Excimer lamps are quasimonochromatic light sources that can operate over a wide range of wavelengths in the ultraviolet (UV) and vacuum ultraviolet (VUV) spectral regions. The operation of excimer lamps is based on the formation of excited dimers (excimers) and the following transition from the bound excited excimer state to a weakly bound ground state resulting to an UV-photon radiation. Maximum of an excimer lamp radiation wavelength is specified by a working excimer molecule (see table below).
Wavelength and photon energy of excimer lamp radiation.
|Working excimer molecule||Wavelength (nm)||Photon energy (eV)|
Excimers are diatomic molecules or complexes of molecules that have stable excited electronic states and an unbound or weakly bound ground state. Originally only homonuclear dimers with a stable excited state but a repulsive ground state were called excimers (excited dimers). The term excimer has since been extended to mean any multi-atom molecule with a thermally unstable (repulsive or only very weakly bonding) ground state. But sometimes one can meet term exciplex meaning an excited complex. It is also excimer molecule but not homonuclear dimer. For example, Xe2*, Kr2*, Ar2* are excimer molecules but XeCl*, KrCl*, XeBr*, ArCl*, Xe2Cl* are exciplex molecules. The best known include rare gas excimers and rare-gas-halogen excimers. Rare-gas-halogen trimers, metal excimers, metal-rare-gas excimers, metal-halogen excimers and rare-gas-oxygen excimers are also known.
Because of these excimer formations are unstable they disintegrate within a few nanoseconds converting their excitation energy to optical radiation. Because of the excimer molecule nature the difference between their stable excited state and weakly bound ground state amounts from 3.5 to 10 eV that is provided radiation of UV and VUV spectral range. The spectrum of excimer lamp radiation is characterized by an intense narrow emission band. The full-width at half maximum of these bands depends on the kind of working molecule and excitation conditions and amounts from 2 to 15 nm. Hence, the excimer lamps are actually quasimonochromatic light sources. Moreover, about 70-80% of the whole radiation power of an excimer lamp is concentrated in this one emission band. Such sources are suitable for tasks of spectral-selective irradiation and can even replace lasers in some cases.
Radiation is released when an excimer molecule in its upper electronic exited state de-excites to its ground state. The excimer or exciplex molecules are not very stable and rapidly decompose, typically within a few nanoseconds, giving up their excitation energy in the form of a UV photon:
excimer molecule emission:
exciplex molecule emission:
Excimer molecule formation
Since the main principle underlying the operation of excimer lamps relies on the spontaneous radiative decomposition of excimer states then the key task is effective generation of excimer molecules. The key role in the excimer molecules formation plays electrons. In order to generate efficiently excimer molecules the active medium should contain a sufficient concentration of electrons with energies that are high enough to produce the precursors of the excimer molecules, which are mainly excited and ionized rare gas atoms. Excitation of working gas mixture leads to form excited and ionized rare gas atoms:
- Rg + e− → Rg* + e−,
- Rg + e− → Rg+ + 2e−,
- Rg* + e− → Rg+ + 2e−,
where Rg* is an excited electronic state of rare gas atom, Rg+ is an ionized state of rare gas atom, e− is an electron.
When active medium accumulates enough quantity of excited rare gas atoms the excimer molecules are formed by the following reaction:
- Rg* + Rg + M → Rg2* + M,
where M is a third particle carrying away the excess energy. As a rule it is a rare gas atom of a working mixture.
Since the formation of the excimer molecules is carried out by a three-body reaction it is profitable the pressure to be high. Higher pressure increases the concentration of atoms and the probability of simultaneous collision of three species that is necessary for excimer producing. But at the same time the increasing pressure leads to intensification of excimer molecule quenching that is radiationless decay of the excimer molecules. The optimal pressure of the working mixture is established by the experimental way. On the practice the pressure of working mixture amounts up to one atmosphere and more.
The reaction mechanisms underlying the formation of exciplex molecules (rare gas halides) are rather complex in which take part ground state atomic and molecular species, ionic species, and excited atomic species.
The formation of exciplex molecules is realized in two main ways. The first way is a three-body ion-ion recombination reaction of the positive rare gas ion and the negative halogen ion:
- Rg+ + X− + M → RgX* + M,
where M is a collisional third partner which in many cases can be an atom or molecule of the working mixture or even of the buffer gas. The formation of the negative halogen ion occurs by the electron interaction with halogen molecules (dissociative attachment reaction):
- X2 + e− → X + X−,
where X is a halogen atom.
The second way is a harpooning reaction which is a binary process. In this case the excited rare gas species transfers its loosely bound electron to the halogen molecule or halogen-containing compound to form an electronically excited state of exciplex molecule RgX*:
- Rg* + X2 → RgX* + X.
Since the harpooning reaction is a two-body process it does not need so high pressure like for three-body reaction. So, the harpooning reaction makes available the excilamps operating at a low pressure of working mixture. As a result, in such case the intensity of excimer molecule quenching is much less than in the excimer lamp in which excimer molecules forms by three-body reaction. That allows the achievement of the maximal energy conversion efficiency to the UV radiation.
It should be mentioned that both harpooning reaction and reaction of ion-ion recombination running simultaneous. The domination of the first or second reaction is determined mainly by the working mixture pressure. High pressure is advantageous for the domination of the ion-ion recombination reaction and the low pressure is advantageous for the domination of the harpoon reaction.
