Editing Population inversion (section)

== Other methods of creating a population inversion ==
Stimulated emission was first observed in the microwave region of the electromagnetic spectrum, giving rise to the acronym [[maser|MASER]] for Microwave Amplification by Stimulated Emission of Radiation. In the microwave region, the Boltzmann distribution of molecules among energy states is such that, at room temperature, all states are populated almost equally.

To create a population inversion under these conditions, it is necessary to selectively remove some atoms or molecules from the system based on differences in properties. For instance, in a [[maser#Hydrogen maser|hydrogen maser]], the well-known [[hydrogen line|21cm wave transition in atomic hydrogen]], where the lone electron flips its spin state from parallel to the nuclear spin to antiparallel, can be used to create a population inversion because the parallel state has a magnetic moment and the antiparallel state does not. A [[Stern–Gerlach experiment|strong inhomogeneous magnetic field]] will separate atoms in the higher energy state from a beam of mixed-state atoms. The separated population represents a population inversion that can exhibit stimulated emissions.