Praseodymium XPS is typically performed on the Pr 3d orbitals. It will strongly overlap with the Cu 2p and I 3p photoemission, as well as weakly overlap with the Bi 4s emission, and augers from Cs, Mn and Ba when using Al X-rays (1486.7 eV). The binding energies of the metal and oxides may be found in Table 1. Note, the peak position of the oxides is that of the ‘m’ peak (see later section in this article).
|Species||Binding energy / eV||Doublet Separation / eV||Ref|
Praseodymium metal comprises of a mostly singular asymmetric 3d5/2 peak with a spin-orbit splitting to the 3d3/2 of 20.2 eV.(1,4) There are, however, small satellite peaks to lower binding energy due to the screened 3d9 4f3 final state.(1) Additionally, the Pr 3d3/2 photoemission does undergo splitting, thought to be a shake-up satellite in which an electron from the sd band during photoemission is excited into an empty 4f state (pulled below the Fermi level following photoemission) with the absence of this transition from the 3d5/2 peak attributed to a lower ratio of this transition between 3d5/2:3d3/2.(1)
Note for below: WordPress does not support underlining characters – so core holes will be described using a bold text-type rather than an underlined text-type. Apologies for the confusion.
The oxides of Pr are complicated by the presence of intense satellite features. Praseodymiuin ions in Pr2O3 are in a 4f2 configuration in the ground state, however twin final states of 4f2 and 4f3L (L = hole in O 2p valence band) cause each 3d photoemission to split into two, induced by a core-hole potential acting on the 4f electrons.(2) PrO2 in the ground state is a mixture of 4f1 and 4f2L.(3)
Distinguishing between Pr3+ and Pr4+ by XPS can be challenging, due to the complex nature of final state effects in such systems, however, careful analysis of the Pr 3d5/2 lineshape and high energy final state effects affords an appropriate methodology. Analysis of the 3d5/2 photoemission reveals the presence of one photoemission peak (m) and one shake-down satellite (s) (Figure 2) at lower binding energy arising from a well screened 4f3 final state.
The ratio between these two peaks may be used as a semi-quantitative probe since it varies between Pr3+ and Pr4+, with Pr2O3 reporting a higher ratio of m:s than PrO2 (table 2).(4)
Further support for the assignment of Pr4+ can be found in an an extra satellite peak, not observed in Pr or Pr2O3, at ~967 eV arising from 3d4f1 final states, although this photoemission is relatively weak and may be hard to justify in materials with a lower praesodymium content, especially since it may overlap with the O KLL auger (note, the Pr 3d5/2 emission does also produce this peak in PrO2, however it overlaps with the 3d4f2L, and 3d4f3L2 peaks).(3)
- Crecelius, G., et al. (1978). “Core-hole screening in lanthanide metals.” Physical Review B 18(12): 6519. Read it online here.
- Ogasawara, H., et al. (1991). “Praseodymium 3d-and 4d-core photoemission spectra of Pr 2 O 3.” Physical Review B 44(11): 5465. Read it online here.
- Bianconi, A., et al. (1988). “Many-body effects in praesodymium core-level spectroscopies of PrO 2.” Physical Review B 38(5): 3433. Read it online here.
- Lütkehoff, S., et al. (1995). “3d and 4d x-ray-photoelectron spectra of Pr under gradual oxidation.” Physical Review B 52(19): 13808. Read it online here.