Ultraviolet Excimer Radiation from Nonequilibrium Gas Discharges and its Application in Photophysics, Photochemistry and Photobiology
ИСТОЧНИКИ СПОНТАННОГО ИЗЛУЧЕНИЯ И ИХ ПРИЛОЖЕНИЯ
ULTRAVIOLET EXCIMER RADIATION FROM NONEQUILIBRIUM GAS DISCHARGES AND ITS APPLICATION IN PHOTOPHYSICS, PHOTOCHEMISTRY AND PHOTOBIOLOGY
© 2012 г. U. Kogelschatz, Doctor of Natural Sciences
Retired from ABB Corporate Research, 5405 Baden, Switzerland
E-mail: u.kogelschatz@bluewin.ch
Narrowband UV and VUV excimer radiation can be generated in a variety of nonequilibrium gas discharges: dielectric barrier discharges, microhollow cathode discharges, arrays of microplasmas, corona discharges. Excimer lamps (excilamps) are now available for a large number of wavelengths and in various geometrical shapes. The availability of nearly monochromatic photon fluxes ranging in energy up to 15 eV resulted in a number of innovative photo-induced processes in photophysics, photochemistry and photobiology. This report focuses on progress made in the last decade.
Keywords: ultraviolet radiation, excimer fluorescence, photochemistry, materials processing, pollution control, phototherapy.
OCIS сodes: 350.5400, 350.5130, 350.5610
Поступила в редакцию 02.05.2012
Introduction
Sources of incoherent excimer radiation, excimer lamps or excilamps, have recently gained considerable importance. They can provide high photon fluxes mainly in the ultraviolet (UV) or vacuum ultraviolet (VUV) part of the spectrum. Typical efficiencies are at least an order of magnitude higher than those of excimer lasers. Progress has been made in understanding and optimizing the discharge physics and reaction kinetics of excimer formation. In addition to the already established technologies of UV curing, materials processing and pollution control novel applications like nanoparticle charging, particle coating and single photon ionization (SPI) of organic molecules emerged. Several manufacturers now offer excimer lamps for various industrial applications and as research tools. This report focuses on developments that took place in the last decade. For the extensive literature on earlier developments the reader is referred to a number of review papers on the subject [1–7]. The most comprehensive information on excimer lamps can be found in a recently published book by Boichenko, Lomaev, Panchenko, Sosnin and Tarasenko [8].
Different Discharges
The most important non-equilibrium discharge used for the generation of excimer radia-
tion is without doubt the dielectric barrier discharge (DBD). In DBDs the requirements for efficient excimer formation (high electron energy at reasonably high pressure and comparatively low gas temperature) can easily be fulfilled. Different electrode geometries for generating filamentary or diffuse volume DBDs with one or two dielectric barriers and coplanar systems with buried adjacent electrodes leading to surface discharges are common. Scale-up is straight forward and cost-effective reliable power supplies are available. This technology, originally developed for industrial ozone generation, has been described in several reviews [1–8] and will not be repeated here. In laboratory studies also radiofrequency [9, 10] and microwave excited DBD excimer lamps [11–13] have been studied. A very simple configuration is the use of two parallel sleeve or ring electrodes placed on the outside of a dielectric tube. At about atmospheric pressure and narrow electrode spacing surface discharges are initiated at the inner surface of the tube [14, 15]. At reduced pressure of about 10 hPa a fairly wide separation of the external sleeve electrodes (up to 38 cm) can be used to sustain a glow discharge inside the tube. This configuration is often referred to as a capacitive excilamp [16, 17]. Straight cylindrical as well as bent U-shaped tubes are used. Power supplies providing bipolar meander pulses at 100 kHz repetition frequency are typically used.
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It should be mentioned that excimer formation has also been obtained in several other types of high-pressure non-equilibrium discharges. Microhollow cathode discharges are important examples [18–20]. In a pulsed Xe microhollow cathode discharge efficiencies of 20% were reached [21]. The discharge behaviour of a Xe microhollow cathode discharge was modeled by Boeuf 2005 [22]. In addition, discharges in metal capillaries can emit excimer radiation [23] and also dc excited cathode boundary layer (CBL) discharges can produce intense Xe excimer radiation [24]. In a similar way microplasmas produced in narrow holes or slits can be used [25, 26]. Also arrays of several microplasmas were investigated [27, 28]. A completely different approach uses dc corona discharges from a number of pins in planar configurations or from a central wire in cylindrical configurations as a simple and efficient source of excimer radiation [29, 30].
