The strength of the zero-phonon transition arises in the superposition of all of the lattice modes. Each lattice mode m has a characteristic vibrational frequency Ω Ω m which leads to an energy difference between phonons ℏ ℏ Ω Ω m . When the transition probabilities for all the modes are summed, the zero-phonon transitions always add at the electronic origin ( E 1 − − E 0 ), while the transitions with phonons contribute at a distribution of energies. Figure 3 illustrates the superposition of transition probabilities of several lattice modes. The phonon transition contributions from all lattice modes constitute the phonon sideband.