ABSTRACT
Luminescence is a general term for the spontaneous emission of electromagnetic
radiation by a material. This results from the transition of molecules from
their electronically excited states down to their ground states. Different types
of luminescence were historically differentiated on the basis of their respective
timescales, but now their primary classification is derived from changes in
spin angular momenta. Luminescence is most often thought of as being initiated
by electromagnetic radiation having wavelengths in the X-ray, ultraviolet (UV),
or visible (VIS) regions of the spectrum, with such processes being grouped
under the category of photoluminescence. However, luminescence is initiated
by a variety of other sources, such as ionizing radiation (scintillation), electron
beams (cathodoluminescence), heat (thermoluminescence), chemical reaction
(chemiluminescence), electrical current (electroluminescence), or mechanical
impact (triboluminescence). Generally, as long as the resulting emission spectrum
originates from transitions among electronic states of the molecule, it will exhibit
the same wavelength dependence regardless of the mode of excitation.
