Asymmetric Peak Shapes

For conductive samples, such as metals and graphite, there is a distribution of unfilled one-electron levels (conduction electrons) that are available for shake-up like events following core electron photoemission. When this occurs, instead of a discrete structure like that seen for shake-up satellites, a tail on the higher binding energy side of the main peak – an asymmetric peak shape is evident[1]. An example of this is shown in Figure 1 for a sputter cleaned vanadium metal surface. It is clear from this figure that the asymmetric tail of the metal peak shape will overlap with higher oxidation state species. As such it is important that the total photoelectron yield contribution from the metal is captured during curve-fitting analysis. The use of standard spectra that is fit with mathematically derived asymmetric peak shapes allows for this.

Figure 1. Asymmetric peak shapes in the V 2p spectrum of an argon ion sputter cleaned surface of vanadium metal [2].

David Morgan at Cardiff University has recently published an excellent insight article [3] on asymmetric peak shapes in XPS.  This article goes into detail about the causes of asymmetry, curve-fitting of asymmetric peaks, implications of using hard X-ray sources (HAXPES), and asymmetry in other materials such as conductive metal oxides, graphitic materials, and polymeric materials. Well worth the read for a more in-depth look.

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] M.C. Biesinger, L.W.M. Lau, A.R. Gerson, R.St.C. Smart, Resolving Surface Chemical States in XPS Analysis of First Row Transition Metals, Oxides and Hydroxides: Sc, Ti, V,Cu and Zn, Applied Surface Science, 257 (2010) 887-898.

[3] D.J. Morgan, XPS insights: Asymmetric peak shapes in XPS, Surface and Interface Analysis, 55 (2023) 567-571.