Photoelectric effect

The photoelectric effect is a fundamental phenomenon in which electrons are emitted from a material when it is exposed to electromagnetic radiation, typically ultraviolet or visible light of sufficiently high frequency. The emitted electrons, known as photoelectrons, provide direct insight into the electronic structure of atoms, molecules, and solids. The effect is of central importance in condensed-matter physics, solid-state chemistry, and quantum theory, and forms the basis of numerous technologies requiring precise light detection or controlled electron emission.

Departure from Classical Expectations

Classical electromagnetic theory posited that light, considered as a continuous wave, should transfer energy gradually to electrons. Under this view, lower-intensity light would require a measurable time for electrons to accumulate enough energy to be ejected, and greater light intensity would increase their kinetic energy. However, empirical observations contradicted these predictions:

• Electron emission occurs only when light exceeds a threshold frequency, regardless of intensity.
• Low-frequency light, even at high intensity, does not eject electrons.
• The kinetic energy of emitted electrons depends on light frequency, not light intensity.
• Electron emission happens with negligible delay.

These anomalies led Albert Einstein in 1905 to propose that light consists of discrete packets of energy—later termed photons—each carrying energy proportional to its frequency. This formulation laid the foundation for quantum theory and supported the concept of wave–particle duality.

Emission Mechanism

Photons possess a definable energy proportional to their frequency. When a photon strikes a material, its energy may be absorbed by an electron. If the photon’s energy exceeds the binding energy of that electron, the electron is liberated.
The kinetic energy of an emitted electron is given by:
Kₘₐₓ = hν − W,
where hν is the photon energy and W is the material’s work function, the minimum energy required for electron emission. Increasing intensity increases the number of photons striking the surface, thereby increasing the number of emitted electrons, but does not affect their individual energies.
Electrons in solids occupy states with varying binding energies, so the emitted photoelectrons display a range of kinetic energies. In metals, electrons near the Fermi level contribute the highest-energy emissions. When electrons escape into vacuum the process is termed external photoemission; when emission occurs within the solid it is referred to as internal photoemission.

Experimental Observation

Photoemission is most readily observed in metals and conductive materials because any loss of charge is immediately compensated by current flow. Oxide layers on metal surfaces often suppress emission, so experiments routinely use clean metal surfaces in evacuated tubes.
A standard experimental setup includes:

• a monochromatic light source;
• a vacuum tube with a photoemitting electrode;
• a collecting electrode with an adjustable potential.

A positive potential attracts photoelectrons, increasing the photoelectric current until it reaches saturation. A negative potential opposes their motion; the stopping potential is the negative voltage required to halt even the most energetic electrons. Its magnitude directly yields the maximum kinetic energy of emitted photoelectrons.
For a given metal, a threshold frequency exists below which no electrons are emitted. Increasing frequency raises the stopping potential and the kinetic energy of ejected electrons. Increasing light intensity at fixed frequency raises the current but does not affect stopping potential.
The emission is essentially instantaneous, occurring in less than 10⁻⁹ seconds. The angular distribution of emitted electrons depends on light polarisation and the electronic structure of the material. Angle-resolved photoemission spectroscopy (ARPES) is a principal technique for studying these distributions.

Additional Light-Induced Charge Phenomena

The photoelectric effect is closely related to other processes in which light influences electronic behaviour, including:

• photoconductivity, where conductivity increases under illumination;
• the photovoltaic effect, where electron–hole pairs generate electricity in devices such as solar cells;
• photoelectrochemical effects, important in catalysis and solar-fuel generation.

Einstein’s Quantum Explanation

Einstein’s model treated light as consisting of energy quanta, each with energy hν. If a single photon’s energy exceeds the material’s work function, an electron is emitted, and the excess energy appears as kinetic energy. This explanation accounted for all experimental observations and earned Einstein the Nobel Prize in Physics in 1921. The concept of light quanta was later aligned with the modern understanding of photons, a term introduced by Gilbert N. Lewis.