Optical Pumping

T he rather odd term optical pumping means just what it says. In general “pumping” is a process of raising matter from lower to higher energy; for example, raising the potential energy of water hy moving it from an underground well to an elevated tank. In this article we shall he concerned w i t h the pumping of individual atoms from lower to higher states of internal energy. The word optical refers to the light energy that is the source of power for the pump. Why pump atoms? The purpose is to prepare them for a special kind of spectroscopic analysis. When we think of a spectrum, we usually visualize bands of color. A c t u a l l y atomic spectra extend far beyond the wavelengths of visible light (a few hundred thousandths of a centim e t e r ) in both directions. Atoms can emit and a b s o r b electromagnetic radiation ranging from radio waves (whose length is measured in hundreds of meters) to X-rays (a thousandth as long as light waves). The visible spectrum has been intensively studied for a century, and the X-ray spectrum for about 50 years . Among the results o f these invest i gat ions is the quantum theory of atomic s t ructure . But spectroscopy in the radiofrequency region is a very recent development, in part made possible by the technique of optical pumping, and only now beginning to he exploited. Paradoxical ly , long-wave studies are now revealing fine detail in the structure of a t o m s t h a t i s i n v i s b l e a t s h o r t e r w a v e lengths. And optical pumping has already led to a number of practical applications. The reason for both the effectiveness and the difficulty of using radio waves for spectroscopy l ies in their low energy. Like a l l other e lectromagnet ic radiat ions , radio waves are divided into discrete