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A photoelectron is an electron emitted from a substance due to the photoelectric effect. The photoelectric effect occurs when a material that is usually metallic in nature absorbs enough light radiation so that this results in the emission of electrons from its surface. The discovery of the photoelectric effect was first made in 1887 by Heinrich Hertz, a German physicist, and subsequently was named the Hertz effect. Many researchers spent time defining its properties over the years, and, in 1905, Albert Einstein published findings that it was caused by quanta of light known as photons. Einstein's clear and elegant explanation of how photoelectrons were produced resulted in his winning the Nobel Prize in Physics in 1921.
In order for photoelectrons to be emitted from a surface, the light wavelength needs to be of a sufficiently low value, such as that of UV light. Photoelectron emission is also a key feature that is used in describing quantum mechanics principles. The process involves a quanta, or single photon of energy being absorbed by a solid material if the energy of the photon is greater than the energy of the top valence band, or outermost electron shell of the material.
Photoelectron spectroscopy is a process where the kinetic energy of photons emitted from a surface is analyzed to study the surface region of a sample material. Two basic types of the process have been used. X-ray spectroscopy studies core-levels of a material using photon energy ranges of 200 to 2,000 electron volts, and ultraviolet photoelectron spectroscopy uses photon energy levels of between 10 to 45 electron volts for studying the outer electron or valence shells of the material. As of 2011, the latest synchrotron equipment, which is a magnetic cyclotron that electrostatically accelerates particles, allows for the study of energy ranges between 5 to over 5,000 electron volts so that separate research equipment is no longer necessary. These machines are expensive and complex, however, so they are not widely used in the field.
As of 2011, photoelectron spectrometer equipment has been developed with an electron detector that can operate in open air and at atmospheric pressure, which is new to the field. It is capable of measuring the thickness of thin films down to levels as fine as 20 nanometers, or 20 billionths of a meter. The machines are desktop models that use an ultraviolet light source and can operate in a range from 3.4 to 6.2 electron volts. They are used to analyze both metals and semiconductors such as silicon.
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