Electron Beam Pumped Excimer Lamp (EBEL)
A compact sealed rare gas excimer VUV source based on electron beam excitation was first described by Wieser in 1997, 1998 [31, 32] and further investigated by Fedenev [33], by Morozov [34, 35] and by Ulrich [36]. An electron beam of about 10–20 keV energy is generated in a small vacuum chamber with the aid of a heated tungsten filament. Accelerated electrons pass through an extremely thin (300 nm thick) vacuum tight silicon nitride (SiNx) membrane into a second chamber filled with a rare gas at about atmospheric pressure. For electron energies above 12 keV the losses in the membrane amount to only a few percent. In the rare gas the electrons are slowed down over a short distance exciting atoms which immediately form excited dimers. The second excimer continua of Ar, Kr, Ne, Xe can be obtained with striking energy efficiency (about 30% for Ne, Ar and about 40% for Kr, Xe). The gas volume excited in 1000 hPa Ar is about 1 mm3, an almost spherical region of bright VUV excimer radiation. The Ar*2, Kr*2, Xe*2 radiation can be extracted through a MgF2 window. This excimer lamp can be run in dc or pulse mode. The SiNx membrane will hold up to a pressure difference of more than 5000 hPa, a dc beam current of 10 A, or a pulse current of 2 A for 100 ns. This compact source of excimer radiation has been commercialized under the name e-luxTM [37] and is used as
a soft ionization source in advanced mass spectrometry.
Pulsed Excimer Lamps
An interesting property of many excimer systems is hat they can be run at high power densities. Bright pulsed excimer flash lamps have been reported in the literature. In 1996 Kubudera described a pulsed DBD using a mixture of Kr and Xe in pulsed DBD to obtain broadband VUV emission ranging from 120 nm to 190 nm from the dimers Kr2*, XeKr* and Xe*2 [38]. A few years later this group reached a radiant power density of 1.5 kW/cm2 VUV output peaking at 147 nm from a pulsed Kr DBD [39]. This was achieved with a filamentary DBD in a quartz tube with external linear strip electrodes on opposite sides (6 mm wide, 600 mm long). Carman focused on short pulses with high repetition rates (up to 50 kHz) and reached VUV efficiencies of 60% in a diffuse Xe DBD [40]. The peak radiant power density was 6 W/cm2. Tarasenko [41] showed that with a nanosecond duration runaway electron preionized diffuse discharge (REP DD) in all atmospheric pressure rare gases VUV radiation from the second excimer continuum could be obtained. Best results were obtained in Xe with a radiant peak power of 300 kW corresponding to a 1.5 kW/cm2 radiant power density at the output window. At an elevated pressure of 1.2 MPa the Xe2* peak radiant power density could be raised to 6 kW/cm2 for an 8 ns pulse. The electrical power density during the pulse was 100 MW/cm3 [41]. The technology of REP DD was discussed in detail by Baksht [42]. Short-pulse excitation of near atmospheric pressure Ne was studied by Carman using a DBD with current pulses of 150 ns duration [43]. Almost monochromatic radiation of Ne*2 peaking at 84 nm was obtained with 5 nm FWHM (full width at half maximum). High values of radiant peak power densities can also be obtained in the rare gas/halogen systems Kr/Cl2, Xe/Cl2, Kr/Br2 and Xe/Br2 [44, 45]. In these systems radiant power densities of a few kW/cm2 were obtained at efficiencies close to 5%. Typical operating parameters were total pressures of 650– 1000 hPa and halogen admixtures of 1 to 2% [41]. In addition to high radiant power densities or high repetition rates pulsed excimer lamps can also be optimized for other characteristics. For example, a miniaturized XeCl lamp with a fast decay of the UV radiation, about three orders
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of magnitude within 5 s, was developed with intended applications in fluorescence spectroscopy for the detection of biomolecules [46, 47].
