XPS Basics: Which X-ray Sources Are Most Common and Why!

Understanding Energy Sources in XPS Analysis

Importance of Energy Sources

• The choice of energy sources in X-ray Photoelectron Spectroscopy (XPS) is crucial as the maximum binding energies of detectable elements are typically below 1200 eV. This ensures effective photoelectron peak detection.

• Photoelectron peaks exhibit binding energy that remains constant regardless of the X-ray source used, whether magnesium or aluminum. In contrast, OJ electron peaks depend on the specific X-ray source employed.

Binding Energy and X-Ray Source

• When X-rays strike a sample, photoelectrons are ejected with kinetic energy determined by the known energy of the X-ray source and the work function of the spectrometer. The intrinsic binding energy does not change with different X-ray sources.

• A minimum threshold for the X-ray source is necessary to emit electrons from core levels; for instance, an energy greater than 1000 eV is required to eject electrons effectively from certain materials. Lower energies may fail to produce detectable photoelectrons.

Commonly Used X-Ray Sources

• Two primary types of X-ray sources are utilized in XPS: Magnesium K-alpha and Aluminum K-alpha, each providing fixed energies essential for generating photoelectrons and OJ electrons within a suitable range for analysis.

• The maximum binding energy for all detectable elements must be below 1200 eV; otherwise, insufficient energy will prevent electron emission from core levels during analysis. For example, using a 900 eV or 1000 eV source would not yield usable results due to inadequate energy levels.

Distinction Between Photoelectron and OJ Electron Peaks

• In XP spectra, two major fixed peaks are observed: one for photoelectrons and another for OJ electrons; importantly, photoelectron binding energies remain independent of the chosen X-ray source while OJ electron binding energies do vary based on this factor.

• An example spectrum using Mg K-alpha shows significant peaks ranging from 0 to 1200 eV where both OJ and photoelectron peaks can be identified clearly within this survey spectrum context. This indicates that Mg K-alpha is sufficient to remove maximum quantities of both types of electrons from tungsten samples analyzed in high resolution.

High Resolution Spectra Observations

• Upon switching from magnesium to aluminum as an X-ray source, it was noted that while some peak positions remained unchanged (e.g., those between 25 to 45 eV), others such as OJ electron peaks shifted significantly indicating their dependence on the type of radiation used during analysis (from around 1050 eV up). This highlights how varying sources affect specific measurements differently even if some aspects remain stable across different conditions.