The term excimer is derived from the short form of " excited dimer " (' excited dimer '). It is a short-lived particles consisting of two or more contiguous atoms or molecules. The special feature is that the excimer can be formed only if a binding partner is in an excited state. Loses this energy particles to separate the binding partner and return to the ground state. This light is often emitted. The emission band is broad and strongly red - shifted, as the emission band of the excited monomer. This property can be used for spectroscopic identification of excimers.

By definition, the atoms consist of an excimer molecule from two or more atoms of the same chemical element. Is it atoms of different elements, one speaks of an exciplex. In the literature, this fact is rarely taken into account and thus exciplexes are often inaccurately called excimers.

In laser technology, the properties of excimers (or nowadays usually exciplexes ) are used by excimer lasers for specific applications. The need for the laser population inversion is already given by the formation of molecules, since in this case the ground state may not be occupied. The decay time is I.A. few nanoseconds ( ns). Practical importance excimer laser obtained, which emit in the ultraviolet spectral range. The laser-active medium consists here mainly of fluorine ( F2) 157 nm argon fluoride ( ArF) 193 nm krypton fluoride (KrF ) 248 nm xenon chloride ( XeCl ) 308 nm or xenon fluoride ( XeF ) 351 nm, and they are used in medical technology and photolithography, a portion of the semiconductor manufacturing.