VUV Radiation Source Research Articles

Experimental and Theoretical Investigations of Dielectric Barrier Discharge (DBD) Lamp in Ne/Xe Mixture

Dielectric barrier discharges (DBDs) are prized due to their excellent performance and their light emission in a UV band at low temperatures. Excimer lamps using DBDs filled with rare gas and under the atmospheric pressure are used in industry as VUV radiation sources or visible light sources in the presence of a phosphor. In this article, we report the experimental and theoretical investigations of Ne-50%/Xe-50% DBD, using a high voltage of 2 kV. The discharge was achieved and studied under the pressure of 250 mbar, at a frequency of 40 kHz. First, we have analyzed experimentally the electrical behavior of DBD Ne/Xe excilamp and have studied the temporal profile of total current density, discharge current density, applied voltage, plasma, and dielectric voltage. Then, we have compared the experimental electrical characteristics with a theoretical work. The computer model used is based on three modules: plasma chemistry, electrical circuit, and Boltzmann equation. The effect of applied voltage and gas pressure on the total current density measured is studied and discussed. Finally, we have simulated the Ne/Xe DBD with the same operating conditions of experience. The results of simulation illustrate the temporal variations of the discharge current density and applied voltage; the power deposited in the plasma by electrons and the species concentrations considered in the kinetic model. The total and partial lamp efficiencies are also investigated and discussed.

Visualization of the gas flows that formed above the thin-film coatings under VUV radiation influence

The plasma focus of a magnetoplasma compressor is a unique source of high-brightness broadband VUV (with photon energy of 5 to 100 electron volt) radiation. When such radiation affects on the surfaces of materials, it is possible to generate rather complex gas-dynamic structures due to the evaporation of the material and ionization of its vapors. A separate task is to study the processes of a gas-dynamic response to the exposure of the specified radiation fluxes on the surface of interference antireflection and reflective multilayers, which are used in modern laser technology, high-power optoelectronics, etc. In this report, we used schlieren photography for studying the features of gas-dynamic structures that arise at irradiation of coatings. Radiation resistant (in the visible and near IR ranges) HfO2/SiO2 and ZrO2/SiO2 multilayer structures (with a total thickness of 140–3700 nanometers and a number of layers of from 2 to 24) were used as prototypes for testing their stability under the VUV exposure.

Lead evaporation by VUV radiation of various spectral ranges

The results of an experimental study of the plasma formed by the evaporation of the lead target in the field of powerful broadband VUV radiation are presented. A pulse light-erosion magnetoplasma compressor (MPC’s) discharge is used as a model source of VUV radiation. “Gas filtration” of radiation is used to control the spectral composition – the discharge takes place in pure inert gases: in argon at 200 torr and neon at 400 torr. This allows us to manage the spectral distribution of radiation energy and to limit the energy of quants which irradiate the lead target with the first ionization potential of buffer gas. Shadow photography, toeplergrathy, double exposure laser holographic interferometry are used for diagnostics. Experimentally established different distribution of parameters in the lead plasma depending on the spectral composition of the impact radiation (the composition of buffer gas). It is shown that when the energy of quants increases (above the lead second ionization potential), a more even heating of the plasma layer is realized.

Applications of the gas discharge sustained by the powerful radiation of THz gyrotrons

The terahertz frequency range currently remains the least studied from the point of view of gas-discharge physics. Recent progress in its mastering is associated, first of all, with the creation of powerful sources of terahertz radiation - gyrotrons. Nevertheless, the discharge sustained by the powerful radiation of the terahertz frequency range is of interest not only from the fundamental point of view (because of its lack of knowledge), but also from the applied one. This paper presents results of investigations aimed at studying the point-like THz discharge as a source of vacuum ultraviolet radiation and at research of CW THz plasma torch existing at atmospheric pressure and therefore having promising applications in various fields: plasma medicine, plasma chemistry, material processing etc. It is shown that it is possible to create a point-like plasma in an inhomogeneous gas flow with the density above the cut-off one (4·1015 cm−3 for 0.67 THz). It has been demonstrated that such plasma is an effective source of VUV and EUV radiation. Investigations of the atmospheric plasma torch, sustained by CW sub-terahertz radiation (1 kW @ 0.26 THz), demonstrated the possibility of the creation dense non-equilibrium plasma (2·1015 cm−3) with electron temperature in the range from 1 to 2 eV.

On the temperature and time dependent photoluminescence of Lu3Al5O12:Gd3+

Lu3Al5O12 (LuAG) is a widely applied host for scintillators, laser gain media, and luminescent materials. This work concerns the photoluminescence (PL) of Gd3+ doped LuAG upon VUV excitation and its dependence on temperature. It also relates to the time dependent PL and thus the determination of the decay constants. It turned out, that LuAG:Gd3+ is a very efficient UV-B emitting phosphor and that the emission spectrum is a sensitive function of temperature. Consequently, this material can be applied as UV-B emitter in VUV radiation sources and as an optical temperature sensor for devices or processes, where VUV radiation is applied as the excitation source.

