Difference between revisions of "Luminescence"

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* Fluorescence
 
* Fluorescence
 
* Phosphorescence
 
* Phosphorescence
* Tenebrescence
 
 
* Triboluminescence
 
* Triboluminescence
 +
* Tenebrescence (not technically a luminescence)
  
 
The causes of luminescence are varied, but are mostly due to impurities ("activators") or due to the molecular packing of the crystal lattice.
 
The causes of luminescence are varied, but are mostly due to impurities ("activators") or due to the molecular packing of the crystal lattice.
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<br clear=all>
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 +
==Triboluminescence==
 +
 +
Triboluminescence is caused by pressure, friction or mechanical stress in any way applied to a gemstone.<br>
 +
Opposed to fluorescence, phosphorescence and tenebrescence, triboluminescence is not caused by light, hence it is not to be named photoluminscence. Instead, it is the result of electric charges, therefore an ''electroluminescence''.<br>
 +
This effect is usually seen in diamond cutting. When the diamond is sawn or cleaved, electric charges break free from the stone and immediately recombine, showing a red or blue glow.
  
 
==Tenebrescence==
 
==Tenebrescence==
 +
 +
Although technically tenebrescence is not a luminescence it shares some common characteristics with phosphorescence. Technically tenebrescence is an unstable color that is caused by low energy artificial irradiation from the UV light source.
  
 
In 1896, a vibrant pink variety of sodalite was discovered in Greenland by L.C. Boergstroem. The pink color of this unusual sodalite faded to colorless when exposed to light. The sodalite will return to its original pink color when it is placed in the dark for an extended period of time, or when exposed to short wave ultraviolet light. This transformation can be repeated endlessly. Tenebrescence is defined by minerals that are able to make this color transformation; minerals that display the ability to change color in this fashion are termed tenebrescent. Tenebrescence is the property that some minerals and phosphors show of darkening in response to radiation of one wavelength and then reversibly bleaching on exposure to a different wavelength. Very few minerals exhibit this phenomenon, also known as reversible photochromism, a word that applies to sunglasses that change color density on exposure to sunlight.
 
In 1896, a vibrant pink variety of sodalite was discovered in Greenland by L.C. Boergstroem. The pink color of this unusual sodalite faded to colorless when exposed to light. The sodalite will return to its original pink color when it is placed in the dark for an extended period of time, or when exposed to short wave ultraviolet light. This transformation can be repeated endlessly. Tenebrescence is defined by minerals that are able to make this color transformation; minerals that display the ability to change color in this fashion are termed tenebrescent. Tenebrescence is the property that some minerals and phosphors show of darkening in response to radiation of one wavelength and then reversibly bleaching on exposure to a different wavelength. Very few minerals exhibit this phenomenon, also known as reversible photochromism, a word that applies to sunglasses that change color density on exposure to sunlight.
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Exposure to a UVP UVG4 SW UV lamp for 15 minutes triggered an almost Santa Maria aquamarine blue color that faded gradually during the following 2-3 minutes in natural daylight.
 
Exposure to a UVP UVG4 SW UV lamp for 15 minutes triggered an almost Santa Maria aquamarine blue color that faded gradually during the following 2-3 minutes in natural daylight.
  
==Triboluminescence==
 
 
Triboluminescence is caused by pressure, friction or mechanical stress in any way applied to a gemstone.<br>
 
Opposed to fluorescence, phosphorescence and tenebrescence, triboluminescence is not caused by light, hence it is not to be named photoluminscence. Instead, it is the result of electric charges, therefore an ''electroluminescence''.<br>
 
This effect is usually seen in diamond cutting. When the diamond is sawn or cleaved, electric charges break free from the stone and immediately recombine, showing a red or blue glow.
 
  
 
==Sources==
 
==Sources==

Revision as of 09:57, 7 December 2006

Luminescence is the phenomenon that shows itself as a "glowing" of a gemstone. This is caused by absorption of energy and the releasing of surplus of this energy in small amounts.
The sources of energy are usually ultraviolet light, X-ray light and even visible light. When energy comes from light, it is referred to as photoluminescence.

In gemology we are usually only concerned with 4 types of luminescence:

  • Fluorescence
  • Phosphorescence
  • Triboluminescence
  • Tenebrescence (not technically a luminescence)

The causes of luminescence are varied, but are mostly due to impurities ("activators") or due to the molecular packing of the crystal lattice. In general, the presence of iron inside the gemstone kills or suppresses luminescence.

