Editing Excimer (section)

==Formation and decay==
[[Image:Molecule HOMO-LUMO diagram.svg|thumb|300px|Molecular orbitals]]
Under the [[molecular orbital]] formalism, a typical ground-state molecule has [[electron]]s in the lowest possible energy levels. According to the [[Pauli principle]], at most two electrons can occupy a given orbital, and if an orbital contains two electrons they must be in opposite [[Spin (physics)|spin states]]. The highest occupied molecular orbital is called the HOMO and the lowest unoccupied molecular orbital is called the LUMO; the energy gap between these two states is known as the [[HOMO/LUMO|HOMO–LUMO]] gap. If the molecule absorbs light whose energy is equal to this gap, an electron in the HOMO may be excited to the LUMO. This is called the molecule's [[excited state]].

Excimers are only formed when one of the dimer components is in the excited state. When the excimer returns to the ground state, its components dissociate and often repel each other. The wavelength of an excimer's emission is longer (smaller energy) than that of the excited [[monomer]]'s emission. An excimer can thus be measured by fluorescent emissions.

Because excimer formation is dependent on a [[bimolecular]] interaction, it is promoted by high monomer density. Low-density conditions produce excited monomers that decay to the ground state before they interact with an unexcited monomer to form an excimer.