Luminescence
With luminescence , a physical system is put into an excited state by externally supplied energy and emits light (including radiation outside the visible range) when it changes to its basic state with the emission of photons . The term luminescence describes either the process (the phenomenon) or the radiation emitted.
If there is no activation process between the absorption of the energy and the emission, then one speaks of fluorescence ; if an excited intermediate state can “freeze” the energy for a certain period of time, it is from phosphorescence .
Differentiation according to the mechanism of excitation of the system
Depending on the type of excitation, a distinction is made between different types of luminescence:
| Types of Luminescence | The system is stimulated by ... | Examples and remarks |
|---|---|---|
| Electroluminescence | an electric field | Light emitting diodes , EL foils or OLEDs . |
| Chemiluminescence | a chemical reaction | Luminol for the detection of blood . |
| Candoluminescence | heterogeneous catalytic recombination of radicals | probably in the mantle . Used in analytics. |
| Bioluminescence | a chemical reaction in living organisms | Oxidation of luciferin in firefly , Shining wood . |
| Cathodoluminescence | Bombardment with electrons | Luminous layer of a cathode ray tube , cathodoluminescence microscope |
| Radioluminescence or ionoluminescence | Exposure to alpha or beta radiation or other high-energy particles | Luminescent markings on hands by adding radium to phosphorescent material |
| Photoluminescence | Photons | A distinction is made according to the type of radiant transition
When the absorbed EM radiation is X-ray radiation , the term X-ray fluorescence is used . |
| Thermoluminescence | Release of energy stored in the material by increasing the temperature | Use in thermoluminescence dating and thermoluminescence dosimeters . |
| Sonoluminescence | Sound waves (in liquids) | |
| Triboluminescence | various, caused or triggered by friction or tearing apart | With sugar crystals or when opening self-adhesive envelopes. |
| Fractoluminescence | Breaking of crystal structures | Special case of triboluminescence. |
| Lyoluminescence | Dissolve some substances | |
| Aquoluminescence | Dissolving crystal structures in water | Special case of lyoluminescence. |
| Crystal luminescence | Crystallization of crystals | Arsenic trioxide |
| Piezo luminescence | Crushing quartz | related to piezoelectricity |
Fluorescence and phosphorescence
The different types of luminescence can also be classified according to the duration of the glow after the end of the excitation. A very short afterglow (usually <a millionth of a second) as a direct consequence and side effect of the excitation is described with the term fluorescence , whereas phosphorescence describes a longer afterglow of at least 1/1000 of a second after the excitation.
Examples of both processes in the band model : When the substance is excited, the electrons move from the valence band to the conduction band. In the case of fluorescence these conduction electrons recombine with the emission of electromagnetic radiation again directly with an electron vacancy in the valence band. In phosphorescence, on the other hand, metastable intermediate levels are generated in the forbidden zone by impurities introduced into the material , the so-called adhesion or activator terms. In the ground state, the activator terms are occupied by electrons, the traps remain empty. After the electrons have been lifted from the valence band to the conduction band by the excitation, the resulting defect electrons are filled with electrons from the activator terms. The free electrons try to recombine with the defect electrons from the activator term. They are caught by the detention centers. It is also possible that the electrons are lifted from the valence band directly into the trap (direct excitation). By applying energy again, these electrons can be lifted back into the conduction band and from there recombine with the emission of light of the energy with defect electrons from the activator term.
The examination of luminescence, for example in crystals, is carried out using the Becquerel phosphoroscope .
Special case: warming releases otherwise deposited energy
The so-called thermoluminescence was discovered by Robert Boyle in 1663 . He reported to the Royal Society on October 28 that year that he made a diamond faint in the dark by holding it in bed by the warmest part of his naked body.
In some substances such as B. quartz or feldspar , the energy of the decay of naturally occurring unstable nuclides and cosmic radiation is stored in the crystal lattice in the form of radiation damage. Electrons are trapped in “electron traps” between the valence and conduction bands. When heated to temperatures around 300 ° C to 500 ° C, thermally stimulated light emission (thermoluminescence) sets in: excited electrons leave their metastable state and fall back to lower energy levels with light emission. Since all excited electrons have fallen to a lower energy level after a relatively short time, this effect, known as thermoluminescence, only occurs when it is first heated. The stored energy can be inferred. This depends on the intensity and the duration of the previous accumulated energy. This gives the possibility of thermoluminescence dating over millions of years ( irradiation age ). The energy of phosphorescent substances is less durable, but often released with a mild increase in temperature.
The depleted energy store can be used to determine the absorbed dose of ionizing radiation by reheating after exposure and measuring the luminescence. Materials with defects that are stable at room temperature, such as B. lithium fluoride , which is also very sensitive to radiation. The result is a thermoluminescence dosimeter . In this way, food irradiation can also be detected.
Thermoluminescence measurements can also provide important information in photosynthesis research. Here, too, after stimulation with light, metastable radical pairs are formed, which recombine through the supply of heat. The peak temperature and the extent of the emitted light allow conclusions to be drawn about the condition of the photosynthetic apparatus.
literature
- Hans Kittel: paint and varnish and plastic dictionary. Scientific publishing company Stuttgart 1952
Individual evidence
- ↑ Presentation and characterization of cadmium sulfide-aluminum oxide nanocomposites , dissertation by Ingo Heim, p. 28 - along with other references there.
- ^ Newton, HE, 1957. A history of luminescence from the earliest times until 1900. Philadelphia, American Philosophical Society., P. 126
- ↑ G. Schwedt: Pocket Atlas of Food Chemistry. 2. completely revised u. exp. Wiley-VCH, Weinheim 2005.
Web links
- Types of luminescence (University of Jena)
- Luminescent dyes , Fluorophores.org database for luminescent dyes, Graz University of Technology
- When minerals glow themselves - phosphorescence, fluorescence and luminescence Explained at school level (on Prof. Blum's educational server for chemistry)