Multiplet splitting arises
when an atom contains unpaired electrons (e.g. Cr(III), 3p63d3).
When a core electron vacancy is created by photoionization, there can be
coupling between the unpaired electron in the core with the unpaired electrons
in the outer shell. This can create a number of final states, which will be
seen in the photoelectron spectrum as a multi-peak envelope[1]. Figure 1 shows the
multiplet structure associated with the Cr 2p3/2
peak for a vacuum fractured Cr2O3 specimen.
Figure
1. Multiplet
structure associated with the Cr 2p3/2 peak for a vacuum
fractured Cr2O3 specimen.
The
early Hartree-Fock calculation of the multiplet structure of core p-valence levels of free ion state first
row transition metals by Gupta and Sen[2] graphically shows their multiplet structures
(Figure 2). These calculations are an excellent starting point for the
examination of multiplet structure observed for transition metal compounds.
However, they apply to free ion states only and, in transition metals and their
compounds, there may be ligand charge transfer effects that will change the
spacing and intensity of the multiplet peaks present in their spectra. These
relative changes can be utilized for transition metal compounds to differentiate
those more closely approximating free ions from those in which charge transfer
from the bonded neighbouring ions may have changed both the effective oxidation
state and multiplet splitting of the core transition metal[3,4,5,6].
This change in local electronic structure has been used to explain the differences
between the XPS spectra of nickel oxide and its oxy/hydroxides [3,7]. De Groot and
Kotani’s text “Core Level Spectroscopy of Solids”[8] provides an excellent advanced analysis of multiplet effects and their use in
the modeling of spectra for both XPS and XAS.
Figure 2. Calculated multiplet structure of 2p ionisation created in the free ions as labelled. The zero energy is arbitrary and the intensity normalization is the same for all spectra shown [From 2]. Table 1 summarizes the various first row transition metal species that show multiplet splitting in their XPS spectra. Sc, Ti, V, Cu and Zn species, where multiplet splitting is not present or, if present, is generally not well resolved or shows as peak broadening only[9]. Cr, Fe, Mn, Co and Ni species show significant multilpet spitting[3,4,5,6,7,10,11].
Table 1. First row transition metal species that show multiplet splitting in their XPS spectra. This is for high spin compounds. For low spin Fe(II) and low spin Ni(II) electrons are paired and no multiplet splitting is observed.
References:
[1] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp, Eden Prairie, MN, 1992.References:
[2] R.P. Gupta, S.K. Sen, Phys. Rev. B 12 (1975) 15.
[3] A.P. Grosvenor, M.C. Biesinger, R.St.C. Smart, N.S. McIntrye, Surf. Sci. 600 (2006) 1771.
[4] N.S. McIntyre, D.G. Zetaruk, Anal. Chem. 49 (1977) 1521.
[5] M.C. Biesinger, C. Brown, J.R. Mycroft, R.D. Davidson, N.S. McIntyre, Surf. Interface Anal. 36 (2004) 1550.
[6] A.P. Grosvenor, B.A. Kobe, M.C. Biesinger, N.S. McIntyre, Surf. Interface Anal. 36 (2004) 1564.
[7] M.C. Biesinger, L.W.M. Lau, A.R. Gerson, R.St.C. Smart, Physical Chemistry Chemical Physics, 14 (2012) 2434.
[8] F. de Groot, A. Kotani, Core Level Spectroscopy of Solids, CRC Press, Boca Raton, 2008.
[9] M.C. Biesinger, L.W.M. Lau, A.R. Gerson, R.St.C. Smart, Appl. Surf. Sci. 257 (2010) 887.
[10] M.C. Biesinger, B.P. Payne, A.P. Grosvenor, L.W.M. Lau, A.R. Gerson, R.St.C. Smart, Appl. Surf. Sci. 257 (2011) 2717.
[11] M.C. Biesinger, B.P. Payne, L.W.M. Lau, A.R. Gerson, R.St.C. Smart, Surf. Interface Anal. 41 (2009) 324.
