Luminescence and Fluorescence
Spectroscopy
Defining light emissive phenomena can often be
difficult. Researchers from different backgrounds often
call it by different names, but they are basically
talking about the same thing. In this note we will try to
present the simple and clear classification of general types
of luminescence based upon the source and the nature of light
excitation (see the chart below).
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Luminescence
Strictly speaking, Luminescence is a
light emission which represents an excess over the thermal
radiation and lasts for a time exceeding the period of
electromagnetic oscilation. This
actually indicates two things. First of all, excess
over thermal radiation brings the distinction between
Luminescence and the light emitted from warm or hot
incandescent bodies. Visible light from thermal
radiation begins emitting at minimum temperatures of a few
hundred degrees K, while Luminescence can be observed at any
temperature. That is why Luminescence is sometimes labeled a
cold light. The second condition, in
which Luminescence should last for a time exceeding
the period of electromagnetic oscilation, distinguishes
it from reflected and stray light. In Luminescence
there are intermediate processes between absorption and
emission duration which exceeds the period of a single
electromagnetic oscilation. As a result, Luminescence looses
correlation between phases of absorbed and emitted light, in
contrast to reflected and stray light, in which the phase
correlation can always be observed.
Luminescent analysis is performed using the
intrinsic Luminescence of materials under observation, or
special markers (luminophors) are added when the material
itself does not demonstrate luminescent properties.
Luminescence applications are so numerous and diverse that
contemporary reviews and books are unnable to accommodate
them all.
Photoluminescence, a Luminescence
stimulated by light absorption in UV-Vis-NIR spectral region,
represents any process in which material absorbs
electromagnetic energy at a certain wavelength and then emits
part of it at a different (usually longer) wavelength.
Therefore, only a part of the absorbed energy is transformed
into luminiscent light. The rest of it ends up as molecular
vibrations, or simply as a heat. Photoluminescence is the most
popular type of Luminescence because a large selection of
reliable and inexpensive excitation sources are available
and also because the effect can often be observed
with the naked eye. Usually an excitation source emits in
UV and the Photoluminescence occurs in Vis or NIR.
There is always a delay between the moment the
material has absorbed the higher energy photon and the moment
the secondary lower energy photon is re-emitted. This delay is
defined by the lifetime of excitation states, or
simply by how long atoms or molecules are able to
stay in excited high-energy conditions. Delay time can vary
many orders of magnitude for different materials. Based on
practical observations, two types of Photoluminescence were
historically established – Fluorescence and Phosphorescence.
Technically, delay time is the only difference between them.
It is shorter for Fluorescence (10-12 to
10-7 s) and much longer for Phosphorescence (up to
a few hours and even days).
Fluorescence is a “fast”
Photoluminescence. The effect is widely used in such everyday
practical applications as industrial and residential lightning
(neon and fluorescent lamps) as an analytical technique in
science and as a quality and process control method in
industry.
Phosphorescence is a “slow”
Photoluminescence. In contrast to Fluorescence, it
demonstrates itself as a glowing that lasts long after the
excitation light is gone. Phosphorescent materials are usually
called “glow-in-the-dark”. This effect is generally used by
the Department of Transportation to attract drivers' attention
to road signs, in adertising campains to produce glowing
stickers and promotion materials, as well as in numerous
industries to notify people of potential hazards and
dangers.
Electroluminescence is a Luminescence
excited in gases and solids by applying an electromagnetic
field. Molecules are excited upon creation of any form of
electric discharge in material.
Electroluminescence of gases is used in
discharge tubes. The electroluminescence effect,
which readily occurs in semiconductors and light emitting
diodes (LEDs), is the most well-known application. Natural
blue diamond emits light when electrical current is passed
through it.
Triboluminescence occurs when a material
is scratched, crushed, rubbed or stressed mechanically in any
way. When a material is subjected to mechanical stress
spatially separated, electrical charges are produced.
Upon recombining these charges, a flash of light emerges
as a result of electric discharge, ionizing the surrounding
space. Since electrical discharge is in the foundation of
Triboluminescence, it can be classified as a part of
Electroluminescence. Blue or red Triboluminescence can be
observed when sawing a diamond during the cutting process.