In principle all rare gases and rare-gas halogens can form a slightly bound excited state and thus can form excimers. In most cases the reaction kinetics leading to the formation of a special excimer complex is selective. Thus, it is possible to construct radiation sources with high intensity within certain narrow spectral regions.
The main benefits of excimer lamps from other sources of UV and VUV radiation are the following:
- high average specific power of UV radiation (up to 1 Watt per cubic centimeter of active medium);
- high energy of emitting photon (from 3.5 to 10 eV);
- quasimonochromatic radiation with the spectral full-width at half maximum from 2 to 15 nm;
- high power spectral density of UV radiation;
- choice of wavelength of UV radiation maximum for specific purposes;
- availability of multiple-wavelength UV radiation by the simultaneous excitation of several kind of working excimer molecules;
- absence of visible and IR radiation;
- instant attainment of operating mode;
- low heating of radiating surface;
- absence of mercury.
Excimer lamp excitation methods
Nevertheless, one of the most usable ways for practical use is excitation by an electric discharge. Actually it is used more types of discharge for excimer lamp pumping (some of them are glow discharge, pulsed discharge, capacitive discharge, longitudinal and transverse discharges, volume discharge, spark discharge, microhollow discharge, etc.). Currently, the excimer lamps with capacitive discharge type of excitation namely dielectric barrier discharge  are the most widely spread. Lamps using this discharge type are already commercially available. In this technology electrodes are not in direct contact with the active medium (plasma) that eliminates any interaction between the discharge and the electrodes, contamination of the active medium with sputtering electrode material, as well as electrode corrosion leading to short operational lifetime of conventional lamp constructions. Moreover, dielectric barrier discharge carries out an effective excitation of gas mixture in wide range of working pressure. Such lamps can be produced in any desired form of radiating surface suitable for any specific tasks.
Radiation sources emitting UV photons are widely used in techniques involving photo-chemical processes, for example drying up adhesive or printing-inks, photolithography, UV induced growth of dielectrics, UV induced surface modification, and cleaning or material deposition. Incoherent sources of UV radiation have several advantages over laser sources because of their lower cost, huge area of irradiation, and simplicity of operation especially when large-scale industrial processes are envisaged.
Mercury lamps (λ = 253.7 nm) are also used as UV sources, but their production, use and disposal of old lamps are harmful to personal and environment. Besides, compared with commonly used mercury lamps, excimer lamps have a number of advantages. A specific feature of the excimer molecules is their stability in the excited electronic state and the absence of a strong bond in the ground state. Thanks to the absence of an absorbing ground state of excimer molecule high intensity UV radiation can be extracted from the plasma without significant self-absorption. It makes available to the UV radiation efficiently convert energy deposited to the active medium.
The excimer lamps are also called cold radiation lamps because of low heating of the lamp radiating surface under operation in contrast with traditional lamps like mercury. Moreover, the excimer lamps attain the operating conditions practically at once after power supply is turned on.
Rare gas and rare gas-halide excimer sources generally radiate in the ultraviolet (UV) and vacuum-ultraviolet (VUV) wavelength range (see table). Their unique broad band emission characteristics (without narrow spectral lines), high internal efficiency and the high energy photons make them suitable for applications such as absorption spectroscopy, UV curing, UV coating, sterilization, ozone generation, destruction of gaseous organic waste, photo-etching and photo-deposition and more other applications.
Light sources emitting UV photons in the energy range of 3.5-10 eV have found a number of interesting applications because they are capable of splitting most chemical bonds. This has led to a number of new photo-initiated surface and volume processes like surface modification and cleaning as well as material deposition processes.
Examples of excimer lamps application include disinfection of drinking water, pool water, air, industrial waste, photochemical synthesis and degradation of organic compounds in flue gases and in water, photopolymerization of organic coatings and paints and photo-enhanced chemical vapour deposition. In all cases UV photons excite or cleave chemical bonds, forming radicals or other chemical species, which initiate the desired reaction.
Excimer lamps have selective action. Intense UV radiation at a specific wavelength can selectively generate certain radicals. Therefore, such lamps may prove useful for industrial chemical and photo-physical processing such as: UV curing of paints, varnishes and adhesives, cleansing and modifying surface properties, polymerization of lacquers and paints, and photo degradation of a variety of pollutants. Photo-etching of polymers is possible using different wavelengths: xenon excimers (172 nm), krypton chloride (222 nm), and xenon chloride (308 nm). Excimer UV sources may find applications in the microstructuring of large area polymer surfaces. XeCl-excimer lamps (308 nm) are especially suitable to get a tan.
Fluorescence spectroscopy is one of the most common methods in detecting biomolecules. In this process biomolecules are labeled with a fluoroprobe, which can be excited with a short pulse of UV light and re-emit radiation in visible spectral range. Thus, the detected fluorescence can be correlated with the density of labeled molecules. Lanthanide complexes are commonly used fluoroprobes. Due to their long lifetimes they play a special role in Forster resonance energy transfer (FRET) analysis.
At present, excilamps are coming into use in ecology, photochemistry, photobiology, medicine, criminalistics, petrochemistry, physics, microelectronics, wide-ranging engineering tasks, technologies, science, varied lines of industry including even food industry, and more else areas.
Mercury lamps are the most common source of UV radiation due to their high efficiency. However, the use of mercury in these lamps poses disposal and environmental problems. On the contrary, excimer lamps based on rare gases are absolutely non-hazardous and the excimer lamps containing halogen are more environmentally benign than mercury ones.
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