In this system, the pumping transition P excites the atoms in the ground state (level 1) into the pump band (level 4). From level 4, the atoms again decay by a fast, non-radiative transition Ra into the level 3. Since the lifetime of the laser transition L is long compared to that of Ra (τ32 ≫ τ43), a population accumulates in level 3 (the "upper laser level"), which may relax by spontaneous or stimulated emission into level 2 (the "lower laser level"). This level likewise has a fast, non-radiative decay Rb into the ground state.