Excimer
Definition and meaning of Excimer in chemistry.
Excimer (short for "excited dimer") is a short-lived species formed when two atoms or molecules of the same kind combine, with at least one in an electronically excited state, producing a complex that is bound only in the excited state.
In more detail
In the ground state the two units repel each other (or interact only weakly), so no stable molecule exists; but promoting one partner to an excited state creates an attractive potential well that lets the pair bind transiently. The excimer relaxes by emitting a photon (fluorescence), and because the ground state is dissociative, this emission collapses back into two separated, unbound ground-state species rather than a stable molecule. This gives excimer fluorescence a broad, red-shifted emission band relative to the absorbing monomer, a signature used to study molecular association and energy transfer. Excimer-forming systems, especially noble-gas mixtures, are also exploited in excimer lasers.
Key facts
| Formula notation | Generic: A2* (e.g., Xe2*, Py2*) |
|---|---|
| Lifetime | Nanoseconds; dissociates upon relaxation |
| Related device | Excimer lasers (e.g., ArF, KrF, XeCl) |
| Field | Physical Chemistry |
Ultraviolet excitation of pyrene solutions produces Py* (excited monomer) that collides with a ground-state pyrene to form the excimer Py2*, which emits a broad, structureless fluorescence band near 470 nm distinct from the sharper monomer emission.
Frequently asked questions
What is the difference between an excimer and an exciplex?
An excimer forms from two identical species (A + A*), while an exciplex forms from two different species (A* + B); both are bound only in an excited state.
Why do 'excimer lasers' actually use exciplexes?
Common excimer lasers use noble-gas halide pairs like ArF or KrCl, which are technically exciplexes since the two partners differ, but the term 'excimer laser' has stuck by convention.