Excimer

An excimer, short for excited dimer, is a molecular species that is bound only when one partner is electronically excited. In the ground electronic state, the two atoms or molecules interact weakly or repulsively, so the pair does not form a stable molecule. When one partner is excited, the excited-state potential surface can support a bound state, allowing the pair to exist briefly as an excimer. After relaxation to the ground state, the excimer dissociates.

Excimers are commonly formed in noble-gas systems such as Ar2*, Kr2*, and Xe2*, and in heteronuclear systems known as exciplexes. They are produced by electronic excitation or collisional energy transfer in dense vapors and plasmas, and their emitting state is typically short-lived, with lifetimes of a few to tens of nanoseconds depending on species and environment. The fluorescence of excimers is usually broad and centered at longer wavelengths than the corresponding ground-state atom or molecule, often in the ultraviolet or vacuum ultraviolet range. For noble-gas excimers, typical emissions include Ar2* around 126 nm, Kr2* near 146 nm, and Xe2* near 172 nm.

Excimers have practical applications in ultraviolet lasers, notably ArF, KrF, and XeCl excimer lasers, where stimulated emission arises from transitions of the excited dimer to a repulsive ground state. They are also used as UV light sources, in spectroscopy and materials processing, and as research tools in gas-phase photochemistry.

Exciplexes are related but distinct; they are excited complexes of two different species that often bind only in the excited state.