Photoelectric Effect

The photoelectric effect is a quantum electronic phenomenon in which electrons are emitted from matter after the absorption of energy from electromagnetic radiation such as x-rays or visible light.   The emitted electrons can be referred to as photoelectrons in this context.   The effect is also termed the Hertz Effect, due to its discovery by Heinrich Rudolf Hertz.   Study of the photoelectric effect led to important steps in understanding the quantum nature of light and electrons and influenced the formation of the concept of wave–particle duality. ^{Wiki n.p.}

When a metallic surface is exposed to electromagnetic radiation above a threshold frequency, which is specific to the surface of the material, the photons are absorbed and current is produced.   No electrons are emitted for radiation with a frequency below that of the threshold because the electrons are unable to gain sufficient energy to overcome the electrostatic barrier presented by the termination of the crystalline surface, the "binding energy" or "work function".   In 1905 it was known that the energy of the photoelectrons increased with increasing frequency of incident light.   However, the manner of the increase was not experimentally determined to be linear until 1915 when Robert Andrews Millikan showed that Einstein was correct.   By the law of conservation of energy, the electron absorbs the energy of the photon and if sufficient, the electron can escape the material with a finite kinetic energy.   A single photon can only eject a single electron because the energy of one photon can only be absorbed by one electron.   The electrons that are emitted are often called photoelectrons.   The photoelectric effect helped further wave-particle duality, whereby physical systems, e.g., photons, display both wave-like and particle-like properties, a concept that was used in quantum mechanics.   Albert Einstein mathematically explained the photoelectric effect and extended the work on quanta that Max Planck developed. ^{Wiki n.p.}

The photons of the light beam have a characteristic energy determined by the frequency of the light.   In the photoemission process, if an electron absorbs the energy of one photon and has more energy than the work function, it is ejected from the material.   If the photon energy is too low, the electron is unable to escape the surface of the material.   Increasing the intensity of the light beam does not change the energy of the constituent photons, only the number of photons.   Thus the energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy of the individual photons, which is a function of their frequency.   Electrons can absorb energy from photons when irradiated, but they follow an "all or nothing" principle.   All of the energy from one photon must be absorbed and used to liberate one electron from atomic binding, or the energy is re-emitted.   If the photon energy is absorbed, some of the energy liberates the electron from the atom, and the rest contributes to the electron's kinetic energy as a free particle. ^{Wiki n.p.}

Laws of photoelectric emission:^{Wiki n.p.}

• 1. For a given metal and frequency of incident radiation, the rate at which photoelectrons are ejected is directly proportional to the intensity of the incident light.
  
  
• 2. For a given metal, there exists a certain minimum frequency of incident radiation below which no photoelectrons can be emitted.   This frequency is called the threshold frequency.
  
  
• 3. Above the threshold frequency, the maximum kinetic energy of the emitted photoelectron is independent of the intensity of the incident light, but it depends on the frequency of the incident light.
  
  
• 4. The time lag between the incidence of radiation and the emission of a photoelectron is very small, less than 10^-9 seconds.

When E-M radiation falls on an insulated conductor connected to a capacitor, the capacitor charges electrically.   Nikola Tesla described the photoelectric effect in 1901. He described such radiation as vibrations of aether of small wavelengths which ionized the atmosphere.   On November 5, 1901, he received the patent that describes radiation charging and discharging conductors (e.g., a metal plate or piece of mica) by "radiant energy".   Tesla used this effect to charge a capacitor with energy by means of a conductive plate, which was a solar cell precursor.   The radiant energy threw off with great velocity minute particles, i.e., electrons, which were strongly electrified.   The patent specified that the radiation (or radiant energy) included many different forms. These devices were called "photoelectric alternating current stepping motors".   In practice, a polished metal plate in radiant energy, e.g. sunlight, will gain a positive charge as electrons are emitted by the plate.   As the plate charges positively, electrons form an electrostatic force on the plate because of surface emissions of the photoelectrons and "drain" any negatively charged capacitors.   As the rays or radiation fall on the insulated conductor connected to a capacitor, the condenser will indefinitely charge electrically. ^{Wiki n.p.}

