Encyclopedia
The
photoelectric effect is the emission of
electrons from matter upon the absorption of
electromagnetic radiation, such as
ultraviolet radiation or
x-rays. An older term for the photoelectric effect was the
Hertz effect, though this phrase has fallen out of current use.
Introduction
Upon exposing a
metallic surface to electromagnetic radiation that is above the threshold frequency , the photons are absorbed and
current is produced. No electrons are emitted for radiation with a frequency below that of the threshold, as the electrons are unable to gain sufficient energy to overcome the electrostatic barrier presented by the termination of the crystalline surface . In 1905 it was known that the energy of the photoelectrons increased with increasing frequency of incident light, but the manner of the increase was not experimentally determined to be linear until 1915 when
Robert Andrews Millikan showed that Einstein was correct.
By conservation of energy, the energy of the photon is absorbed by the electron and, if sufficient, the electron can escape from the material with a finite kinetic energy. A single photon can only eject a single electron, as the energy of one photon may only be absorbed by one electron. The electrons that are emitted are often termed
photoelectrons.
The photoelectric effect helped further
wave-particle duality, whereby physical systems display both wave-like and particle-like properties and behaviours, a concept that was used by the creators of
quantum mechanics. The photoelectric effect was explained mathematically by
Albert Einstein, who extended the work on quanta developed by
Max Planck.
Explanation
The photons of the light beam have a characteristic energy given by the wavelength 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, however, 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 their number, and thus the energy of the emitted electrons does not depend on the intensity of the incoming light.
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 is absorbed, some of the energy is used to liberate it from the atom, and the rest contributes to the electron's kinetic energy as a free particle.
Equations
In analysing the photoelectric effect quantitatively using Einstein's method, the following equivalent equations are used:
Energy of
photon = Energy needed to remove an
electron + Kinetic energy of the emitted electron
Algebraically:
where
- h is Planck's constant,
- f is the frequency of the incident photon,
- is the work function, or minimum energy required to remove an electron from atomic binding,
- 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, and
- is the velocity of the ejected electron.
Note: If the photon's energy is not greater than the work function , no electron will be emitted. The work function is sometimes denoted .
History
Early observations
In 1839,
Alexandre Edmond Becquerel observed the photoelectric effect via an electrode in a conductive solution exposed to light. In 1873, Willoughby Smith found that
selenium is photoconductive.
Hertz's spark gaps
Heinrich Hertz, in 1887, made observations of the photoelectric effect and of the production and reception of electromagnetic waves, published in the journal Annalen der Physik. His receiver consisted of a coil with a
spark gap, whereupon a spark would be seen upon detection of EM waves. He placed the apparatus in a darkened box in order to see the spark better; he observed, however, that the maximum spark length was reduced when in the box. A glass panel placed between the source of EM waves and the receiver absorbed ultraviolet radiation that assisted the electrons in jumping across the gap. When removed, the spark length would increase. He observed no decrease in spark length when he substituted quartz for glass, as
quartz does not absorb UV radiation.
Hertz concluded his months of investigation and reported the results obtained. He did not further pursue investigation of this effect, nor did he make any attempt at explaining how the observed phenomenon was brought about.
JJ Thomson: electrons
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". In the research, Thomson enclosed a metal plate in a vacuum tube, and exposed it to high frequency radiation. It was thought that the oscillating electromagnetic fields caused the atoms' field to resonate and, after reaching a certain amplitude, caused a subatomic "corpuscle" to be emitted, and current to be detected. The amount of this current varied with the intensity and color of the radiation. Larger radiation intensity or frequency would produce more current.
