= Ammonia maser
An ammonia maser uses an <ammonia> <molecular beam>, <maser state selection> and a <maser cavity> to obtain amplification or sustained <microwave> <oscillation>. In the 1954 experiment by <James Power Gordon>, <Herbert Jack Zeiger> and <Charles Hard Townes>, a quadrupolar static <electric field> focused upper-state <molecules> toward the cavity and deflected lower-state <molecules> away. The reported $J=K=3$ <ammonia inversion transition> lies near $23.87\,\mathrm{GHz}$.<Molecular Microwave Oscillator and New Hyperfine Structure in the Microwave Spectrum of NH3 by James Power Gordon and others>[pp. 282–283, Fig. 1 and Fig. 2 caption; 23 870 Mc/sec]{ref}
\Image[/ammonia-maser-state-selection-and-cavity.png]
{title=How <maser state selection> feeds a <maser cavity>}
{description=Upper states enter the <maser cavity>; lower states are rejected. Paths and spacing are illustrative. The inset shows the selector end view.<Chapter 9: The Ammonia Maser>[§§9–2–9–3, separation and cavity feedback]{ref}}
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