ammonia clock

A form of atomic clock in which the frequency of a quartz oscillator is controlled by the vibrations of excited ammonia molecules. The ammonia molecule (NH3) consists of a pyramid with a ni­trogen atom at the apex and one hy­drogen atom at each corner of the triangular base. When the molecule is excited, once every 20.9 microsec­onds the nitrogen atom passes through the base and forms a pyra­mid the other side: 20.9 microsec­onds later it returns to its original position. This vibration back and forth has a frequency of 23 870 hertz and ammonia gas will only absorb excitation energy at exactly this fre­quency. By using a crystal oscillator to feed energy to the gas and a suit­able feedback mechanism, the oscil­lator can be locked to exactly this frequency.