Excimer lasers are pulsed gas lasers that emit ultra-short pulses (pulses with picosecond or femtosecond durations). They emit high-energy ultraviolet light having wavelengths shorter than 360 nm. The source of ultraviolet emission is a fast electrical discharge in a high-pressure mixture of a noble gas (for example, helium, neon, argon, krypton, etc.) and a halogen gas (for example, fluorine, chlorine, bromine, etc), taken in equal ratios.
They commonly use noble gases containing molecules as they are non-reactive chemical compounds under normal circumstances. The wavelength of the laser emission depends on the constituent halogen and the rare gas present in the lasing medium. The word excimer is derived from the word dimer which means a diatomic molecule formed by the combination of two atoms. If such a dimer is in an excited state, then it is known as an excited dimer or an excimer molecule. Excimer lasers are also known as exciplex lasers.
Excimer molecules contain noble gas that does not form chemical compounds under normal conditions but forms unstable compounds only when they are in an excited state. Such molecules dissociate when in the ground state.
A gas mixture containing a noble gas and a halogen (or oxides) gas is excited using an electric discharge. This results in an excited molecule that contains halogen gas and the noble gas, or the excimer molecule. The lifetime of this excited state is higher when compared to the ground state. Thus a stimulated emission occurs and the two gases de-excites to the ground state and get separated with the emission of a photon.
Some of the common dimer molecules are oxides (Argon oxide, ArO) and halides (Argon halide, ArF; Krypton fluoride, KrF, etc) of noble gas which participate in the formation of excimer lasers. For example, krypton fluoride which is a gas mixture containing krypton and fluorine is excited in a pulsed electrical discharge. A chain of complex processes takes place while excitation of krypton fluoride, i.e., a KrF* metastable excited state is formed. The asterisk (*) denotes that the excited state molecule. The metastable excited state remains intact for a short duration before dissociating. It is to be noted that excimer lasers cannot produce continuous-wave, partly because it is not possible to obtain a stable electric discharge with suitable properties. The pulse duration obtained from the excimer laser is often a few nanoseconds, but sometimes longer, of the order of 100 ns.
The reaction involved is:
Exposing Nobel gas ( Argon, Ar2; krypton, Kr2; xenon Xe2) to a pressure of about 10 atm and using a high energy electrical beam or a pulsed electrical discharge. Modern industrial excimer lasers used Electric-discharge excimer lasers because they are less expensive and small in size. Additionally, the energy extraction capabilities of pulsed electric discharge are less compared to electron beam devices.
Earlier, excimer lasers used to have limited lifetimes due to problems like (1) corrosion of used gas, (2) ablation of electrode materials, (3) Optical materials degradation by the strong UV light, (4) contamination by the electric discharge with chemical byproducts and dust.
The above problem is solved by a regular exchange of the gas mixture (after 30 million pulses). A buffer gas (typically neon or helium) is used as these lasers are highly corrosive. Excimer lasers are often designed using stainless steel, polyvinyl, and Teflon materials to prevent corrosion.
Different Excimer lasers, their associated wavelength, and application
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