VUV Spectral Irradiance Measurements in H2/He/Ar Microwave Plasmas and Comparison with Solar Data

Abstract Microwave plasmas with H2 and H2/rare gas mixtures are convenient sources of VUV radiation for laboratory simulations of astrophysical media. We recently undertook an extensive study to characterize microwave plasmas in an H2/He gas mixture in order to optimize a VUV solar simulator over the 115–170 nm spectral range. In this paper, we extend our investigation to the effect of the addition of Ar into H2/He plasma on the VUV spectral irradiance. Our study combines various optical diagnostics such as a VUV spectrometer and optical emission spectroscopy. Quantitative measurements of the spectral irradiance and photons flux in different mixtures are accomplished using a combination of VUV spectrometry and chemical actinometry. Results show that the Ar addition into H2/He plasma largely affects the predominant emissions of the hydrogen Lyα line (121.6 nm) and H2 (B1Σu–X1Σg) band (150–170 nm). While a microwave plasma with 1.4% H2/He is required to mimic the entire VUV solar spectrum in the 115–170 nm range, the combination with 1.28% H2/35% Ar/He is the best alternative to obtain a quasi-monochromatic spectrum with emission dominated by the Lyα line. The maximum of the spectral irradiance is significantly higher in the ternary mixtures compared to the binary mixture of 1.4% H2/He. Further Ar increase yielded lower spectral irradiance and absolute photon fluxes. Our measured spectral irradiances are compared to VUV solar data in the 115–170 nm range, emphasizing the use of microwave plasmas in astrophysical studies and laboratory simulations of planetary atmospheres.

Efficient UV and VUV Radiation Sources – Excilamps and Photoreactors on Their Basis

Results of investigations of UV and VUV radiation sources (excilamps) based on inert gases and mixtures of inert gases with halides are presented. Conditions are described in which maximum radiation parameters are attained. Photoreactors and irradiators promising for application in medicine, agriculture, and gas industry have been developed based on the excilamps.

STUDY OF ATOMIC SYSTEMS IN STRONG LASER FIELDS: SPECTRAL HIERARCHY, DYNAMICAL STABILISATION AND GENERATION OF ULTRA-SHORT VUV AND X-RAY PULSES

A new method for sensing a spectral hierarchy and dynamical stabilization in atomic systems in the intense laser field is presented. It is carried out modeling generation of the atto-second VUV and X-ray pulses under ionization of atomic systems by femto-second optical pulse. The theory of studied phenomena is the physical basis for construction of the new nano-atomic elements and devices, including sensors, sources of VUV and X-ray radiation, quantum Carnot engine, single-atomic lasers, quantum computers elements etc.

Creation of inverse population in the 229Th ground-state doublet by means of a narrowband laser

A new method for obtaining inverse population between quadrupole sublevels of the ground state 5/2+(0.0) and the isomeric state 3/2+(7.6 eV) of the 229Th nucleus in a dielectric crystal with a large band gap by means of an external source of VUV radiation is proposed. The method is based on an efficient depopulation of the upper sublevels of the ground state of 229Th by resonant photons from a narrowband laser or a broader tunable free electron laser. The sublevels of the isomeric state play the role of intermediate states. The proposed scheme of optical pumping (1) results in inverse population of nuclear sublevels without using extremely low temperatures, and (2) shows a new means of creating a gamma-ray laser of optical range at the nuclear transition in 229Th (a) in crystals with isovalent substitution of host ions (e.g. Si4+ ions in the SiO2 crystal replaced by the 229Th4+ ions) and (b) in crystals such as Na2ThF6, where the substitution is not necessary.

Barrier-discharge excilamps: history, operating principle, prospects∗∗To the radiant memory of Galina Arkad’evna Volkova (1935–2011)

This paper describes the operating principle of modern sources of spontaneous UV and VUV radiation based on excimer and exciplex molecules—excilamps. Citations to pioneering papers are given, in which a barrier discharge was used to excite the radiation of excilamps. The design of barrier-discharge excilamps and irradiating modules based on them are considered, along with efficient operating regimes and the emission spectra of the most promising excilamps. Examples are given of the use of barrier-discharge excilamps in science and engineering.

Development and production of UV and VUV sources

This paper discusses gas-discharge sources of ultraviolet and vacuum ultraviolet radiations developed at the S. I. Vavilov State Optical Institute. The main characteristics of lamps with arc, glow, rf, and barrier discharges are presented, along with areas in which they can be used. It is indicated that krypton, xenon, argon, hydrogen, deuterium, neon, helium, and combinations of these gases are used as a fill gas. It is pointed out that a recent breakthrough in VUV engineering was to use magnesium fluoride and lithium fluoride windows. After the technique for attaching windows made from these materials to glass bulbs was mastered, it became possible to create a new class of VUV sources. It is emphasized that the structure of the laboratory of gas-discharge light sources assumes that the development of new types of UV and VUV lamps is to be combined with their production. This makes it possible to carry out small-scale production and to develop and fabricate sources (lamps) as single samples to meet individual technical requirements, and this is important when developing new devices that use UV and VUV radiation sources.