Open Discharge Configurations
The Xe excimer lamp is by far the most important VUV lamp because its photon energy of 7.2 eV is high enough to initiate various photochemical processes and special sorts of fused silica glass (quartz) have high transmission at 172 nm, so that sealed lamps of high life expectancy can be produced. For shorter wavelengths special windows have to be used (e. g. CaF2, MgF2, LiF) which are expensive and available only in smaller sizes. To circumvent this problem and still make use of the radiation of the Ar2* excimer radiation at 126 nm open discharge configurations, “windowless excimer lamps”, were designed, in which the DBD for generating the VUV radiation and the substrates to be treated are both situated inside a through-flow reactor. The idea was first published by Kogelschatz in 1992 [1]. Photo-induced metal deposition was demonstrated starting from thin Pd acetate films [48]. The design of windowless excimer systems was followed up by several authors [49–51]. The most sophisticated design was recently published by Sobottka [52]. In this publication an atmospheric pressure glow discharge in Ar is run between parallel dielectriccovered electrodes of 540 mm length. The operating frequency was 680–840 kHz, the maximum electric power 20 kW. Substrates could be placed on a moving conveyor belt below the discharge. With well designed aerodynamic seals and an Ar flow of 4–6 m3/h the residual O2 content was 1 bar) dielectric barrier discharge lamps generating short pulses of high-peak power vacuum ultraviolet radiation // J. Phys. D: Appl. Phys. 2004. V. 37. № 17. P. 2399–2407.
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ULTRAVIOLET EXCIMER RADIATION FROM NONEQUILIBRIUM GAS DISCHARGES AND ITS APPLICATION IN PHOTOPHYSICS, PHOTOCHEMISTRY AND PHOTOBIOLOGY
© 2012 г. U. Kogelschatz, Doctor of Natural Sciences
Retired from ABB Corporate Research, 5405 Baden, Switzerland
E-mail: u.kogelschatz@bluewin.ch
Narrowband UV and VUV excimer radiation can be generated in a variety of nonequilibrium gas discharges: dielectric barrier discharges, microhollow cathode discharges, arrays of microplasmas, corona discharges. Excimer lamps (excilamps) are now available for a large number of wavelengths and in various geometrical shapes. The availability of nearly monochromatic photon fluxes ranging in energy up to 15 eV resulted in a number of innovative photo-induced processes in photophysics, photochemistry and photobiology. This report focuses on progress made in the last decade.
Keywords: ultraviolet radiation, excimer fluorescence, photochemistry, materials processing, pollution control, phototherapy.
OCIS сodes: 350.5400, 350.5130, 350.5610
Поступила в редакцию 02.05.2012
Introduction
Sources of incoherent excimer radiation, excimer lamps or excilamps, have recently gained considerable importance. They can provide high photon fluxes mainly in the ultraviolet (UV) or vacuum ultraviolet (VUV) part of the spectrum. Typical efficiencies are at least an order of magnitude higher than those of excimer lasers. Progress has been made in understanding and optimizing the discharge physics and reaction kinetics of excimer formation. In addition to the already established technologies of UV curing, materials processing and pollution control novel applications like nanoparticle charging, particle coating and single photon ionization (SPI) of organic molecules emerged. Several manufacturers now offer excimer lamps for various industrial applications and as research tools. This report focuses on developments that took place in the last decade. For the extensive literature on earlier developments the reader is referred to a number of review papers on the subject [1–7]. The most comprehensive information on excimer lamps can be found in a recently published book by Boichenko, Lomaev, Panchenko, Sosnin and Tarasenko [8].
Different Discharges
The most important non-equilibrium discharge used for the generation of excimer radia-
tion is without doubt the dielectric barrier discharge (DBD). In DBDs the requirements for efficient excimer formation (high electron energy at reasonably high pressure and comparatively low gas temperature) can easily be fulfilled. Different electrode geometries for generating filamentary or diffuse volume DBDs with one or two dielectric barriers and coplanar systems with buried adjacent electrodes leading to surface discharges are common. Scale-up is straight forward and cost-effective reliable power supplies are available. This technology, originally developed for industrial ozone generation, has been described in several reviews [1–8] and will not be repeated here. In laboratory studies also radiofrequency [9, 10] and microwave excited DBD excimer lamps [11–13] have been studied. A very simple configuration is the use of two parallel sleeve or ring electrodes placed on the outside of a dielectric tube. At about atmospheric pressure and narrow electrode spacing surface discharges are initiated at the inner surface of the tube [14, 15]. At reduced pressure of about 10 hPa a fairly wide separation of the external sleeve electrodes (up to 38 cm) can be used to sustain a glow discharge inside the tube. This configuration is often referred to as a capacitive excilamp [16, 17]. Straight cylindrical as well as bent U-shaped tubes are used. Power supplies providing bipolar meander pulses at 100 kHz repetition frequency are typically used.
