Excimer
Definition and meaning of Excimer in chemistry.
An excimer is a short-lived pair of atoms or molecules. The pair only bonds together while at least one partner is in an energized, or excited, state. The name comes from excited dimer. Once the energy is gone, the pair falls apart.
In more detail
In their normal, low-energy state, the two units barely attract each other, so no stable molecule can form. But when one partner absorbs energy and becomes excited, a new attractive force appears between the two, letting them bond for a brief moment. The excimer loses its energy by giving off a flash of light called fluorescence.
Since the ground state is not stable, this light emission breaks the pair back into two separate, unbonded pieces instead of leaving a lasting molecule. This produces a wide, shifted glow of light that differs from the sharper light given off by a single excited molecule.
Scientists use this pattern to study how molecules interact and pass energy to each other. Excimers built from noble gases power excimer lasers, tools used in eye surgery and computer chip manufacturing. A common misconception is that excimers behave like ordinary stable molecules that simply happen to glow.
Unlike a normal molecule, an excimer has no stable ground state to settle into, so it cannot exist for more than a brief moment. This unusual behavior is exactly what chemists exploit in excimer lasers. The burst of light released as the pair falls apart produces a highly controlled beam of ultraviolet light, used for tasks like laser eye surgery.
Key facts
| Formula Notation | Generic: A2* (e.g., Xe2*, Py2*) |
|---|---|
| Lifetime | Nanoseconds, falls apart upon relaxation |
| Related Device | Excimer lasers (e.g., ArF, KrF, XeCl) |
| Field | Physical Chemistry |
| Meaning | Short for excited dimer |
Shining ultraviolet light on a solution of pyrene creates excited pyrene molecules. When one of these bumps into a normal pyrene molecule, they form the excimer Py2*, which glows with a broad, smooth light near 470 nanometers. This glow looks noticeably different from the sharper light given off by lone excited pyrene molecules.
Frequently asked questions
What is the difference between an excimer and an exciplex?
An excimer forms from two identical species. An exciplex forms from two different species. Both only exist while at least one partner is 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. The term excimer laser has stuck by convention anyway.
Why do excimers only exist briefly?
The two units are attracted to each other only while one holds extra energy. As soon as the excimer releases that energy as light, the attraction disappears and the pair flies apart.