Einstein`s Photoelectric Law

Formula: Kmax = hf - pfi. Under the right circumstances, light or other electromagnetic radiation directed on to the surface of a material can cause electrons to be ejected from the material. This process is sometimes called `(electron) photoemission`, along with an assortment of alternative names, in particular the (external) photoelectric effect. In the above equation, Kmax represents the maximum kinetic energy that a photoelectron (emitted from a particular material whose work function is ? and subject to radiation of frequency f) can have. h represents Planck`s constant. The work function is the minimum energy needed to extract an electron within the material to a point in a vacuum immediately outside the surface. For metals, work functions range from 1.95 eV for caesium to 5.63 eV for platinum.

The most usual context in which this phenomenon is discussed is when light (UV or visible) is incident on a metal, with a single valence electron being ejected. Some non-metals, mostly metalloids, can demonstrate weak photoelectric effects if the energy of the incident photons is high enough, i.e. in the X or gamma ray region.

When incident on a metal, these X or gamma rays can cause the emission of a core electron, with secondary effects ensuing. If the frequency is high enough, the nucleus itself may absorb the radiation, resulting in photofission or the nuclear photoelectric effect.

Electron photoemission can occur in a gas as well as a solid or liquid, with the electron being ejected from an atom or molecule.

Note that the photo effect can happen alongside other interactions in which a photon is incident on a material. These include both photon-absorbing and photon-scattering phenomena.

More Terminology

A photoemissive material is a material that can exhibit the photoelectric effect. The ejected electrons are, for convenience, referred to as photoelectrons. The photoelectric threshold is the point at which the energy of photons is just sufficient to cause photoemission. The threshold frequency and threshold wavelength of a material refer to the frequency and wavelength at this point. (Most elements have threshold frequencies in ultraviolet and only a few dip into the visible region. The materials with the lowest threshold frequencies - even down in the infrared - are all semiconductors.) The internal photoelectric effect is a phenomenon in which the absorption of a photon in a semiconductor results in an excitation, whereby an electron from the valence band jumps to the conduction band, rather than being ejected from the material entirely.

The photoelectric yield is the number of electrons ejected per photon incident on the surface. For most metals, this value is low (roughly 10^-4 near the photoelectric threshold) because (a) their surface is strongly reflective thus only a small fraction of the incident radiation penetrates the metal, and (b) only photoelectrons produced at a depth of less than a few nanometres have enough energy left to escape.

History

Some time after Becquerel discovered the `photovoltaic effect`, Heinrich Hertz discovered the photoelectric effect in 1887 whilst investigating the effect of UV radiation on spark length. A number of scientists made contributions to knowledge about the effect, including Philipp Lenard, who observed that the energy of individual emitted electrons increased with the frequency of the light, in contravention of Maxwell`s wave theory of light. In 1905, Einstein solved this seeming paradox by describing light as composed of discrete `quanta`, now called `photons`, rather than continuous waves. He theorized that the energy in each quantum of light was equal to the frequency multiplied by a constant (Planck`s constant as it is now known). A photon above a threshold frequency has the necassary energy to eject a single electron, producing the photo effect. This observation and theory led to the quantum revolutionand earned Einstein the Nobel Prize in Physics in 1921.

Applications

Photoemission is utilised in devices such as solar cells, photo-multiplier tubes, charge coupled devices, photocopiers, light meters, photodiodes, phototransistors and scintillators.