Excimer
An excimer (originally short for excited dimer) is a short-lived polyatomic molecule formed from two species that do not form a stable molecule in the ground state. In this case, formation of molecules is possible only if such atom is in an electronic excited state.[1] Heteronuclear molecules and molecules that have more than two species are also called exciplex molecules (originally short for excited complex). Excimers are often diatomic and are composed of two atoms or molecules that would not bond if both were in the ground state. The lifetime of an excimer is very short, on the order of nanoseconds.
Formation and decay
Under the
Excimers are only formed when one of the dimer components is in the excited state. When the excimer returns to the ground state, its components dissociate and often repel each other. The wavelength of an excimer's emission is longer (smaller energy) than that of the excited monomer's emission. An excimer can thus be measured by fluorescent emissions.
Because excimer formation is dependent on a
Usage note
The term excimer (excited state dimer) is, strictly speaking, limited to cases in which a true dimer is formed; that is, both components of the dimer are the same molecule or atom. The term exciplex refers to the heterodimeric case; however, common usage expands excimer to cover this situation.
Examples and use
Laser | Reagents | Emission peak |
---|---|---|
XeCl |
Xe + Cl 2 |
308 nm |
KrF | Kr + NF 3 |
248 nm |
ArF | Ar + F 2 |
193 nm |
Heterodimeric diatomic complexes involving a noble gas and a halide, such as xenon chloride, are common in the construction of excimer lasers, which are excimers' most common application. These lasers take advantage of the fact that excimer components have attractive interactions in the excited state and repulsive interactions in the ground state. Emission of excimer molecules is also used as a source of spontaneous ultraviolet light (excimer lamps).[2]
The molecule
In
As a general rule, the regioselectivity is in favor of the ortho adduct at the expense of the meta adduct when the amount of charge transfer taking place in the exciplex increases.
Generation techniques
It takes a noble gas atom in an
Fluorescence quenching
Exciplexes provide one of the three dynamic mechanisms by which
As an exception to the conventional
See also
- Excimer lamp – Ultraviolet source based on spontaneous emission of excimer molecules.
- Excimer laser – Type of ultraviolet laser important in chip manufacturing and eye surgery
- Förster resonance energy transfer – Photochemical energy transfer mechanism
- Krypton fluoride laser – type of excimer laser
- Noble gas compound – Chemical compound containing a noble gas element