Luminescence

Luminescence is a physical phenomenon that consists in the emission of photons of visible or invisible light from materials excited by causes other than temperature rise. Property of some substances (solid, liquid, aeriform) to re-emit, in the form of electromagnetic waves, energy previously absorbed in the form of electromagnetic radiation.

The emitted radiation can be constituted by infrared rays, luminous rays (and in this case we speak also of photoluminescence), ultraviolet, X-rays, gamma rays. The phenomenon is produced by an atomic or molecular mechanism independent from the exciting agent, but it is influenced by temperature, in the sense that as the temperature increases, the emission decreases and the decay becomes more rapid. The duration of the phenomenon (characterized by the fact that the emission occurs after the excitatory agent has been removed) ranges from time intervals of less than 10-8 s to some hours (at the limit, in some substances, some days); if the duration is less than 10-8 s we talk about fluorescence, if it is longer we talk about phosphorescence.

In molecular gases, liquids and solids, luminescence follows a more complex mechanism, in the sense that transitions are not between levels that represent discrete values of energy, but between energy bands; moreover in many cases the band of emitted radiation is at lower level, therefore shifted to higher wavelengths, than the band of absorbed radiation. This happens because between the first transition, which brings the atom to a higher level, and the second transition, with luminescence emission, each atom gives energy to neighboring atoms in the form of mechanical vibrations, that in solids are lattice vibrations. In other cases the opposite happens, i.e. in the interaction with neighboring atoms the atom receives vibrational energy, so the radiation is of higher frequency and lower wavelength than absorbed radiation (anti-Stokes stripes).

Luminescent materials

Most applications of luminescence phenomena are based on the use of solid materials with particularly long durations of the phenomenon. The name of phosphors, initially attributed to them, was later extended to all materials in which the phenomenon occurs, but it is particularly used for artificially made luminescent materials.

In the case where the luminescence is produced by the passage of ionized particles through solids, or liquids, or aeriforms, we generally speak of scintillators. In many substances luminescence is artificially induced by introducing small percentages of impurities, called activators. Manganese, copper, silver, thallium, lead, cerium, chromium, titanium, antimony, and tin are generally used as activators; they are introduced into such substances as silicates, selenides, alkali halides, calcium oxides, magnesium, barium, and zinc, which thus become artificial phosphors.

Sensitized luminescence

A particular type of luminescence, called sensitized, or indirect luminescence, occurs when the incident radiation, of wavelength λ₁, is absorbed by the mixture of two activators, for one of which the resonance corresponds to λ₁. The emitted radiation is, in this case, a mixture of λ₁ and λ₂, where λ₂ is the wavelength of the radiation emitted by the second activator, to which the atoms of the first one have given energy by collision. The atoms of the second activator emit their radiation returning to their fundamental energy state. This phenomenon can be observed, for example, with a mixture of mercury and thallium vapors, on which the mercury resonance radiation affects.

Another type of luminescence, called luminescence with movement of electric charges, occurs in solid insulators in which there are impurities of luminescent substances. The energetic phenomena in such a material can be represented with a band model, consisting, for increasing energies, of a valence band, a forbidden band (but with the possibility of the presence of impurities) and an empty conduction band. The luminescence can then be schematized as due to the motion of electrons I from the valence band to the conduction band (1-4), due to the effect of irradiation, and subsequent fall (4-5) to the point M of the forbidden band previously occupied by an electron (constituting an impurity), which in the meantime has gone to neutralize itself in B in the electronic hole created by the displacement of the electron I.

Types

The following are types of luminescence:

  • Chemiluminescence, the emission of light as a result of a chemical reaction;
    • Bioluminescence, a result of biochemical reactions in a living organism;
    • Electrochemiluminescence, a result of an electrochemical reaction;
    • Lyoluminescence, a result of dissolving a solid (usually heavily irradiated) in a liquid solvent;
    • Candoluminescence, is light emitted by certain materials at elevated temperatures, which differs from the blackbody emission expected at the temperature in question;
  • Crystalloluminescence, is the effect of luminescence produced during crystallization. The phenomenon was first reported in the 1800s from the rapid crystallization of potassium sulfate from an aqueous solution;
  • Electroluminescence, a result of an electric current passed through a substance;
    • Cathodoluminescence, a result of a luminescent material being struck by electrons;
  • Mechanoluminescence, a result of a mechanical action on a solid;
    • Triboluminescence, generated when bonds in a material are broken when that material is scratched, crushed, or rubbed;
    • Fractoluminescence, generated when bonds in certain crystals are broken by fractures;
    • Piezoluminescence, produced by the action of pressure on certain solids;
    • Sonoluminescence, a result of imploding bubbles in a liquid when excited by sound;
  • Photoluminescence, a result of absorption of photons;
    • Fluorescence, photoluminescence as a result of singlet–singlet electronic relaxation (typical lifetime: nanoseconds);
    • Phosphorescence, photoluminescence either as a result of triplet–triplet electronic relaxation or of persistent luminescence (typical lifetime: microseconds to hours);
  • Radioluminescence, a result of bombardment by ionizing radiation;
  • Thermoluminescence, the re-emission of absorbed energy when a substance is heated;
    • Cryoluminescence, the emission of light when an object is cooled (an example of this is wulfenite).
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