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Excimer

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The name excimer (originally short for excited dimer) was given to a short-lived molecule that bonds two molecules when in an electronic excited state. But in current usage there remains some confusion, and the term is a (sometimes vague) concept that describes a particular type of spectroscopic behavior used to explain various types of spectroscopic data. In this way, an excimer can be:

an excited state interaction between two molecules of the same type,
an excited state interaction between two molecules of varying type,
excited polymer conformations of varying types of molecules,
dimer formation signatures within certain kinds of polymers.

The point is that, an excimer is an excited state interaction between molecules that is distinguishably different from the corresponding stable interaction of ground state molecules as observed in the emmission spectra.

If returned to the ground state, the components of the molecular excimer often strongly disassociate and repel each other. The lifetime of an excimer is typically on the order of nanoseconds. Excimers can be used as the gain medium of a type of powerful ultraviolet laser known as an excimer laser.

Compare exciplex, one made from two different molecules. An example is xenon chloride (XeCl), where the xenon atom will only form a molecule with the chorine atom while in an excited state (Xe^*), and on reverting to the ground state returns to an unreactive noble gas electronic structure. Unfortunately, ecxiplexes are very commonly miscalled excimers, and exciplex lasers excimer lasers, which should be noted when reading the literature.

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