An excimer lamp (or excilamp) is a source of ultraviolet light based on spontaneous emission of excimer (exciplex) molecules.

Excimer lamps are quasimonochromatic light sources operating over a wide range of wavelengths in the ultraviolet (UV) and vacuum ultraviolet (VUV) spectral regions. Operation of an excimer lamp is based on the formation of excited dimers (excimers), which spontaneously transiting from the excited state to the ground state result in the emission of UV photons. The spectral maximum of excimer lamp radiation is specified by a working excimer molecule:

Excimers are diatomic molecules (dimers) or polyatomic molecules that have stable excited electronic states and an unbound or weakly bound (thermally unstable) ground state. Initially, only homonuclear diatomic molecules with a stable excited state but a repulsive ground state were called excimers (excited dimers). The term "excimer" was later extended to refer any polyatomic molecule with a repulsive or weakly bound ground state. One can also come across the term "exciplex" (from "excited complex"). It is also an excimer molecule but not a homonuclear dimer. For instance, Xe₂*, Kr₂*, Ar₂* are excimer molecules, while XeCl*, KrCl*, XeBr*, ArCl*, Xe₂Cl* are referred to exciplex molecules. Dimers of rare gases and rare-gas–halogen dimers are the most spread and studied excimers. Rare-gas–halide trimers, metal excimers, metal–rare-gas excimers, metal–halide excimers, and rare-gas–oxide excimers are also known, but they are rarely used.

An excimer molecule can exist in an excited electronic state for a limited time, as a rule from a few to a few tens of nanoseconds. After that, an excimer molecule transits to the ground electronic state, while releasing the energy of internal electronic excitation in the form of a photon. Owing to a specific electronic structure of an excimer molecule, the energy gap between the lowest bound excited electronic state and the ground state amounts from 3.5 to 10 eV, depending on a kind of an excimer molecule and provides light emission in the UV and VUV spectral region. A typical spectral characteristic of excimer lamp radiation consists mainly of one intense narrow emission band. About 70–80% of the whole radiation power of an excimer lamp is concentrated in this emission band. The full width at half maximum of the emission band depends on a kind of an excimer molecule and excitation conditions and ranges within 2 to 15 nm. In fact, excimer lamps are sources of quasimonochromatic light. Therefore, such sources are suitable for spectral-selective irradiation and can even replace lasers in some cases.

Radiation is produced owing to the spontaneous transition of an excimer molecule from an excited electronic state to the ground state. Excimer and exciplex molecules are not long-living formations. They rapidly decompose, typically within a few nanoseconds, releasing their excitation energy in the form of a UV photon:

where Rg₂* is an excimer molecule, RgX* is an exciplex molecule, Rg is an atom of rare gas, and X is an atom of halogen.

It is convenient to generate excimer molecules in a plasma. Electrons play an important role in a plasma and, in particular, in the formation of excimer molecules. To efficiently generate excimer molecules, the working medium (plasma) should contain sufficient concentration of electrons with energies that are high enough to produce the precursors of the excimer molecules, which are mainly excited and ionized rare gas atoms. Introduction of power into a gaseous mixture results in the formation of excited and ionized rare gas atoms as follows:

Electron excitation