Ghost peaks occur when internal X-rays are produced within the sample, ejecting a detectable number of core-level photoelectrons. Since these internally produced X-rays have lower energy than the source X-rays, they eject photoelectrons with a lower kinetic energy (higher apparent binding energy). While ghost peaks in many cases appear with low intensity and therefore do not generally interfere with XPS analysis, they may cause confusion during peak assignment.
Ghost peaks are often observed for magnesium-rich samples from the Mg Kα emission lines (X-rays produced when a valence electron fills the Mg 1s core hole). The average photon energy of these lines is 1253.6 eV, and the position of the ghost peaks on the binding energy scale is dependent on the X-ray source. For example, the Mg Kα induced O 1s ghost peak is seen in the XPS spectra of Mg(OH)2 at 765.7 eV, when using an Al Kα source (1486.7 eV). This peak position is shifted by 233.1 eV, which is the energy difference between internal and source X-rays (Figure 1).
Alternatively, this can also be calculated as follows: the Mg K X-ray induced peak for O 1s is found at a binding energy of ~ 765.7 eV, as the kinetic energy of these photoelectrons would be 1253.6 eV (Mg Kα X-rays) - 532.6 eV (O 1s B.E.) = 721.0 eV. Thus, they would then be seen as a peak at a binding energy of 1486.7 ( Al K(α) X-rays) - 721.0 eV = 765.7 eV. Similarly, the C 1s Mg X-ray induced peak would be at a binding energy of ~517 eV.
Also of note for spectra of magnesium-rich samples: Mg Auger structure can be found at ~242 eV, which is not a ghost peak and often not noted in most libraries (Figure 1, Mg KLL-2) and may cause confusion during survey scan peak assignment.
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| Figure 1. Survey spectrum of Mg(OH)2 powder where the O 1s ghost peak is clearly visible at 765.7 eV. The figure is reproduced from reference [1]. |
