An excimer (originally short for excited dimer) is a short-lived dimeric or heterodimeric molecule formed from two species, at least one of which is in an electronic excited state. A dimer is a Chemical or Biological entity consisting of two subunits called Monomers which are held together by either Intramolecular forces In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J Excitation is an elevation in energy level above an arbitrary baseline energy state Excimers are often diatomic and are formed between two atoms or molecules that would not bond if both were in the ground state. In Quantum mechanics, a stationary state is an Eigenstate of a Hamiltonian, or in other words a state of definite energy The lifetime of an excimer is very short, on the order of nanoseconds. A nanosecond ( ns) is one billionth of a second See also times of other orders of magnitude. Binding of a larger number of excited atoms form Rydberg matter clusters the lifetime of which can exceed many seconds. Rydberg matter is a metastable state of highly excited atoms (see Rydberg atom) which are condensed in a solid- or liquid-like very low density matter
Under the molecular orbital formalism, a typical ground-state molecule has electrons in the lowest possible energy levels. In Chemistry, a molecular orbital (or MO) is a region in which an Electron may be found in a Molecule. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J According to Hund's rule, at most two electrons can occupy a given orbital, and if an orbital contains two electrons they must be in opposite spin states. In Atomic physics, Hund's rules refer to a simple set of rules used to determine which is the Term symbol that corresponds to the ground state of a multi- Electron In Quantum mechanics, spin is a fundamental property of atomic nuclei, Hadrons and Elementary particles For particles with non-zero spin The highest occupied molecular orbital is called the HOMO and the lowest unoccupied molecular orbital is called the LUMO; the energy gap between these two states is known as the HOMO/LUMO gap. HOMO and LUMO are Acronyms for highest occupied Molecular orbital and lowest unoccupied Molecular orbital, respectively If the molecule absorbs light whose energy is larger than this gap, an electron in the HOMO may be excited to the LUMO. This is called the molecule's excited state. Excitation is an elevation in energy level above an arbitrary baseline energy state
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. A monomer (from Greek mono "one" and meros "part" is a small Molecule that may become chemically bonded to other An excimer can thus be measured by fluorescent emissions.
Because excimer formation is dependent on a bimolecular interaction, it is promoted by high monomer density. Molecularity in Chemistry is the number of colliding molecular entities that are involved in a single Reaction step. Low-density conditions produce excited monomers that decay to the ground state before they interact with an unexcited monomer to form an excimer.
The term excimer (excited state complex) 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.
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. History Noble gas is translated from the German noun de ''Edelgas'' first used in 1898 by Hugo Erdmann to indicate their extremely low level of reactivity A halide is a Binary compound, of which one part is a Halogen Atom and the other part is an element or radical that is less An excimer laser (sometimes and more correctly called an exciplex laser) is a form of ultraviolet laser which is commonly used in Eye surgery and Semiconductor These lasers take advantage of the fact that excimer components have attractive interactions in the excited state and repulsive interactions in the ground state. The molecule pyrene is another canonical example of an excimer that has found applications in biophysics to evaluate the distance between biomolecules. Pyrene is a Polycyclic aromatic hydrocarbon (PAH consisting of four fused Benzene rings resulting in a large flat Aromatic system A biomolecule is any organic Molecule that is produced by living Organisms including large Polymeric molecules such as Proteins
In organic chemistry many reactions occur through an exciplex for example those of simple arene compounds with alkenes:  The reactions of benzene and their products depicted are a [2+2]cycloaddition to the ortho product (A) . Organic chemistry is a discipline within Chemistry which involves the scientific study of the structure properties composition reactions, and preparation Benzene, or benzol, is an organic Chemical compound and a known Carcinogen with the molecular formula C 6 H 6 , a [2+3]cycloaddition to the meta product (B)  and the [2+4]cycloaddition to the para product (C)  with simple alkenes such as the isomers of 2-butene. Arene substitution patterns are part of Organic chemistry IUPAC nomenclature and pinpoint the position of Substituents other than Hydrogen in Isomers Among the molecules which have the Chemical formula 48 four Isomers are Alkenes. In these reactions it is the arene that is excited.
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