The basic principle of hard x-ray photoelectron spectroscopy is similar to that of general XPS, in which a sample is irradiated with photons, and the count and kinetic energy of emitted photoelectrons is measured. While the photon energy of the monochromatic Al Kα x-ray source most commonly used in traditional XPS instruments is 1486.6 eV, the photon energy of the excitation source used in hard x-ray photoelectron spectroscopy is 5 to 8 keV, more than triple. “Hard x-ray photoelectron spectroscopy” is abbreviated as HX-PES or HAXPES. HAXPES is traditionally performed at synchrotron facilities.

Due to the higher energy of the primary x-ray source compared to XPS, information from deeper sampling depths can be obtained. Moreover, the photoelectron spectrum obtained using a hard x-ray contains additional spectroscopic information not available with general XPS. Photoelectrons from deeper core levels can be excited by hard x-ray compared to soft Al Kα x-ray. For example, for Si, soft x-ray can excite up to the 2s orbital only, but hard x-ray can excite up to the 1s orbital. Figure 1 shows additional transitions accessible using a Cr Kα source at 5414.7 eV photon energy.

Figure 1. Electronic transitions available for analysis using Al Kα and Cr Kα  x-ray sources​

Photoionization cross-section is a probability of photoelectron excitation, and the intensity of the spectrum is proportional to this value. As the excitation energy increases, the photoionization cross-section for electrons that have high kinetic energy (their binding energy is much lower than excitation energy) decreases significantly. Figure 2 shows the photoionization cross-section of Si and Ag for different excitation energies. For the commonly-used Si 2p3/2 and Ag 3d5/2 transitions, photoionization cross-section decreases by 1-2 orders of magnitude when a Cr x-ray source is used instead of Al. At the same time, higher binding energy Si 1s and Ag 2p3/2  transitions, which have higher cross-sections, can be measured using a CrKα x-ray source.

Figure 2. Photoionization cross-section as a function of excitation energy​

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