Atomic Clocks: An Application of Spectroscopy
The article focuses on the discovery of the atomic clock as an application of spectroscopy. Early atomic clocks based on hydrogen, rubidium and cesium were developed at the U.S. National Bureau of Standards and at the National Physical Laboratory in Great Britain. However, the cesium atomic clocks are the ones in which the modern definition of the second is based. The cesium clocks were chosen because of its relatively low boiling point and fairly heavy atom.
Ball, David W., "Atomic Clocks: An Application of Spectroscopy" (2007). Chemistry Faculty Publications. 132.