Relative Sensitivity Factors #
Relative sensitivity factors (RSFs) in X-ray photoelectron spectroscopy (XPS) are calibration coefficients that account for differences in the efficiency of detecting electrons from various elements and orbitals. These factors are crucial for converting XPS peak intensities into meaningful quantitative elemental concentrations. Since each element and electron energy level responds differently to X-ray excitation and electron detection, RSFs correct for these variations, enabling reliable comparison of element abundances in sample surfaces. Typically, RSFs are determined experimentally or provided by instrument manufacturers based on standardized measurements, ensuring consistent and accurate quantification in XPS analysis.
RSFs are typically calculated using:
RSFx ∝ σx, nl(hv) . λx(Ek) . T(Ek) . A(θ)
Where:
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σx, nl(hv) : Photoionization cross-section for element X, subshell nl, at photon energy hν
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λx(Ek): Inelastic mean free path for kinetic energy Ek
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: Transmission function of the spectrometer for electrons of energy Ek
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: Angular factor depending on take-off angle and geometry
If high accuracy or non-standard conditions are required, RSFs should be measured using calibration samples:
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Select pure standards or well-characterized thin films: Choose materials with known stoichiometry and a flat, homogeneous surface.
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Measure XPS spectra: Acquire spectra under the same conditions as you will use for your actual samples (same analyzer settings, geometry, etc.).
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Calculate integrated peak areas: Correct these areas for background and instrumental effects.
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Apply known compositions: Use the known atomic concentration of each element to calculate the RSF using the formula:
RSFx = Ax / Cx
Where Ax is the measured (corrected) peak area for element , and is the atomic fraction of element in the standard.
The Scofield Library refers to a widely used set of atomic data, specifically calculated atomic photoionization cross-sections and related parameters, developed by J.H. Scofield using quantum mechanical methods such as Hartree-Slater or Dirac-Slater calculations.
