Excitation temperature
Encyclopedia
The Excitation Temperature (
) (often called the spin temperature for the 21 cm line) is defined for a population of particles via the Boltzmann factor
. It satisfies
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
).
) (often called the spin temperature for the 21 cm line) is defined for a population of particles via the Boltzmann factorBoltzmann factor
In physics, the Boltzmann factor is a weighting factor that determines the relative probability of a particle to be in a state i in a multi-state system in thermodynamic equilibrium at temperature T...
. It satisfies
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
Maser
A maser is a device that produces coherent electromagnetic waves through amplification by stimulated emission. Historically, “maser” derives from the original, upper-case acronym MASER, which stands for "Microwave Amplification by Stimulated Emission of Radiation"...
).