Upon pumping the medium, an appreciable number of atoms will transition to level 3, such that "N"3 > 0. To have a medium suitable for laser operation, it is necessary that these excited atoms quickly decay to level 2. The energy released in this transition may be emitted as a photon (spontaneous emission), however in practice the 3→2 transition (labeled R in the diagram) is usually "radiationless", with the energy being transferred to vibrational motion (heat) of the host material surrounding the atoms, without the generation of a photon.