Different fluorescent reactions of Diamonds in UV light

Fluorescence

Basic

Fluorescence is the emission of visible light by a gemstone when exposed to a light source whose light we normally cannot see. When the gemstone is exposed to ultraviolet light (UV), which falls outside the range of light that we can see, the UV light is absorbed by the gemstone. Due to processes inside the gemstone, it will lose energy. This loss of energy causes the UV light to change to a color in the visible light range (red, orange, yellow, green, blue, indigo or violet).

Although some people understand this as a speeding up of light, this is incorrect. (Neither is it caused by the slowing down of light inside the gemstone because the slowing down results in shorter wavelengths, not longer ones. See refraction).

All rays of light carry a specific amount of energy. Light with a lower wavelength has higher energy. This energy is expressed in eV (electron Volts). For instance red light has energy of around 1.8eV while violet light has 3.1eV of energy.
When the loss of energy of UV light (with an energy of -- let's say -- 4eV) is 2.2eV, this results in 1.8eV, hence red light (4 - 2.2 = 1.8).

Fig.1 Simplified diagram showing cause of fluorescence

This might best be explained with a ball that gets tossed upward onto a staircase.
If you threw the ball up onto the stairs, that motion would require energy (the energy coming from your arm). Let's say that the ball now carries 4eV of energy and this is just enough to get it from the ground state (1) to the 3rd board (4). As it then drops from level 4 to level 3, it would loose part of that energy (in this example 0.5eV). So the ball still has 3.5eV of energy. It will then drop to level 2, loosing an additional 0.3eV of energy. After this it will drop down to the ground again while carrying only 1.8eV of energy. When it reaches the ground state again, the ball looses all its surplus energy.

Now imagine the ball being an electron and the source of energy (formerly your arm) is ultraviolet light. As the electron gets 4eV of energy from the UV light source we can't see it as light (we can only see it as it reaches 3.1eV, which corresponds to violet light), consequently it will loose more and more energy as it drops down level by level. When it reaches level 2, it has an energy of 1.8eV which corresponds with red light, so the electron will now emit red light.

As long as energy is fed to the electrons (in the form of UV light), this process is continuous and this process only takes a fraction of a second (a femtosecond or 10-15 seconds). How much energy that is required for a gemstone to fluoresce varies from stone to stone, for Ruby that is 3eV and explains why the best Rubies appear to glow like hot coil in daylight.
Not all gemstones will show this phenomenon and those gemstone loose the extra energy in another way.

The fluorescence lifespan is relative to the UV light source, meaning that if you turn off the lightsource the fluorescence is gone.

The electromagnetic spectrum and the place of ultraviolet light


For day-to-day use, we use two different types of UV light:

  • Shortwave ultraviolet light, or S-UV (with a wavelength of about 254nm)
  • Longwave ultraviolet light, or L-UV (with a wavelength of about 366nm)

Warning: When using UV light, make sure to protect your eyes as they are damaging! This is particularly true for S-UV.


Some colors that might be seen in a UV viewing cabinet:

Fluorescence
L-UV S-UV Produced color
Ivory Synth. white Spinel White
Opal
Ruby Red
Red Spinel
Synth. Emerald
Nat. blue Sapphire
Alexandrite
Diamond Synth. white Spinel Blue
Moonstone
Apatite Green
Fluorite Violet
Kunzite Orange
Lapis lazuli
Sodalite
Zircon Yellow
Topaz

Advanced

Crossed filters technique

Copper Sulphate solution in a flask and a red filter

The "crossed filters" technique should not be confused with "crossed polars" or "crossed polaroids" as they have to do with polarization, not luminescence.
A flask is filled with hydrous copper sulphate and white light is being passed through the solution. The exiting light will be blue. During the illumination of the gemstone with this blue light, a red filter is placed between the eye of the observer and the stone. When the stone appears red, when viewed through the red filter, this is clear proof that the stone is fluorescent in daylight.
The activator in the gem which causes this is the presence of Chromium (Cr) in the crystal lattice and this effect is predominantly seen in Ruby, Alexandrite, Emerald, red Spinel and pink Topaz. It should be noted that Iron (Fe) can greatly diminish or completely eliminate this fluorescence effect. As synthetic materials usually carry more Cr and little to none Fe, this glowing of red light is more intense than in their natural counterparts (in general).

The hassle of carrying hydrous copper sulphate is luckily eliminated by the invention of blue LED pocket (or keychain) torches that may be purchased for just a few USD at your local hardware shop. One could use a sheet of red selenium glass as the red filter, or even your Chelsea Color Filter. Other sheets like plastics could also serve as crossed filters.
With a sheet of blue material in front of one'slightsource, one can mimic the copper sulphate solution and/or the LED torch.