Von Lenard's observations
In 1902,
Philipp von Lenard observed the variation in electron energy with light frequency. He used a powerful electric arc lamp which enabled him to investigate large changes in intensity, and had sufficient power to enable him to investigate the variation of potential with light frequency. His experiment directly measured potentials, not electron kinetic energy: he found the electron energy by relating it to the maximum stopping potential in a phototube. He found that the calculated maximum electron kinetic energy is determined by the frequency of the light. For example, an increase in frequency results in an increase in the maximum kinetic energy calculated for an electron upon liberation -
ultraviolet radiation would require a higher applied stopping potential to stop current in a phototube than blue light. However Lenard's results were qualitative rather than quantitative because of the difficulty in performing the experiments: the experiments needed to be done on freshly cut metal so that the pure metal was observed, but it oxidised in a matter of minutes even in the partial vacuums he used. The current emitted by the surface was determined by the light's intensity, or brightness: doubling the intensity of the light doubled the number of electrons emitted from the surface. Lenard did not know of photons.
Einstein: light quanta
Albert Einstein's mathematical description in 1905 of how it was caused by absorption of what were later called
photons, or
quanta of light, in the interaction of light with the
electrons in the substance, was contained in the paper named "
On a Heuristic Viewpoint Concerning the Production and Transformation of Light". This paper proposed the simple description of "light quanta" and showed how they could be used to explain such phenomena as the photoelectric effect. The simple explanation by Einstein in terms of absorption of single quanta of light explained the features of the phenomenon and helped explain the characteristic frequency. Einstein's explanation of the photoelectric effect won him the
Nobel Prize of 1921.
The idea of light quanta was motivated by
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 = hf, where h is Planck's constant. Einstein, by assuming that light actually
consisted of discrete energy packets, wrote an equation for the photoelectric effect that fit experiments . This was an enormous theoretical leap and 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 there was resistance to the idea of photons, since it appeared to contradict Maxwell's equations, which were believed to be well understood and well verified.
Einstein's work predicted that the energy of the ejected electrons would increase linearly with the frequency of the light. Perhaps surprisingly, that had not yet been tested. 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 -- but the manner of the increase was not experimentally determined to be linear until 1915 when
Robert Andrews Millikan showed that Einstein was correct.
Effect on wave-particle question
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. The idea of the pre-loaded state was discussed in Millikan's book
Electrons and in Compton and Allison's book
X-Rays in Theory and Experiment. These ideas were abandoned.
Uses and effects
Photodiodes
Solar cells 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.
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 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 . 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 , and measuring the kinetic energy distribution of the electrons emitted.
This must be done in a high
vacuum environment, since the electrons would be scattered by air.
A typical electron energy analyzer is a concentric hemispherical analyser , which uses an electric field to divert electrons different amounts depending on their kinetic energies. For every element and core
atomic orbital there will be a different binding energy. The many electrons created from each will then show up as spikes in the analyzer, and can be used to determine the elemental composition of the sample.
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 from nearby plasma, and the imbalance can discharge through delicate electrical components. The static charge created by the photoelectric effect is self-limiting, though, because a more highly-charged object gives up its electrons less easily.
Moon dust
Light from the sun hitting lunar dust causes it to become charged through the photoelectric effect. The charged dust then repels itself and lifts off the surface of the
Moon by electrostatic levitation. This manifests itself almost like an "atmosphere of dust", visible as a thin haze and blurring of distant features, and visible as a dim glow after the sun has set. This was first photographed by the
Surveyor program probes in the 1960s. It is thought that the smallest particles are repelled up to kilometers high, and that the particles move in "fountains" as they charge and discharge.
See also
External links and references
General- Nave, R., "". HyperPhysics.
- Jpaul's "". .
- "". Physics 2000. University of Colorado, Boulder, Colorado.
- ACEPT W3 Group, "". Department of Physics and Astronomy, Arizona State University, Tempe, AZ.
- Haberkern, Thomas, and N Deepak "". , Chapter 3.
- Department of Physics, "". Physics 320 Laboratory, Davidson College, Davidson.
- Fowler, Michael, "". Physics 252, University of Virginia.
- Brandl, Michael, "" , .
-
Applets- Curull, Xavi Espinal, "".
- Fendt, Walter, and Taha Mzoughi, "".
- "". Open Source Distributed Learning Content Management and Assessment System.
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