Accumulating atomic electrons in certain energy levels by irradiation with light or radiofrequency waves at the transition frequency. Assume two fine-structure energy states A1 and A2 (≈10–8 eV apart) separated because of electron spin alignments; these states are apt to be equally populated. If we irradiate a sample with light from which the line A2B has been filtered out, we can raise the A1 population to an elevated state B; then in falling back to the A states, an equal number will fall to A1 as to A2. As the cycle is repeated, A2 will become overpopulated at the expense of A1. This principle is involved in masers, lasers, and optically pumped magnetometers. See Figure O-4.