Core levels in XPS use the
nomenclature nlj where n is the principal quantum number, l is the angular momentum quantum number
and j = l + s (where s is the spin angular momentum number
and can be ±½). All orbital levels except the s levels (l = 0) give rise to a doublet with the two possible states having
different binding energies. This is known as spin-orbit splitting (or j-j
coupling)[1]. The peaks will also have specific
area ratios based on the degeneracy of each spin state, i.e. the number of
different spin combinations that can give rise to the total j (see Table 1). For example, for the
2p spectra, where n is 2 and l is 1, j will be 1/2 and
3/2. The area ratio for the two spin orbit peaks (2p1/2:2p3/2) will be 1:2 (corresponding to 2
electrons in the 2p1/2 level
and 4 electrons in the 2p3/2
level). These ratios must be taken into account when analyzing spectra of the p, d
and f core levels. An example of this
splitting for the Sc 2p peak for Sc2O3
is shown in Figure 1. Spin-orbit splitting values (eV) can be found in a
variety of databases[2,3]. These values will be needed when
fitting spectra where the chemical shifts are larger than the spin-orbit splitting.
For example, the As 3d spectrum for
an oxidized GaAs surface in Figure 2 shows that all spin-orbit doublets must
be fit in order to properly identify the species present. The 3d5/2 and 3d3/2 doublet for each
chemical specie is constrained to have 3:2 peak area ratios, equal FWHM, and a
peak separation of 0.69 eV.
Table 1. Spin-orbit splitting j values and peak area ratios.
Figure 1. An example of spin-orbit splitting in the Sc 2p
spectrum of Sc2O3.
Figure 2. The As 3d spectrum of a sample of oxidized GaAs. Each chemical specie is
fit with the 3d5/2 and 3d3/2 doublet that is
constrained to have a 3:2 peak area ratio, equal FWHM for the two peaks of the
doublet, and a peak separation of 0.69 eV.
References:
[1] D. Briggs, XPS: Basic Principles, Spectral Features and Qualitative Analysis, in: D. Briggs, J.T. Grant (Eds.), Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, IM Publications, Chichester, 2003, pp. 31-56.
[2] 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.
[3] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R. Jr. Rumble, NIST Standard Reference Database 20, Version 3.4 (Web Version) (http:/srdata.nist.gov/xps/) 2003.