Using the same lightsource in conjunction with a spectroscope, one can then easily distinguish between Ruby and red Spinel.


Jablonski energy diagram

Fig.2 Jablonksi energy diagram

in Fig.2 "s0" consists of 3 vibrational levels and "s1" of 4. The "s" stands for "singlet state", meaning that all electrons are spin-paired.

An absorbed photon will cause an electron to be excited from the groundstate (s0) to higher energy levels (s1 or higher). As the electron drops from the higher vibrational states in s1 to the lower s1 levels, it looses energy through internal conversion. After relaxation in s1 (at a time rate of 10-12 seconds) it will then drop to groundstate s0 with lower energy (10-9 seconds).

Due to the fact that quantum energy is opposite proportional to wavelength, the emitted photon (which carries lower energy as the absorbed photon) must travel at a longer wavelength. This difference in wavelengths is known as Stokes Shift.

Relationship between wavelength and quantum energy
<-------- wavelength --------
wavelength 800nm 700nm 600nm 500nm 400nm 300nm 200nm
energy 1.55eV 1.77eV 2.07eV 2.48eV 3.1eV 4.13eV 6.2eV
--------- energy --------->


Phosphorescence

Basic

Phosphorescence is similar to fluorescence but differs in the "lifetime" the glow fades. In Phosphorescent materials the glow, or better the "after-glow" can range from a fraction of a second to several hours (although the latter is usually not observed in minerals).

The "trap" diagram

When an electron gets enough energy from an energy source as UV light, the electron will jump from its groundstate (1) to a higher energy level (4). The electron will fall into a gap (2) rather than to the energy level just below 4 (3).
As a result the electron will be "trapped" in the gap and needs extra energy (also provided by the UV light source) to jump out. While the UV energy source feeds the electrons, fluorescence is seen, yet when the source is shutdown the electrons stay in the trap till other energy is provided to free them.
The energy required to get the electron out of the trap comes from white light (with thermal energy, or heat at room temperature).
As white light doesn't carry as much energy as UV light, the release of the electron goes at a slower rate, thus creating an after-glow. During this period the phenomenon is named phosphorescence.

One can look at the freeing of the electron as "bleaching".

Tenebrescence (see below) can be explained by this as well, yet the energy required to free the electron is higher. You can visualize it as a deeper gap if you will.


Advanced

The Jabonksi diagram to explain phosphorescence.

Jablonski energy diagram (phosphorescence)

An electron absorbs a photon and gets exited from groundstate s0 to the excited state s1. After loosing energy going from the upper levels in s1 to the lower levels it looses energy and through intersystem crossing it will drop to the "forbidden" triplet state t1.
After relaxation in t1 it will drop down to groundstate s0 at a considerably lower time rate (10-3 to 100 seconds).


Triboluminescence

Triboluminescence is caused by pressure, friction or mechanical stress in any way applied to a gemstone.
Opposed to fluorescence, phosphorescence and tenebrescence, triboluminescence is not caused by light, hence it is not to be named photoluminscence. Instead, it is the result of electric charges, therefore an electroluminescence.
This effect is usually seen in diamond cutting. When the diamond is sawn or cleaved, electric charges break free from the stone and immediately recombine, showing a red or blue glow.

Tenebrescence

Although technically tenebrescence is not a luminescence it shares some common characteristics with phosphorescence. Technically tenebrescence is an unstable color that is caused by low energy artificial irradiation from the UV light source.

In 1896, a vibrant pink variety of sodalite was discovered in Greenland by L.C. Boergstroem. The pink color of this unusual sodalite faded to colorless when exposed to light. The sodalite will return to its original pink color when it is placed in the dark for an extended period of time, or when exposed to short wave ultraviolet light. This transformation can be repeated endlessly. Tenebrescence is defined by minerals that are able to make this color transformation; minerals that display the ability to change color in this fashion are termed tenebrescent. Tenebrescence is the property that some minerals and phosphors show of darkening in response to radiation of one wavelength and then reversibly bleaching on exposure to a different wavelength. Very few minerals exhibit this phenomenon, also known as reversible photochromism, a word that applies to sunglasses that change color density on exposure to sunlight.

