Excimer Lamp As A Perspective Photonic Instrument
2015 marks the 25th anniversary since the beginning of studies of a new class of gas-discharge light sources, excimer lamps, in the optical radiation laboratory of the Institute of High Current Electronics SB RAS. During this time, it has been created 18 samples of excimer lamps and photoreactors and demonstrated their wide applicability in solving various problems (photochemistry, photomedicine, gas industry etc.).
The term "excimer lamp" is a generic name of a class of devices emitting spontaneous ultraviolet (UV) and/or vacuum ultraviolet (VUV) radiation of excimer and exciplex molecules. Today there is a large variety of these light sources (LSs): they are classified according to both their working molecules (Table 1) and gaseous medium excitation techniques and design [1, 2].
Unlike fluorescent and thermal UV and VUV LSs, the radiation flux of excimer lamps is mostly focused in the UV or VUV ranges, in a relatively narrow spectral region with a half-width of 2 to 15 nm for exciplex molecules RgX* (Fig. 1) and to 30 nm for inert-gas excimers X2* [1–3]. This ensures the selectivity of various photochemical reactions and, accordingly, wide application of LSs in problems where narrowband is required and there is no need in radiation directivity and coherence .
Specific radiant flux (W/cm3) of excimer lamps exceeds the values typical for the low-pressure mercury lamps at the resonant transitions of atoms. Radiant emittance (up to 100 mW/cm2) and, unlike exciplex and excimer lasers, lack of self-absorption at the working wavelengths in the majority of excimer lamps.
Diversity in the design concept is described separately below.
Easy switching with a quick achievement of maximum power after switching (less than 1 s), that mostly does not require equipping LSs with control gears. Thus, the power supply of barrier discharge (BD) excimer lamps is performed by voltage pulses with amplitude of up to several hundred kilovolts and a frequency of up to several hundred kHz that is why these devices do not need special starters. This also determines the relative electrical safety of BR excimer lamps, as the bulb discharge is limited by a dielectric barrier, and the discharge current is limited to tens of milliamps.
The bulbs are heated to moderate temperatures. Due to this, in foreign literature excimer lamps are called "cold emission sources". This feature is useful for irradiation of objects sensitive to thermal stress.
The useful service life in the best samples of excimer lamps (trk period of time when the radiant flux of excimer lamps decreases by k percent) is tr 15–20 > 10,000 hours for chlorine excimer lamps and tr5 > 10,000 hours for the inert-gas excimer lamps. This parameter is often crucial for the use of LSs in various applications.
Efficiency (ratio of useful power radiated by the lamp to the power supplied thereto) is 7 to 40% (depending on the working molecule) [1, 2].
Finally, an important feature of excimer lamps is currently absence of mercury in the bulb (excluding halide mercury excimer lamps HgX*, which have not gained ground). As rightly pointed out in , when broken a mercury lamp containing 80 mg of metal and provided its complete evaporation, it causes indoor air pollution to MPC level of 300,000 m3. Therefore, the EU- counties have adopted Directive 2011/65/EU on the withdrawal of mercury-containing LSs from the economic turnover and/or restriction of the use of mercury for manufacturing LSs. As compared with, for example, low pressure mercury lamps, excimer lamps currently have better parameters (see. P. 2), but do not contain mercury that provides them with the prospect for the introduction to various kinds of bactericidal facilities .
It has recently became apparent that new companies and research groups engaged in research and production of excimer lamps, or using them as components for various equipment appear worldwide every year.
In the optical radiation laboratory of the Institute of High Current Electronics SB RAS, the excimer lamps investigations began 25 years ago. The first studies of the conditions of excimer and exciplex molecule formation was conducted in 1990–1997. Further development of this topic was stimulated by the appearance of powerful, serial, and low-cost field and bipolar transistors for BD electric drive circuits, getting access to high-quality quartz glass of domestic production  and finding modes which ensure long service life of excimer lamps.
Created excimer lamps have turned out to be relatively inexpensive (the price of excimer lamps at least substantially lower than the price of UV or VUV laser), have shown their applicability in almost all known photo processes required UV and/or VUV radiation. Thus, in many cases when it is required to subject extended objects to the narrow-band radiation, to meet contemporary environmental standards while radiation coherence does not matter, excimer lamps serve as an alternative to laser radiation. In comparison with UV LEDs (in the wavelength range of 200–300 nm), in relation to the cost of one watt of radiation and lifetime, excimer lamps are also currently beyond competition.
Here are some examples of LSs models based on excimer lamps developed at the Institute of High Current Electronics SB RAS, which received commercial and semi-commercial value. By "semi-commercial", we mean models manufactured by small enterprises and found their consumers in the market.
BD_P Series (barrier discharge, portable) is portable coaxial radiators with excimer lamps placed in the casing equipped with an air-cooling system and a reflector (Fig. 2, Table 2). These LSs have a relatively small size of 240 × 80 × 80 mm, weight 700 g, are suitable for scientific studies and popular in Russia and abroad.
