X-ray photoelectron spectroscopy (XPS) is a highly surface-specific chemical analytical technique used to probe the elemental composition and bonding states in the outermost 2-10 nm of a solid surface.

XPS makes use of the photoelectric effect to stimulate electron emission from the sample surface with a high-energy X-ray source. The photoelectrons which escape the sample surface by this process have kinetic energies uniquely correlated to the elemental species of the atom they emit from. An energy analyzer is used to tally and measure the kinetic energies of the photoelectrons. This data is computationally transformed into a binding-energy scale. The system then outputs a final spectrum contrasting binding energy (BE) with measured signal intensity, called a survey scan allowing for quantitative elemental composition determination. High-energy-resolution scans of selected elemental peaks are also done to analyze and quantify bonding states of the elements present. To characterize composition as a function of penetration depth in the sample, depth-profiling tests can be performed to iteratively access sub-surface compositional layers.