excitation temperature
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The excitation temperature (T_) is defined for a population of particles via the Boltzmann factor. It satisfies : \frac = \frac \exp, 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). 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 {{thermodynamics-stub