Photoluminescence is the emission of light from a material
under optical excitation. Absorption of visible or UV radiation
raises molecule to an excited state.
Electron absorbs quantum of energy and jumps to a higher energy orbital.
When electron drops back to the ground state, excitation energy can be
What is Phosphorescence?
Phosphorescence is the result of a radiative (light emitting) transition involving a change in teh spin multiplicity of (in most cases) a molecule from excited state singlet to excited state triplet. This transition is quantum mechanically forbidden as is the transition that leads to light emission. These forbidden transitions are kinetically slow, whcih introduces a delay between photo-excitation (exposure to light of one wavelength) and emission (release of light of a different wavelength). So-called "glow in the dark" materials are phosphorescent materials with a very long (seconds, minutes, even hours) delay between excitation and emission. Most phosphorescent compounds have triplet lifetimes on the order of milliseconds.
Usually re-emission of radiation, if it occurs, is within nanoseconds (10-9
sec)of the excitation= FLUORESCENCE.
PHOSPHORESCENCE is a much slower (and rarer)
process. After source of excitation radiation is switched off,
phosphorescence emission continues for periods that vary from milliseconds
to weeks. Phosphorescence is caused by electron becoming transferred into
a triplet state. (Electrons of the same spin in the one orbital). Triplet
states have long lifetime so phosphoresence persists. Mostly requires low
temperatures - at higher T, triplet state is
de-activated by quenching.
The Morse curves (see above) display energy transitions for an electron, in a covalent bond A-B, excited by hn, then re-emitting the energy as photoluminescence
What will affect fluorescence spectra? Fluorescence spectra affected by:
What do we mean with "brightness"?
brightness: An attribute of visual perception in which a source appears to emit a given amount of light. Note 1: "Brightness" should be used only for nonquantitative references to physiological sensations and perceptions of light. Note 2: "Brightness" was formerly used as a synonym for the photometric term "luminance" and (incorrectly) for the radiometric term radiance.
What can we say about Radioactivity?
What is Excitation wavelength?Excitation wavelength in fluorometry is the wavelength of radiant energy that is absorbed by a fluorophore molecule. This incident light excites the molecule and results in the emission of radiation at a higher wavelength (the emission wavelength).
What is of phosphor?
A phosphor is a substance that exhibits the phenomenon of phosphorescence (sustained glowing without further stimulus). The chemical element phosphorus (Greek. Phosphoros, meaning "light bearer") was discovered by German alchemist Hennig Brand in 1669 through a preparation from urine. Working in Hamburg, Brand attempted to distill salts by evaporating urine, and in the process produced a white material that glowed in teh dark. Since that time, the term phosphorescence has been used to describe substances that shine in the dark without burning. Phosphorus itself is NOT a phosphor; it is highly reactive and gives-off a faint glow upon uniting with oxygen. The glow observed by Brand was actually caused by the very slow burning of the phosphorus, but as he saw no flame nor felt any heat he did not recognize it as burning.
Phosphors are transition metal compounds or rare earth compounds of various types. The
most common uses of phosphors are in CRT displays and fluorescent lights.
CRT phosphors were standardized beginning around World War II and
designated by the letter "P" followed by a number.
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