Phosphorescence is a specific type of photoluminescence related to fluorescence.
Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation
it absorbs.
The slower time scales of the re-emission are associated with "forbidden" energy state
transitions in quantum mechanics.
As these transitions occur very slowly in certain materials, absorbed radiation may
be re-emitted at a lower intensity for up to several hours after the original excitation.
Commonly seen examples of phosphorescent materials are the glow-in-the-dark toys, paint, and
clock dials that glow for some time after being charged with a bright light such as
in any normal reading or room light.
Typically the glowing then slowly fades out within minutes in a dark room.
The study of phosphorescent materials led to the discovery of radioactivity in 1896.
Ironically, white phosphorus does not actually exhibit this property, but rather chemiluminescence.
Explanations Simple
In simple terms, phosphorescence is a process in which energy absorbed by a substance is
released relatively slowly in the form of light.
This is in some cases the mechanism used for "glow-in-the-dark" materials which are "charged"
by exposure to light.
Unlike the relatively swift reactions in a common fluorescent tube, phosphorescent materials
used for these materials absorb the energy and "store" it for a longer time as the processes
required to re-emit the light occur less often.
Quantum mechanical
Most photoluminescent events, in which a chemical substrate absorbs and then re-emits a photon
of light, are fast, on the order of 10 nanoseconds.
Light is absorbed and emitted at these fast time scales in cases where the energy of the
photons involved matches the available energy states and allowed transitions of the substrate.
In the special case of phosphorescence, the absorbed photon energy undergoes an unusual
intersystem crossing into an energy state of higher spin multiplicity, usually a triplet
state.
As a result, the energy can become trapped in the triplet state with only classically
"forbidden" transitions available to return to the lower energy state.
These transitions, although "forbidden", will still occur in quantum mechanics but are kinetically
unfavored and thus progress at significantly slower time scales.
Most phosphorescent compounds are still relatively fast emitters, with triplet lifetimes on the
order of milliseconds.
However, some compounds have triplet lifetimes up to minutes or even hours, allowing these
substances to effectively store light energy in the form of very slowly degrading excited
electron states.
If the phosphorescent quantum yield is high, these substances will release significant
amounts of light over long time scales, creating so-called "glow-in-the-dark" materials.
Equation
where S is a singlet and T a triplet whose subscripts denote states.
Transitions can also occur to higher energy levels, but the first excited state is denoted
for simplicity.
Chemiluminescence
Some examples of glow-in-the-dark materials do not glow by phosphorescence.
For example, glow sticks glow due to a chemiluminescent process which is commonly mistaken for phosphorescence.
In chemiluminescence, an excited state is created via a chemical reaction.
The light emission tracks the kinetic progress of the underlying chemical reaction.
The excited state will then transfer to a dye molecule, also known as a sensitizer or
fluorophor, and subsequently fluoresce back to the ground state
Materials Common pigments used in phosphorescent materials
include zinc sulfide and strontium aluminate.
Use of zinc sulfide for safety related products dates back to the 1930s.
However, the development of strontium aluminate, with a luminance approximately 10 times greater
than zinc sulfide, has relegated most zinc sulfide based products to the novelty category.
Strontium aluminate based pigments are now used in exit signs, pathway marking, and other
safety related signage.
Phosphorescent pigments – comparison ZnS vs. aluminate
Phosphorescent pigments red and blue
See also Luminous paint
Microsphere Persistent luminescence
Phosphor Phosphoroscope
Tritium References
External links Luminosity in Watches
SPECTRA OF GITD MATERIALS