Rami Arieli: " The
Laser Adventure" Chapter 2.4 page 1
Relaxation Processes
Spontaneous emission is one of a family
of processes, called relaxation
processes, by which the excited
atoms return to equilibrium (ground state).
This "classic" explanation assumes that the specific frequencies emitted
by an excited atom are the same as the characteristic frequencies of the
atom, which means that the emission
spectrum is identical to the absorption spectrum.
Some physical phenomena can not be explained by this "classic" approach,
such as:
1. Fluorescence:
A process in which the emitted wavelengths from the atom are longer
than the absorbed wavelengths. Emission stops
the moment the excitation stops.
2. Phosphorescence:
A process in which emission lasts longer than the absorption process.
An example is the special colors used to "Glow
in the dark" after being exposed to light earlier.
In the next chapters we shall see that laser
action can be possible only at wavelengths in which the materials have
a fluorescence emission. That is the basic technique used in
research for new laser materials, by spectroscopically examining the energy
levels to find fluorescence.