State transition - Aetherwisp

A state transition, in quantum mechanics, is the transition of a particle from one energy eigenstate to another, which may be higher or lower energy. During a transition to a higher energy state, the particle must absorb energy from a source (for example, an incident Photon); conversely, when transitioning to a lower energy state, the particle must emit the excess energy, usually in the form of a photon. The absorbed or emitted energy is always equal to the difference in energy between the states: calling $E_{i}$ and $E_{f}$ the initial and final state energies, the transferred energy is

E_{\text{transferred}}=-\Delta E=E_{i}-E_{f}