Phosphorescence

Phosphorescence is the property of a substance nachzuleuchten in the dark after illumination with a ( visible or UV ) light. Cause is radiant deactivation. This phenomenon observed alchemists in the 17th century.

Phosphorescence / fluorescence

The phosphorescence is a special form of luminescence ( cold light fixtures). It differs from the similar phenomenon of fluorescence is that the fluorescence decays rapidly after the end of irradiation, usually within a millionth of a second, whereas there is an afterglow in the phosphorescence, which can last from a fraction of a second to hours. Phosphorescent materials are also referred to as luminophores, as they save the light apparently.

• Fluorescent lights off = lights (eg under irradiation with UV light - black light in the disco )
• Phosphorescence: glows in the dark for many hours (eg, exit signs )

Phosphorus

A " phosphorescence " was a long-lasting afterglow during the chemical element phosphorus ( light bearer ) modification observed for the first time in his white ( highly reactive ). Since this afterglow based on the chemical reaction of atmospheric oxygen with phosphorus, it is here strictly speaking, a chemiluminescence. The actual phosphorescence describes a quantum physical effect in the light excitation.

Colloquially in the technical area all the materials that can be excited by radiation to glow, called " phosphors ". Strictly speaking, it is therefore here by "Glow pigments " (or fluorescent dyes). Thus, the inner coating of a Braun tube, for example, doped zinc sulfides, which can be excited by electron beams to emit light. This inner coating was called " phosphorus " in black and white televisions.

Explanation

The process of phosphorescence can be described with the help of quantum physics: If a phosphorescent material with light quanta ( photons) lit so give these photons transfer their energy to the electrons of the substance from the switch to a higher energy level ( quantum leap ). The corresponding molecules do not go from the ground state to an excited state, while preserving the spin multiplicity ( selection rules ). This is clear if one puts it in a Jablonski diagram. In this, the individual energy and vibrational levels are presented that accept the electrons when they absorb or emit energy.

The electron can release its newly gained energy when it returns to its unexcited state; This can be done after the excited molecule colliding with other particles and thus transfers part of its energy to other particles. On the Jablonski diagram, this is indicated by the lowering of the electron in lower vibrational levels. The energy is usually dissipated as heat is referred to this process as vibrational relaxation.

Can the particles do not completely submit to their environment their energy recovered, it happens that the electrons emit their excess energy in the form of a photon, ie as radiation (light in the broad sense ). In phosphorescence, the emission of light does not end as in the fluorescence at the end of the irradiation - afterglow occurs up to several hours.

It occurs, according to an ordinary residence time of about 10-8 seconds for another quantum leap of electrons in a metastable energy level. It changes this the spin of the electrons, which singlet changes in the molecule by changing the multiplicity of a triplet state; This process is called intersystem crossing (English: intersystem crossing ). The residence time in this state, is longer by a few; it is milliseconds to hours or even thousands of years.

This triplet state occurs also to the vibrational, but the molecule is "trapped" in the excited state, as a discharge energy to the surroundings is impossible. The triplet state can not be transferred to a singlet state basically, as a spin reversal is not possible. But here there is an exception: it occurs again a forbidden Inter combination process on - just like with the transfer of the singlet to the triplet state. However, the transition probability for this is much less, so that the life of the state is correspondingly longer (if the molecule does not change by radiationless transitions to the ground state ), which explains the typical ( long ) afterglow.

The duration of the phosphorescence is temperature dependent, the colder the longer. The intensity of the phosphorescence can with the temperature increasing or decreasing, depending on whether the intersystem crossing or radiationless transitions grow stronger.

Phosphorescent materials

Phosphorescent materials are generally crystals having a small admixture of an impurity that disturbs the lattice structure of the crystal. Generally used to sulfides of metals of the second group as well as zinc and mixed small amounts of heavy metal salts in ( for example, zinc sulphide with traces of heavy metal salts ). In there is an example of a Cu -doped zinc sulfide pigment, the wavelength ranges of the excitation and the emission and the afterglow time. By fusing boric acid with fluorescein doped phosphorescent crystal structures can be caused to decay by means of a UV light source. A long burn time achieved europium -doped strontium aluminate, which was developed in 1998 and is offered under the brand Luminova.

Applications

Postal services

For the automated processing of mail (sorting, raise stamp ) were from the second half of the 1950s used different forms of luminescence. For this purpose, graphite strips and phosphor stripes prints were used on stamps and fluorescent strip next to postal stationery stamp impressions and phosphorescent and fluorescent paper. First, there were examples in the UK from November 1957 with two graphite strips printing on brand backs. In the Federal Republic of Germany first postage stamps definitives Heuss was I and II sold with fluorescent paper on 1 August 1960 by post offices in the Darmstadt. In the manufacture of postage stamps to the pulp to be mixed for a few decades, phosphorescent substances or the material is coated subsequently. With UV light irradiated Stamps then light up in the dark. Postmark machines can thus see where the glue to be canceled stamps on the letter and knock off the postmark on the right place. With this method, both unstamped letters and postcards can be sorted as bad fake tokens are identified.

Safety Technology

Besides phosphorescent signs phosphorescent colors, and tapes are used for marking escape routes. With stairs here the first and last step is marked over the entire width. Especially used only as an emergency exit tunnels and corridors, this is an economical and clear ausfallsicherere alternative to battery -based emergency lighting. Even during the Second World War, the walls were painted with phosphorescent colors to prevent a panic when the power fails in the otherwise totally dark, often heavily overcrowded bunker rooms in many air -raid shelters. Today you can find such phosphorescent markings often in metro stations.

Signal character and persistence

Phosphorescence can also be good to use as a signal character. In many cases it is required that information be provided in the dark. So phosphorescent materials for luminescent hands with watches, on light switches or in some stickers (safety signs, decorative items, auto parts, PC, fishing accessory) can be used. Until the 1950s, radium -containing phosphorescent were common for hands and numerals of watches and measuring instruments.

Other

Phosphorescent colors form a stylistic feature in the psychedelic art.

Special radar picture tubes ( for example, the B23G3 ) have previously been used for display in radars. They have a very high persistence to show targets until the next round of the radar antenna.

The creation of a shadow silhouettes of oneself on a phosphorescent wall by an electronic flash is an attraction in some science centers.