My watch list  

Excitation temperature

The Excitation Temperature (Tex) (often called the spin temperature for the 21 cm line) is defined for a population of particles via the Boltzmann factor. It satisfies

\frac{n_u}{n_l} = \frac{g_u}{g_l} \exp{(\frac{\Delta E}{k T_{ex}})},

where nu and nl represent the number of particles in an upper (e.g. excited) and lower (e.g. ground) state, and gu and gl 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 thermodynamical equilibrium. The excitation temperature can even be negative for a system with inverted levels (such as a maser).

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Excitation_temperature". A list of authors is available in Wikipedia.
Your browser is not current. Microsoft Internet Explorer 6.0 does not support some functions on Chemie.DE