Another example includes sugar crystals, which produce
tiny electrical sparks while crushing. Other substances
exhibiting Triboluminescence include minerals fluorite
(CaF2) and sphalerite (ZnS).
Crystalloluminescence is a
type of Luminescence generated during crystallization, used to
determine the critical size of the crystal nucleus. There is a
theory that the light from
crystalloluminescence emerges through the micro-fracture
of growing crystallites. Separation of electrical charges may
occur on the fracture facets on the surface of micro-fractures
and their following recombination. This effectively classifies
Crystalloluminescence as a type of
Triboluminescence and a subtype of Electroluminescence. Let us
note that electrically charged micro-fractures may be
developed due to multiple processes such as the movement of
charged dislocations, piezoelectrification, etc.
Sonoluminescence is the emission of short
bursts of light from imploding bubbles in a liquid when
excited by sound. It is believed that when a bubble starts
imploding, extremely high pressures inside the bubble cause
the water to form ice-like structures. At the moment when the
opposite sides of an imploding bubble collide, the very strong
mechanical stress causes the ice to fracture. The growth of
ice micro-fractures results in separation of electrical
charges and their following recombination, which generates
light. Therefore, Sonoluminescence is a part of
Triboluminescence phenomenon.
Sonoluminescence light flashes from a single
bubble and lasts from a few tens to a few hundred
picoseconds. It is emitted at relatively short wavelengths,
which can reach into the ultraviolet. The emitting bubble size
is averaged at about 1 mkm in diameter. The addition of a
small amount of noble gas (such as helium, argon, or xenon) to
the gas in the bubble enhances the intensity of the emitted
light dramatically. A possible reason for this is an
increase in the ice fracturing ability.
Chemoluminescence is conversion of
chemical energy directly into light as a result of a chemical
reaction. In brief, given reactants A and
B are transformed into an excited
intermediate I. The decay of the excited
intermediate I to a lower energy level is
responsible for the emission of light.
[A] + [B] --> [I] --> [Products] +
Light
Theoretically, each molecule of reactant should
produce one photon of light, or Avogadro's number of
photons-per-mole. In practice, non-catalytic reactions usually
generate about 1 photon-per-100 reactant molecules with
quantum efficiency of about 1%. Standard laboratory
applications of Chemoluminescence include the forensic
test for locating blood, even if it has been cleaned or
removed. The chemical substance luminol emits blue
light upon contact with the iron in haemoglobin if blood is
present. The glow lasts for about 30 seconds. Lightsticks is
another well-known Chemoluminescence application.
Bioluminescence is Chemoluminescence
produced by living organisms. Bioluminescence observed at the
surface of the sea is produced by microscopic plankton. Other
examples of bioluminescence include glow-worms, fireflies and
various fungi and bacteria found on rotting wood or
decomposing flesh.
Radioluminescence (Scintillation)
is a Luminescence resulting from excitation by high-energy
particles or radiation. Excitation sources include a-particles
- helium nuclei (a-Luminescence), ß-particles -
electrons and positrons (Cathodoluminescence),
accelerated protons and neutrons, ions
(Ionoluminescence), ?-radiation and X-Ray radiation
(X-Ray Luminescence). Radioluminescence is widely
used in medical physics, dosimetry, in television and
radar screens.
Thermoluminescence light emerges from a
heated material as a result of high-energy electrons
previously trapped within the material being released. In
other words, heat frees the electrons, which produces light.
The intensity of Thermoluminescence is proportional to the
energy absorbed by the mineral as a result of its previous
exposure to ionizing radiation. A very important application
of Thermoluminescence is archeological and geological dating.
The natural flux of ionizing radiation— both from cosmic
radiation and natural radioactivity— creates excited states in
crystalline structures. A very small fraction of the radiation
energy can stay in these excited states for a long time. When
such material is heated, the stored energy is released as weak
light or Thermoluminescence. After cooling, re-heating the
material will not generate light, as no excited states remain.
This method is used when materials do not contain carbon
atoms, therefore eliminating the possibility of radiocarbon
dating. It is frequently used for authenticating the age of
old ceramic wares, for which it gives the approximate date of
the last firing.
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