the increased peak width of the Fe 2p_3/2 peak in figure 3. This broadening was found to be largely due to coupling between the 2p core-hole and the unpaired 3d electrons of the photoionized Fe cation, as well as electrostatic and crystal field interactions.^(3),(4)

As with any first row TM multiplet (and a general rule for XPS analysis), be careful fitting and only fit what is necessary. Oxidation states may be obtained through a simple process of lineshape analysis – similar to that found in XANES.

If, however, fitting is required, detailed procedures may be found in the work of Biesinger et al.⁽⁵⁾

The intensity of the resultant satellite may be influenced by the identity of the ligand, with increasing electronegativity diminishing the intensity of the satellite – due to a larger barrier for the relaxation process.⁽⁷⁾ Furthermore, as electronegativity decreases, as does the energy separation between the satellite and core-line envelope centre. Since a decrease in the electronegativity of the ligand increases the charge density around the metal centre, promoting an electron from the 3d orbital to an unfilled 4s state becomes easier due to the increased shielding and thus the photoelectron loses a smaller amount of kinetic energy during the shake-up process.⁽⁸⁾

Crystal field splitting energy #

Grosvernor et al note that the crystal field splitting energy plays an integral role on eventual peak shape, with stronger crystal field spitting ligands causing a greater perturbation from the Gupta and Sen free ion multiplet structure than that observed from weaker crystal field splitting ligands.⁽⁹⁾

Experimental Advice #

Iron is a complex region to understand, and it is often advisable to record a slightly extended energy region (695 – 760 eV) than you might typically think to, to ensure appropriate backgrounds may be selected.

Data Analysis Guidance #

1. Data acquired by HarwellXPS
2. Data acquired by HarwellXPS, view it online here, https://doi.org/10.5281/zenodo.4322801
3. Gupta, R. and S. Sen (1974). “Calculation of multiplet structure of core p-vacancy levels.” Physical Review B 10(1): 71. Read it online here.
4. Gupta, R. and S. Sen (1975). “Calculation of multiplet structure of core p-vacancy levels. II.” Physical Review B 12(1): 15. Read it online here.
5. Biesinger, M. C., et al. (2011). “Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni.” Applied Surface Science 257(7): 2717-2730. Read it online here.
6. Data acquired by HarwellXPS
7. Sarma, D., et al. (1982). “Satellites in the X-ray photoelectron spectra of transition-metal and rare-earth compounds.” Chemical Physics 73(1-2): 71-82. Read it online here.
8. Pauling, L. (1960). The Nature of the Chemical Bond, Cornell university press Ithaca, NY. Read it online here.
9. Grosvenor, A., et al. (2004). “Investigation of multiplet splitting of Fe 2p XPS spectra and bonding in iron compounds.” Surface and Interface Analysis: An International Journal devoted to the development and application of techniques for the analysis of surfaces, interfaces and thin films 36(12): 1564-1574. Read it online here.

Reference Datasets #

Coming soon

References #