The absorption of laser light by the alkali metal is the first process in SEOP. Left-circularly polarized light tuned to the D 1 wavelength of the alkali metal excites the electrons from the spin down S 1/2 (m s =-1/2) state into the spin up P 1/2 (m s =+1/2) state, where collisional mixing then occurs as the noble gas atoms collide with the alkali metal atoms and the m s =-1/2 state is partially populated (Figure 3). Circularly polarized light is necessary at low magnetic fields because it allows only one type of angular momentum to be absorbed, allowing the spins to be polarized. Relaxation then occurs from the excited states (m s =±1/2) to the ground states (m s =±1/2) as the atoms collide with nitrogen, thus quenching any chance of fluorescence and causing the electrons to return to the two ground states in equal populations. Once the spins are depolarized (return to the m s =-1/2 state), they are excited again by the continuous wave laser light and the process repeats itself. In this way, a larger population of electron spins in the m s =+1/2 state accumulates. The polarization of the rubidium, P Rb, can be calculated by using the formula below: