To explain the processes of excitation, optical pumping, and spin exchange easier, the most common alkali metal used for this process, rubidium, will be used as an example. Rubidium has an odd number of electrons, with only one in the outermost shell that can be excited under the right conditions. There are two transitions that can occur, one referred to as the D 1 line where the transition occurs from the 5 S 1/2 state to the 5 P 1/2 state and another referred to the D 2 line where the transition occurs from the 5 S 1/2 to the 5 P 3/2 state. The D 1 and D 2 transitions can occur if the rubidium atoms are illuminated with light at a wavelength of 794.7 nm and 780 nm, respectively (Figure 1). While it is possible to cause either excitation, laser technology is well-developed for causing the D 1 transition to occur. Those lasers are said to be tuned to the D 1 wavelength (794.7 nm) of rubidium.