Excimer is an abbreviation for Excited Dimer, which literally means a double atom in an excited state.
Excimer (originally abbreviated as excited dimer) is a short-lived dimer or heterodimer molecule formed by two substances, in which the valence shell of at least one substance is completely filled with electrons (e.g. inert gas). In this case, it is only possible to form a molecule when the atom is in an electronically excited state. Heteronuclear molecules and molecules with two or more substances are also known as exciton complex molecules (originally abbreviated as exciton complexes). Excimer molecules are usually diatomic and composed of two atoms or molecules. If both are in the ground state, they will not bind. The lifetime of quasi molecular molecules is very short, about nanoseconds. The combination of a large number of excited atoms forms Rydberg matter clusters, whose lifetimes can exceed several seconds.

These two atoms are an inert gas atom and a halogen atom. When inert gas atoms are excited to an ionic state, they become quasi molecules, which only bind to halogen atoms in the excited state. This type of excimer has a short lifespan, emits ultraviolet light and returns to its basic form, becoming the original inert gas and halogen atom.

Excimer laser is a pulsed gas laser that can emit ultra short pulses (pulse duration of picoseconds or femtoseconds). They emit high-energy ultraviolet radiation with wavelengths shorter than 360 nm. The ultraviolet emission source is a rapid discharge in a high-pressure mixture of rare gases (such as helium, neon, argon, krypton, etc.) and halogen gases (such as fluorine, chlorine, bromine, etc.) in equal proportions.
They typically use rare gases containing molecules because they are non reactive chemical compounds under normal circumstances. The wavelength of laser emission depends on the composition of halogens and the presence of rare gases in the laser medium. The term "excimer" originates from a dimer, which refers to a diatomic molecule formed by the combination of two atoms. If this dimer is in an excited state, it is called an excited dimer or quasi molecular molecule. Excimer laser is also known as exciton complex laser.
working principle
Excimer molecules contain inert gases and do not form chemical compounds under normal conditions, but only form unstable compounds when they are in an excited state. Such molecules will dissociate in the ground state.
Use discharge to excite a mixed gas containing rare gases and halogen (or oxide) gases. This will generate excited molecules containing halogen gases and rare gases, or quasi molecular molecules. Compared to the ground state, this excited state has a longer lifetime. Therefore, stimulated emission occurs, and the two gases are excited back to the ground state and separated by photon emission.

Some common dimer molecules are oxides (argon oxide, ArO) and halides (argon halide, ArF) of inert gases involved in the formation of excimer lasers; KrF, etc.). For example, krypton fluoride is a gas mixture containing krypton and fluorine that is excited during pulsed discharge. Under the excitation of krypton fluoride, a series of complex processes occur to form the metastable excited state of KrF *. The asterisk (*) represents excited state molecules. The metastable excited state remains for a brief period of time before dissociation. It should be noted that excimer lasers cannot generate continuous waves, partly because it is impossible to obtain stable discharges with appropriate characteristics. The pulse duration obtained from an excimer laser is usually a few nanoseconds, but sometimes longer, about 100 nanoseconds.
Pump Source：
Inert gas (argon, Ar2; Krypton gas, Kr2; Xenon gas (Xe2) is exposed to a pressure of approximately 10 atmospheres and discharged using high-energy electron beams or pulses. Modern industrial excimer lasers use discharge excimer lasers because they are inexpensive and small in size. In addition, compared with electron beam devices, the energy extraction capability of pulse discharge is lower.
The lifespan of the laser
Previously, the lifespan of excimer lasers was limited due to (1) corrosion from used gases, (2) erosion of electrode materials, (3) degradation of optical materials caused by strong ultraviolet radiation, and (4) chemical byproducts and dust pollution generated by discharge.
The above problem is solved by regularly replacing the mixed gas (after 30 million pulses). Due to the strong corrosiveness of these lasers, buffer gases (usually neon or helium) are used. Excimer lasers are typically designed with stainless steel, polyethylene, and polytetrafluoroethylene materials to prevent corrosion.
Different excimer lasers, their related wavelengths, and applications

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