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X-ray Photoelectron Spectroscopy (XPS, ESCA)

Introduction

X-ray photoelectron spectroscopy (XPS, also called electron spectroscopy for chemical analysis, ESCA) is a electron spectroscopic method that uses X-rays to eject electrons from inner-shell orbitals. The kinetic energy, Ek, of these photoelectrons is determined by the energy of the X-ray radiation, h, and the electron binding energy, Eb, as given by:

Ek = h - Eb

The experimentally measured energies of the photoelectrons are given by:

Ek = h - Eb - Ew

where Ew is the work function of the spectrometer.

Energy-level diagram for XPS

The electron binding energies are dependent on the chemical environment of the atom, making XPS useful to identify the oxidation state and ligands of an atom.


Instrumentation

XPS instruments consist of an X-ray source, an energy analyzer for the photoelectrons, and an electron detector. The analysis and detection of photoelectrons requires that the sample be placed in a high-vacuum chamber. Since the photoelectron energy depends on X-ray energy, the excitation source must be monochromatic. The energy of the photoelectrons is analyzed by an electrostatic analyzer, and the photoelectrons are detected by an electron multiplier tube or a multichannel detector such as a microchannel plate.


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