Spin Orbit Splitting

Core levels in XPS use the nomenclature nl_j 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 (2p_1/2:2p_3/2) will be 1:2 (corresponding to 2 electrons in the 2p_1/2 level and 4 electrons in the 2p_3/2 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 3d_5/2 and 3d_3/2 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 3d_5/2 and 3d_3/2 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.