An experiment is described in which microwave resonance lines have been obtained having widths substantially less than the normal Doppler width. The line shape obtained depends on the properties of a radiating gas when contained in a cavity in the form of a pillbox, of diameter large compared to height. Collisions of molecules with the walls take place in such a manner that the collisions narrow the line instead of broadening it. The 1.25-cm inversion transition in ammonia was observed, and lines one quarter the normal Doppler width of 73 kc/sec obtained; further narrowing appears feasible. The line shapes and widths are in good agreement with the calculations.

• Received 17 March 1955

DOI:https://doi.org/10.1103/PhysRev.99.532