Albert Einstein's showed a mathematical description in 1905 of how the photoelectric effect was caused by absorption of quanta of light (photons).   This paper proposed the simple description of "light quanta," or photons, and showed how they explained such phenomena as the photoelectric effect.   His simple explanation in terms of absorption of single quanta of light explained the features of the phenomenon and the characteristic frequency.   Einstein's explanation of the photoelectric effect won him the Nobel Prize in Physics in 1921.   The idea of light quanta began with Max Planck's published law of black-body radiation by assuming that Hertzian oscillators could only exist at energies E proportional to the frequency f of the oscillator by E = h•f, where h is Planck's constant.   By assuming that light actually consisted of discrete energy packets, Einstein wrote an equation for the photoelectric effect that fit experiments.   This was an enormous theoretical leap, but the reality of the light quanta was strongly resisted.   The idea of light quanta contradicted the wave theory of light that followed naturally from James Clerk Maxwell's equations for electromagnetic behavior and more generally, the assumption of infinite divisibility of energy in physical systems.   Even after experiments showed that Einstein's equations for the photoelectric effect were accurate, resistance to the idea of photons continued, since it appeared to contradict Maxwell's equations, which were well understood and verified.   Einstein's work predicted that the energy of the ejected electrons increases linearly with the frequency of the light.   In 1905 it was known that the energy of the photoelectrons increased with increasing frequency of incident light and independent of the intensity of the light.   However, the manner of the increase was not experimentally determined to be linear until 1915 when Robert Millikan showed that Einstein was correct. ^{Wiki n.p.}

The photoelectric effect helped propel the then-emerging concept of the dual nature of light, that light exhibits characteristics of waves and particles at different times.   The effect was impossible to understand in terms of the classical wave description of light as the energy of the emitted electrons did not depend on the intensity of the incident radiation.   Classical theory predicted that the electrons could 'gather up' energy over a period of time, and then be emitted.   For such a classical theory to work, a pre-loaded state would need to persist in matter.   These ideas were abandoned. ^{Wiki n.p.}

Uses of the photoelectric effect:^{Wiki n.p.}

• Photodiodes and phototransistors:   Solar cells used in solar power and light-sensitive diodes use a variant of the photoelectric effect, but not ejecting electrons out of the material.   In semiconductors, light of even relatively low energy, such as visible photons, can kick electrons out of the valence band and into the higher-energy conduction band, where they can be harnessed, creating electric current at a voltage related to the bandgap energy.
  
  
• Image sensors:   Video camera tubes in the early days of television used the photoelectric effect; newer variants used photoconductive rather than photoemissive materials.   Silicon image sensors, such as charge-coupled devices, widely used for photographic imaging, are based on a variant of the photoelectric effect in which photons knock electrons out of the valence band of energy states in a semiconductor, but not out of the solid itself.
  
  
• Electroscopes:   Electroscopes are fork-shaped, hinged metallic leaves placed in a vacuum jar, partially exposed to the outside environment.   When an electroscope is charged positively or negatively, the two leaves separate, as charge distributes evenly along the leaves causing repulsion between two like poles.   When ultraviolet radiation (or any radiation above threshold frequency) shines onto the metallic outside of the electroscope, a negatively charged scope will discharge and the leaves will collapse, while nothing will happen to a positively charged scope (besides charge decay).   The reason is that electrons will be liberated from the negatively charged one, gradually making it neutral, while liberating electrons from the positively charged one will make it even more positive, keeping the leaves apart.
  
  
• Photoelectron spectroscopy:   Since the energy of the photoelectrons emitted is exactly the energy of the incident photon minus the material's work function or binding energy, the work function of a sample can be determined by bombarding it with a monochromatic X-ray source or UV source (typically a helium discharge lamp), and measuring the kinetic energy distribution of the electrons emitted.
  
  
• Spacecraft:   The photoelectric effect will cause spacecraft exposed to sunlight to develop a positive charge.   This can get up to the tens of volts.   This can be a major problem, as other parts of the spacecraft in shadow develop a negative charge (up to several kilovolts) from nearby plasma, and the imbalance can discharge through delicate electrical components.   However, the static charge created by the photoelectric effect is self-limiting because a more highly-charged object gives up its electrons less easily.
  
  

1830-1839

In 1839, Alexandre Edmond Becquerel observed the photoelectric effect via an electrode in a conductive solution exposed to light.

1840-1849

1850-1859

1860-1869

1870-1879

In 1873, Willoughby Smith found that selenium is photoconductive.

1880-1889

In 1887,Heinrich Hertz observed the photoelectric effect and the production and reception of electromagnetic (EM) waves.

1890-1899

In 1899, Joseph John Thomson investigated ultraviolet light in Crookes tubes.   Influenced by the work of James Clerk Maxwell, Thomson deduced that cathode rays consisted of negatively charged particles, later called electrons, which he called "corpuscles".

1900-1909

Nikola Tesla described the photoelectric effect in 1901. He described such radiation as vibrations of aether of small wavelengths which ionized the atmosphere.   On November 5, 1901, he received the patent that describes radiation charging and discharging conductors (e.g., a metal plate or piece of mica) by "radiant energy".

In 1902, Philipp von Lenard observed the variation in electron energy with light frequency.   Lenard did not know of photons.

Albert Einstein's showed a mathematical description in 1905 of how the photoelectric effect was caused by absorption of quanta of light (photons).

1910-1919

1920-1929

Einstein's explanation of the photoelectric effect wins him the Nobel Prize in Physics in 1921.

1930-1939

1940-1949

1950-1959

1960-1969

1970-1979

1980-1989

1990-1999