The excitation temperature (
) is defined for a population of particles via the
Boltzmann factor
Factor, a Latin word meaning "who/which acts", may refer to:
Commerce
* Factor (agent), a person who acts for, notably a mercantile and colonial agent
* Factor (Scotland), a person or firm managing a Scottish estate
* Factors of production, suc ...
. It satisfies
:
where ''n''
u and ''n''
l represent the number of particles in an upper (''e.g.'' excited) and lower (''e.g.'' ground) state, and ''g''
u and ''g''
l their statistical weights respectively.
Thus the excitation temperature is the temperature at which we would expect to find a system with this ratio of level populations. However it has no actual physical meaning except when in local thermodynamic equilibrium. The excitation temperature can even be negative for a system with inverted levels (such as a
maser
A maser (, an acronym for microwave amplification by stimulated emission of radiation) is a device that produces coherent electromagnetic waves through amplification by stimulated emission. The first maser was built by Charles H. Townes, James ...
).
In observations of the
21 cm line
The hydrogen line, 21 centimeter line, or H I line is the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation has a precise frequency of , w ...
of hydrogen, the apparent value of the excitation temperature is often called the "spin temperature".
References
Temperature
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