XPS is an abbreviation for X-ray Photoelectron Spectroscopy
ESCA is an acronym for Electron Spectroscopy for Chemical Analysis. This could apply to any electron spectroscopy but is usually reserved for XPS

Photoelectrons
When light strikes an atom an electron may be ejected if the energy of the light is high enough. The energy in the light is determined by its wavelength or frequency (short wavelength = high energy and high frequency = high energy)
X-rays have high energy. When X-rays strike a solid electrons are always ejected from the near-surface region of the solid.

XPS
If we measure the energy of the ejected photoelectrons we can calculate its Binding Energy which is the energy required to remove the electron from its atom. From the binding energy we can learn some important facts about the sample under investigation:

• The elements from which it is made     
• The relative quantity of each element     
• The chemical state of the elements present

Modern XPS instruments can also produce images or maps showing the distribution of the elements or their chemical states over the surface. A good instrument would have a spatial resolution of a few microns.

See also parallel imaging

Electrons can only escape from the solid if they originate close to the surface. If they originate from too deep within the bulk of the solid then they will be reabsorbed or suffer inelastic collisions. The information available from XPS is, therefore, highly specific to the surface of the material.

Components of an XPS Instrument
An XPS instrument has two main components:

• An X-ray source, preferably monochromatic     
• An electron energy analyser, usually a spherical sector analyser

The measurements must be made in ultra-high vacuum (uhv), for two reasons: 

• To allow the photoelectrons to travel from the surface of the sample to the detector without striking a gas atom     
• If a clean surface is prepared for analysis, it would become contaminated if it were not under uhv.

Other, optional, items may also be present on an XPS spectrometer: 

• A low energy electron flood gun which must be used to prevent insulating samples from becoming charged during analysis     
• An ion source which is used both to clean a surface prior to analysis and to erode the surface of the sample so that concentration depth profiles can be measured.

Many XPS spectrometers have other analytical techniques associated with them for example: 

• Auger Electron Spectroscopy (AES)     
• Ultraviolet photoelectron spectroscopy (UPS)

XPS instruments from Thermo VG Scientific are the ESCALAB 250 and the Sigma Probe.