To ensure a larger area of radiation, several models of LSs were developed. Thus, in BD_2P Series, a pair of bulbs is installed into the housing instead of a single bulb. This LS has been originally developed for use in dermatology, so here radiation of molecules XeCl and a timer to set photo treatment time are applied. Such a source with dimensions of 125 × 130 × 270 and a window size of 120 120 mm provides a radiant emittance up to 34 mW/cm2 at a power consumption of up to 80 W.
BD_EL Series (barrier discharge, extra-large) is LSs with a long coaxial excimer lamps placed in the casing equipped with an air-cooling system and a reflector (Fig. 3). Based on this model, excimer lamps on the working molecules KrCl*, XeCl* and XeBr* are produced. The radiation rays escape through a flange with a size of 85 ×10 cm.
One of the parameters that affects the efficiency and service life of excimer lamps is their temperature regime. Increasing of temperature leads to the reduction of both efficiency and service life of the lamp bulbs. This necessitates the forced cooling of the bulb. In case of the above mentioned models and specified irradiance emittance levels, it is enough to use a relatively simple air cooling system. Note that, in practice, other manufacturers usually use a water cooling system. In this case it is possible to increase specific radiant flux of KrCl- and XeCl-excimer barrier discharge lamps up to 0.1 W/cm3. We have shown that in case of a powerful Xe2-excimer lamp, it’s enough to use air cooling that has been realized in BD_Cs model (barrier discharge, cascade) that is an area LS comprising coaxial excimer lamps equipped with an air cooling system and a common reflector, assembled in a cascade with an output window 20 × 20 cm (Fig.4). This light source provides a radiant flux of 50 W and a radiant emittance on the xenon dimmer strip (172 nm) up to 120 mW/cm2 .
The Institute also develops various photoreactors for irradiation of solutions and gases. For laboratory purposes, when irradiating relatively small volumes of liquids, it is enough to use BD_P model . To intensify the irradiation of solutions and gases, a flow-through photoreactor is used, in which the irradiated medium goes through a quartz tube placed in the internal cavity of the excimer lamp. BD_R Series (barrier discharge, reactor) works according to this principle. The overall view and parameters of the model are shown in Fig. 5 and Table 3.
The institute has also gained a unique experience in developing photoreactors for irradiation of dense gases (up to 40 atm) on the basis of Xe2 – excimer lamps .
The description of these and other models of excimer lamps can be found at www.hcei.tsc.ru in Section "Technology" . Due to their widespread use, it was possible to conduct broad-scale studies of the excimer lamp radiation effects on different system (tab.4), in particular, on various aqueous solutions of toxic compounds for their removal [4, 9]; on microorganisms for their inactivation [7, 12]; on natural gas to clean it from the water impurities before injecting into the pipeline ; for sample preparation in analytical chemistry ; on the human skin for photo treatment of psoriasis, vitiligo, atopic dermatitis and eczema [13, 14], etc.
Here are some examples of the results obtained in recent years. In 2012–2013, in collaboration with Tomsk Agricultural Institute and ZAO "Siberian Agrarian Group", the question of the physiological effect of XeCl-excimer lamp radiation on animals was investigated . Topicality of the study is connected with the fact that the animals of the contemporary large livestock complexes spend the entire life cycle indoors. On the one hand, it prevents the spread of epidemic diseases and on the other hand, animals are deprived of access to sunlight, including the short-wavelength part of the solar UV radiation (approximately 290–320 nm), that in natural conditions stimulates the physiological activity of the animals. It has been studied the physiological effect of radiation on outbred white mice and sows during their farrowing. The results revealed no toxicity and embryotoxicity, skin-desorptive and allergic effects of radiation, but showed an increase in body weight by 2.6–3.1%. In addition, a small irradiation doses for sows can reduce mortality of newborn piglets more than doubled. The facts found give hope for the future wide application of XeCl excimer lamps to provide technological processes in animal breeding.
In 2002, the staff of the Institute in the course of research on irradiation of living cells with excimer lamps, a threshold effect was found: the dependence of the number of survived cells of Chinese hamster on the administered dose of XeBr-excimer lamp ultraviolet radiation has a threshold nature, that is absent when inactivating microorganisms . Due to this, now it is possible to choose such a dose of excimer lamp radiation that causes bacteria inactivation without violating the functional activity of fibroblasts in the living tissue. For example, it is important for surgery, for post-operative treatment of wounds. In particular, it has been hypothesized about reducing the risk of carcinogenesis in living tissues under the treatment of KrCl- or KrBr-excimer lamps. This hypothesis was confirmed in 2013, when the stuff of Columbia University Medical Center (USA) conducted its experimental verification using BD_P excimer lamp, in particular, it was shown that: 1) the narrowband ultraviolet radiation of KrBr- excimer lamp is effective against antibiotic-resistant bacteria; 2) the radiation is almost harmless for the genetic material of human cells; 3) KrBr excimer lamp radiation is much comparatively safer than the standard mercury lamp radiation, traditionally used for disinfection, as it causes significantly less number of mutations .
All of the above review allows us to predict the future widespread application of photonics devices such as excimer lamps.