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It should be mentioned that excimer formation has also been obtained in several other types of high-pressure non-equilibrium discharges. Microhollow cathode discharges are important examples [18–20]. In a pulsed Xe microhollow cathode discharge efficiencies of 20% were reached [21]. The discharge behaviour of a Xe microhollow cathode discharge was modeled by Boeuf 2005 [22]. In addition, discharges in metal capillaries can emit excimer radiation [23] and also dc excited cathode boundary layer (CBL) discharges can produce intense Xe excimer radiation [24]. In a similar way microplasmas produced in narrow holes or slits can be used [25, 26]. Also arrays of several microplasmas were investigated [27, 28]. A completely different approach uses dc corona discharges from a number of pins in planar configurations or from a central wire in cylindrical configurations as a simple and efficient source of excimer radiation [29, 30].
Electron Beam Pumped Excimer Lamp (EBEL)
A compact sealed rare gas excimer VUV source based on electron beam excitation was first described by Wieser in 1997, 1998 [31, 32] and further investigated by Fedenev [33], by Morozov [34, 35] and by Ulrich [36]. An electron beam of about 10–20 keV energy is generated in a small vacuum chamber with the aid of a heated tungsten filament. Accelerated electrons pass through an extremely thin (300 nm thick) vacuum tight silicon nitride (SiNx) membrane into a second chamber filled with a rare gas at about atmospheric pressure. For electron energies above 12 keV the losses in the membrane amount to only a few percent. In the rare gas the electrons are slowed down over a short distance exciting atoms which immediately form excited dimers. The second excimer continua of Ar, Kr, Ne, Xe can be obtained with striking energy efficiency (about 30% for Ne, Ar and about 40% for Kr, Xe). The gas volume excited in 1000 hPa Ar is about 1 mm3, an almost spherical region of bright VUV excimer radiation. The Ar*2, Kr*2, Xe*2 radiation can be extracted through a MgF2 window. This excimer lamp can be run in dc or pulse mode. The SiNx membrane will hold up to a pressure difference of more than 5000 hPa, a dc beam current of 10 A, or a pulse current of 2 A for 100 ns. This compact source of excimer radiation has been commercialized under the name e-luxTM [37] and is used as
a soft ionization source in advanced mass spectrometry.
Pulsed Excimer Lamps
An interesting property of many excimer systems is hat they can be run at high power densities. Bright pulsed excimer flash lamps have been reported in the literature. In 1996 Kubudera described a pulsed DBD using a mixture of Kr and Xe in pulsed DBD to obtain broadband VUV emission ranging from 120 nm to 190 nm from the dimers Kr2*, XeKr* and Xe*2 [38]. A few years later this group reached a radiant power density of 1.5 kW/cm2 VUV output peaking at 147 nm from a pulsed Kr DBD [39]. This was achieved with a filamentary DBD in a quartz tube with external linear strip electrodes on opposite sides (6 mm wide, 600 mm long). Carman focused on short pulses with high repetition rates (up to 50 kHz) and reached VUV efficiencies of 60% in a diffuse Xe DBD [40]. The peak radiant power density was 6 W/cm2. Tarasenko [41] showed that with a nanosecond duration runaway electron preionized diffuse discharge (REP DD) in all atmospheric pressure rare gases VUV radiation from the second excimer continuum could be obtained. Best results were obtained in Xe with a radiant peak power of 300 kW corresponding to a 1.5 kW/cm2 radiant power density at the output window. At an elevated pressure of 1.2 MPa the Xe2* peak radiant power density could be raised to 6 kW/cm2 for an 8 ns pulse. The electrical power density during the pulse was 100 MW/cm3 [41]. The technology of REP DD was discussed in detail by Baksht [42]. Short-pulse excitation of near atmospheric pressure Ne was studied by Carman using a DBD with current pulses of 150 ns duration [43]. Almost monochromatic radiation of Ne*2 peaking at 84 nm was obtained with 5 nm FWHM (full width at half maximum). High values of radiant peak power densities can also be obtained in the rare gas/halogen systems Kr/Cl2, Xe/Cl2, Kr/Br2 and Xe/Br2 [44, 45]. In these systems radiant power densities of a few kW/cm2 were obtained at efficiencies close to 5%. Typical operating parameters were total pressures of 650– 1000 hPa and halogen admixtures of 1 to 2% [41]. In addition to high radiant power densities or high repetition rates pulsed excimer lamps can also be optimized for other characteristics. For example, a miniaturized XeCl lamp with a fast decay of the UV radiation, about three orders
56 “Оптический журнал”, 79, 8, 2012
of magnitude within 5 s, was developed with intended applications in fluorescence spectroscopy for the detection of biomolecules [46, 47].
Open Discharge Configurations
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