Population inversion and pumping

If there are more atoms in the upper level (N2) than in the lower level (N1), the system is not at equilibrium. In fact, at thermodynamic equilibrium, the distribution of the atoms between the levels is given by Boltzmann's Law:

In this case, N2 is always less than N1. A situation not at equilibrium must be created by adding energy via a process known as “pumping” in order to raise enough atoms to the upper level.