Photoelectric effect

When electromagnetic radiation, like ultraviolet light, strikes a material, it can cause electrons to be released. These released electrons are known as photoelectrons. This process, called the photoelectric effect, is examined in fields such as condensed matter physics, solid-state physics, and quantum chemistry to learn about the properties of atoms, molecules, and solids. Practically, it is used in electronic devices designed for detecting light and for producing precisely timed electron emissions.

Classical electromagnetism cannot explain the experimental results. According to classical theory, continuous light waves transfer energy to electrons, which would be emitted once they gather enough energy. Changing the light's intensity would then alter the kinetic energy of the emitted electrons, and very dim light would cause a delay in emission. However, experiments show that electrons are only released when the light's frequency exceeds a certain threshold, regardless of how intense or how long the light shines. A high-intensity beam with a low frequency cannot build up the energy needed to produce photoelectrons, as would happen if light energy accumulated over time from a continuous wave. To explain this, Albert Einstein proposed that light is not a continuous wave but consists of discrete packets of energy, later named photons by Gilbert N. Lewis.

For typical metals, emitting conduction electrons requires light quanta with a few electron-volts (eV) of energy, which corresponds to short-wavelength visible or ultraviolet light. In extreme cases, emissions can happen with photons near zero energy, such as in systems with negative electron affinity or from excited states, or with photons of a few hundred keV for core electrons in elements with a high atomic number. Studying the photoelectric effect was crucial for understanding the quantum nature of light and electrons and helped shape the concept of wave–particle duality. Related phenomena where light affects electric charge movement include the photoconductive, photovoltaic, and photoelectrochemical effects.

**Emission mechanism** Each photon in a light beam has a characteristic energy proportional to the light's frequency. During photoemission, if an electron inside a material absorbs a photon's energy and gains more than its binding energy, it is likely to be ejected. If the photon energy is too low, the electron cannot escape. Increasing the intensity of low-frequency light only adds more low-energy photons, none of which have enough energy to dislodge an electron. The energy of the emitted electrons depends only on the energy of the individual photons, not on the light's intensity at a given frequency.

Free electrons can absorb any energy when irradiated, as long as they immediately re-emit it (as in the Compton effect). But in quantum systems, an electron either absorbs all the energy from one photon (if quantum mechanics allows it) or none at all. Part of that energy frees the electron from its atomic binding, and the rest becomes the electron's kinetic energy as a free particle. Since electrons in a material occupy many quantum states with different binding energies and can lose energy on their way out, the emitted electrons have a range of kinetic energies. Those from the highest occupied states have the highest kinetic energy; in metals, these electrons come from the Fermi level. When a photoelectron is emitted into a solid rather than a vacuum, it is called internal photoemission, while emission into a vacuum is external photoemission.

**Experimental observation of photoelectric emission** Photoemission can occur from any material, but it is most easily observed from metals and other conductors. This is because the process creates a charge imbalance; if not neutralized by current flow, the increasing potential barrier will stop the emission entirely. Nonconductive oxide layers on metal surfaces usually raise the energy barrier for photoemission, so most practical experiments and devices use clean metal surfaces inside evacuated tubes. The vacuum also helps by preventing gases from interfering with the flow of electrons between the electrodes.

Sunlight is an inconsistent source of ultraviolet light, as cloud cover, ozone, altitude, and surface reflection all affect the amount of UV. In the lab, UV sources include xenon arc lamps or, for more uniform but weaker light, fluorescent lamps. More specialized sources are ultraviolet lasers and synchrotron radiation.

The classic setup to observe the photoelectric effect includes a light source, filters to make the light monochromatic, a vacuum tube transparent to UV, an emitting electrode (E) exposed to the light, and a collector (C) with an externally controlled voltage (VC). A positive external voltage directs the photoemitted electrons to the collector. With fixed light frequency and intensity, the photoelectric current (I) increases as the positive voltage rises, since more electrons are guided to the electrode. When no more photoelectrons can be collected, the current reaches a saturation value, which can only increase if the light intensity increases. Applying an increasing negative voltage prevents all but the highest-energy electrons from reaching the collector. When no current flows through the tube, the negative voltage has reached a point that stops even the most energetic electrons.