SODALITE that shows this behavior has been given the variety name Hackmanite. The pink color in this mineral is unstable because it fades very quickly when exposed to light. There are other examples of minerals that lose or gain color when exposed to light:

TUGTUPITE, some light colored varieties of tugtupite, especially pale pink material, will intensify in color as a result of exposure to shortwave UV—or even strong sunlight (but not artificial light).
SPODUMENE, a darkening of color to pink or purple can be achieved with exposure to high-energy radiation.
CHAMELEON DIAMONDS are olive colored diamonds that temporarily change color after having been stored in darkness or when gently heated. Chameleon diamonds display hues and tones from light to dark olive (stable color phase) through light to medium yellow (unstable color phase). After one to two days in darkness, exposure to light changes the color of a chameleon diamond from the unstable yellow color back to the stable olive . This is observed as an infinitely repeatable process.
AMETHYSTS from Globe, Arizona, and some SHERRY-COLORED TOPAZ are reported to loose their color in the sun. This loss of color is irreversible.
WHITE BARITE from the Gaskin Mine in Pope County Illinois, will change to blue, and yellow barite to grey-green when exposed to ultraviolet light.

The pink color of hackmanite may be restored in two different ways. One is by leaving the specimen in the dark for a few hours to several weeks, or, exposure to short-or long wave ultraviolet will also restore color. Short wave ultraviolet is the most efficient for this purpose. The speed with which this is accomplished and the depth of the color achieved varies from specimen to specimen.

In some specimens, long exposure to ultraviolet light is required to produce a faint degree of pink color. In other specimens, exposure to shortwave ultraviolet will almost instantly produce a pink color. In the latter specimens, additional exposure to ultraviolet light for several minutes to a few hours will produce a deep pink to raspberry-red color in which a weak blue component is evident. This can be seen in some specimens from Mont Saint-Hilaire and Khibina. If the specimen is now put in the dark, the deep red color will exhibit phosphorescence also known as "after glow". Visible light (wavelengths between 480-720 nanometers) will quickly reverse the process and render the specimen colorless once again.

This photochromic effect can be repeated indefinitely, although any heating of the mineral destroys tenebrescence forever.

Research indicates that F-Centers are the cause, at least partially, for the tenebrescence in hackmanite. The term F-Centers is derived from the German word Farbe, meaning color. An F-center is a defect in an ionic lattice that occurs when an anion leaves as a neutral species, leaving a cavity and a negative charge behind. This negative charge is then shared by the neighboring positive charges in the lattice. F-Centers are responsible for coloring a variety of minerals, including fluorite and barite.(Nassau, 1983) In hackmanite, it is proposed that some of the negatively charged chlorine atoms are missing. A negative electric charge is required at such vacancies to provide charge balance, and any free electrons in the vicinity become drawn to such vacancies and are trapped there. Such a trapped electron is the typical basis of an F-Center. It appears that this center in hackmanite absorbs green, yellow, and orange light and varying amounts of blue. When the hackmanite is seen in white light, red and some blue are returned to the eye, giving the hackmanite colors.

A mineral may produce a certain color that depends on different, but fixed arrangements of electrons (Nassau, 1983). Hackmanite absorbs the energy from the ultraviolet radiation and many electrons get stuck in a new, high-energy position in atoms (F-centers); this is what causes the mineral to have a different color when the lights are turned on. But when we turn the room lights on, the new color fades. White light (the visible spectrum) also energizes electrons, just not as much as ultraviolet light. The white light has the necessary energy to "unstick" the electrons from the F-Centers, thus returning the mineral to colorless.

A fairly recent find (2005) in Badakhshan, Afghanistan is tenebrescent scapolite. This colorless to silvery material is unearthed near the hackmanite deposits and shows an aquamarine color after exposure to SW UV light. The intensity of this color (blue) depends on the length of time it has been exposed to the UV lighting.
Exposure to a UVP UVG4 SW UV lamp for 15 minutes triggered an almost Santa Maria aquamarine blue color that faded gradually during the following 2-3 minutes in natural daylight.


Sources

  • Gemology - Peter Read, 3rd edition (2005)
  • Gem-A Diploma Syllabus (1987)
  • Crossed Filters revisited - D.B.Hoover and B. Williams, The Journal of Gemmology, July/October 2005
  • A Status Report on Gemstones From Afghanistan - Gems & Gemology, Winter 1985, Gary Bowersox
  • Update on Hackmanite - Gems & Gemology, Winter 1989, Gem News
  • The Physics & Chemistry of Color - Kurt Nassau, 1983
  • An Introduction to Rock Forming Minerals - Deer, Howie & Zussman 1966
  • Hackmanite - Brochure supplied by SoCalNevada, on Hackmanite from the Kola Peninsula, Russia
  • The origins of color in minerals - Kurt Nassau, American Mineralogist Volume 63, pages 219-229, 1978 [1]

External links