The frequency of microwave photons is Hz, corresponding to an energy of the order of 0.01 eV. The energy is of the same order as that of the thermal energy of air molecules. In such a case, the population of the excited states is comparable to that of the ground state at room temperatures. The process of stimulated emission can then be used to amplify microwave signal. MASER is an acronym for Microwave Amplification by Stimulated Emission of Radiation .

Ammonia Maser is such a device for generating electromagnetic waves. Ammonia molecule has two resonant states with a small energy difference eV. Geometrically, the two states may be picturised as follows. The three hydrogen atoms are at the vertices of an equilateral triangle which forms the base of a pyramid with the nitrogen atom at the apex of the pyramid. The nitrogen atom may be in two possible positions, either above the hydrgen plane or below it. (Physically, the two states are distinguished by the direction of their dipole moment in the presence of an electric field.) The molecules make transition from one state to another by absorption or emission of radiation.

The principle of ammonia maser is to separate the two types of molecules which have different energies and . This is done by subjecting the beam to an inhomogeneous electric field in a transverse direction.