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1. Submitted by :
Preeti Choudhary (17/MAP/016)

2.  A hot body that emits radiation solely because
of its high temperature is said to exhibit
incandescence. All other forms of light
emission are called luminescence.
 When luminescence occurs, the system
loses energy and if the emission is to be
continuous, some form of energy must be
supplied from elsewhere.
 When the external energy supply is by means
of the absorption of infrared, visible or
ultraviolet light, the emitted light is called
photoluminescence and this is the process
that takes place in any fluorimetric analysis.
Luminescent materials

4. The emission of light by a substance that has absorbed light
or other electromagnetic radiation. It is a form of
luminescence.
The emitted light has a longer wavelength, and therefore
lower energy, than the absorbed radiation. The most striking
example of fluorescence occurs when the absorbed radiation
is in the ultraviolet region of the spectrum, and thus
invisible to the human eye, while the emitted light is in the
visible region, which gives the fluorescent substance a
distinct color that can only be seen when exposed to UV
light.

6. For example
 Commonly employed sources in
fluorescence spectrometry have
spectral outputs either as a
continuum of energy over a wide
range or as a series of discrete
lines. An example of the first type
is the tungsten-halogen lamp and
of the latter, a mercury lamp.
 It is advantageous to employ
a source whose output is a
continuum and the most
commonly employed type is
the xenon arc.
Fluorescent mineralsemit visible
lightwhen
exposed to ultraviolet light

7.  The indirect process of conversion from the excited state
produced by absorption of ewith an even number of electrons,
to an excited trinergy, the singlet state, to a triplet state, is
known as intersystem crossing.
 Direct transition from the ground state, usually a singlet state,
for a molecule plet state is theoretically forbidden, which means
that the reverse transition from triplet to ground state will be
difficult.
 Thus, while the transition from an excited singlet state, for
example, S1, to the ground state with the emission of
fluorescence can take place easily and within 10-9 - 10- 6
seconds, the transition from an excited triplet state to the
ground state with the emission of phosphorescence requires at
least 10-4 seconds and may take as long as 102 seconds.

8. 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 there-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 ata 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 forsome timeafter being charged with
a bright light such as in any normal reading
or room light. Typically the glowing then
slowly fades out within minutes to up to a
few hours in a darkroom.
Phosphorescent bird figure

10.  The triplet state of a molecule has a lower energy than its
associated singlet state so that transitions back to the ground
state are accompanied with the emission of light of lower
energy than from the singlet state. Therefore, we would
typically expect phosphorescence to occur at longer
wavelengths than fluorescence.

11. Photoluminescence(abbreviated as PL) is light
emission from any form of matterafter theabsorption of
photons (electromagnetic radiation).
It is one of many forms of luminescence (light
emission) and is initiated by photo excitation(excitation
by photons).
Time periods between absorption and emission may
vary: ranging from short femtosecond for emission
involving free-carrier plasma in inorganic
semiconductors up to milliseconds forphosphorescent
processes in molecular systems; and under special
circumstances delay of emission may even span to
minutes orhours.

12. Thankyou