Excimer Laser
Definition and meaning of Excimer Laser in chemistry.
An excimer laser is a gas laser that emits ultraviolet light through stimulated emission from excimers, short-lived diatomic molecules that exist only in an electronically excited state and immediately dissociate upon returning to the ground state.
In more detail
The gain medium is typically a mixture of a noble gas (argon, krypton, or xenon) and a halogen-containing gas (fluorine or hydrogen chloride), excited by a high-voltage electrical discharge. Colliding excited noble-gas atoms and halogen atoms bond transiently to form species such as ArF*, KrF*, or XeCl*, which are stable only in the excited electronic state; their ground state is repulsive, so the molecule falls apart the instant it emits a photon. Strictly speaking, these noble-gas halide species are 'exciplexes' (excited complexes between two different atoms) rather than true excimers (excited dimers of two identical atoms, such as Xe2* or Kr2*), but the name 'excimer laser' is conventionally retained for both types. Because the dissociated ground-state products cannot reabsorb the emitted light, population inversion is maintained easily, making excimer lasers efficient, high-power ultraviolet sources spanning the vacuum UV through near UV.
Key facts
| Field | Physical Chemistry |
|---|---|
| Common gain species | ArF*, KrF*, XeCl*, XeF* |
| Emission range | ~157-351 nm (vacuum UV to near UV) |
| Excitation method | Electrical discharge or electron beam |
The krypton fluoride (KrF*) excimer laser emits at 248 nm and is a workhorse light source for deep-UV photolithography used to pattern integrated circuits; the argon fluoride (ArF*) laser at 193 nm is used in LASIK and PRK corneal eye surgery.
Frequently asked questions
Why can't the excimer molecule exist in its ground state?
Noble-gas atoms have closed electron shells and form no stable ground-state bond with a halogen atom, so the ground-state potential energy curve is repulsive (dissociative). A bound excimer forms only when one atom is promoted to an excited electronic state that allows a weak, short-lived attractive interaction.
What are excimer lasers used for?
Major applications include deep-UV photolithography in semiconductor manufacturing, refractive eye surgery (LASIK/PRK), micromachining, and dermatological phototherapy (XeCl at 308 nm for psoriasis and vitiligo).