effect The photoemission or photoelectric effect is a quantum electronic phenomenon in which electrons (photoelectrons) are emitted from matter after the absorption of energy from electromagnetic radiation such as UV light or X-ray. When UV light or X-ray is absorbed by matter, electrons are excited from core levels into unoccupied states, leaving empty core states. Secondary electrons are generated by the decay of the core hole. Auger processes and inelastic electron scattering create a cascade of low-energy electrons. Some electrons penetrate the sample surface and escape into vacuum. A wide spectrum of electrons is emitted with energies between the energy of the illumination and the work function of the sample. This wide electron distribution is the principal source of image aberration in the microscope. Quantitative analysis Using Einstein's method, the following equations are used: Energy of photon=Energy needed to remove an electron + Kinetic energy of the emitted electron h is Planck's constant; f is the frequency of the incident photon; is the work function; is the maximum kinetic energy of ejected electrons; f0 is the threshold frequency for the photoelectric effect to occur; m is the rest mass of the ejected electron; vm is the speed of the ejected electron. Electron emission microscopy Electron emission microscopy is a type of electron microscopy in which the information carrying beam of electrons originates from the specimen. The source of energy causing the electron emission can be heat (thermionic emission), light (photoelectron emission), ions, or neutral particles, but normally excludes field emission and other methods involving a point source or tip microscopy. Photoelectron imaging Photoelectron imaging includes any form of imaging in which the source of information is the distribution of points from which electrons are ejected from the specimen by the action of photons. The technique with the highest resolution photoelectron imaging is presently photoelectron emission microscopy using UV light. Photoemission electron microscope A photoemission electron microscope is a parallel imaging instrument. It creates at any given moment a complete picture of the photoelectron distribution emitted from the imaged surface region. Light sources The viewed area of the specimen must be illuminated homogeneously with appropriate radiation (ranging from UV to hard x-rays). UV light is the most common radiation used in PEEM because very bright sources are available, like Mercury lamps. However, other wavelengths (like soft x-rays) are preferred where analytical information is required. Electron optical column and resolution The electron