Atmospheric pressure photoionization mass spectrometry of guanine using tunable synchrotron VUV radiation

Atmospheric pressure photoionization (APPI) source coupled to a krypton discharge lamp permits to specifically generate either radical cation or protonated species for guanine depending on the dopant molecule. In order to scrutinize the consecutive or competitive chemical reactions occurring in the gas phase, a tunable synchrotron VUV radiation source was coupled for the first time to the APPI source of a hybrid quadrupole time-of-flight mass spectrometer. Experiments were achieved at different photon energies close to the ionization energy of the different reagents. This unique set-up allowed the confirmation of theoretical models developed by several groups over the last ten years concerning ionization processes underlying APPI, such as proton or charge transfer reactions.

Combining two VUV lasers with the time-sliced velocity mapped ion imaging method for studying the photodissociation of small molecules

A new experimental setup is described that allows us to study the state-selected photodissociation dynamics of small molecules of astrophysical interest using two independently tunable VUV radiation sources.

UV and VUV Excilamps with High Peak Power

Design and output parameters of pulse sources of spontaneous UV and VUV radiation based on the non-equilibrium discharge plasma of high-pressures gases are presented in this paper. Emission characteristics of plasma of diffuse discharge preionized by runaway electrons and barrier discharge in rare gases and mixtures of rare gases with halogens have been investigated in gases at pressure up to 15 atm at excitation power up to 100 MW/cm3. The radiant power from 4π solid angle up to 1.2 MW with pulse duration of - 8 ns on Xe2* dimers were obtained at 12 atm pressure xenon. A high-voltage (up to 150 kV) short pulsed excitation mode was tested on dielectric barrier discharge KrCl- and XeBr-excilamps.

Laser desorption postionization for imaging MS of biological material.

Vacuum ultraviolet single photon ionization (VUV SPI) is a soft ionization technique that has the potential to address many of the limitations of matrix-assisted laser desorption/ionization (MALDI) for imaging MS. Laser desorption postionization (LDPI) uses VUV SPI for postionization and is experimentally analogous to a MALDI instrument with the addition of a pulsed VUV light source. This review discusses progress in LDPI-MS over the last decade, with an emphasis on imaging MS of bacterial biofilms, analytes whose high salt environment make them particularly resistant to imaging by MALDI-MS. This review first considers fundamental aspects of VUV SPI including ionization mechanisms, cross sections, quantum yields of ionization, dissociation and potential mass limits. The most common sources of pulsed VUV radiation are then described along with a newly constructed LDPI-MS instrument with imaging capabilities. Next, the detection and imaging of small molecules within intact biofilms is demonstrated by LDPI-MS using 7.87 eV (157.6 nm) VUV photons from a molecular fluorine excimer laser, followed by the use of aromatic tags for detection of selected species within the biofilm. The final section considers the future prospects for imaging intact biological samples by LDPI-MS.

VUV-Stimulated MO CVD Processes of Copper Layer Deposition

VUV-stimulated processes of copper layer deposition were investigated. A conventional compound, copper bis-hexafluoroacetylacetonate, was used as a precursor. Excimer lamps emitting light within the wavelength range 116-172 nm were used as the sources of VUV radiation. The results of quantum-chemical calculations were compared with the electronic spectra of copper bis-hexafluoroacetylacetonate in the gas phase. Calculations were carried out using the quantum chemical method of the density functional theory with Slater type of atomic wave functions (DFT/TZP). On the basis of thus determined electronic structure, allowed dipole-dipole electron transitions within the wavelength region 100-1200 nm were calculated; they were compared with the experimental spectra. Investigation of copper deposition processes was carried out under variation of substrate temperature and precursor, process time, presence of reactant gas, spectral characteristics of the excimer lamps. The resulting layers were characterized by means of SEM, ESCA, ellipsometry.

Electron energy distribution function and electronic rates of plasma in mixture Ar-Kr-Xe-Cl<sub>2</sub>

Numerical modelling of Boltzmann kinetic equation forthe electron energy distribution function was performed, being subsequently used for obtaining the transport characteristics of the plasma electrons and specific loss of the discharge power at various values of reduced electric field strength. The plasma parameter modelling was performed for a four-component mixture, being optimal for the multiwave VUV and UV radiation source operation.

High-power short-pulse xenon dimer spontaneous radiation source

A high-power VUV radiation source based on a self-sustained nanosecond volume discharge in an inhomogeneous electric field is developed. It is shown that the volume discharge can be formed at high xenon and helium pressures without using a preionisation source. The 8-ns (FWHM), 172-nm, 1-MW radiation pulses emitted into a total solid angle are obtained in xenon at a pressure of 12 atm.

Vacuum-ultraviolet photochemically initiated modification of polystyrene surfaces: morphological changes and mechanistic investigations

Vacuum-ultraviolet (VUV) irradiation (lambda(exc): 172 +/- 12 nm) of polystyrene films in the presence of oxygen produced not only oxidatively functionalized surfaces, but generated also morphological changes. Whereas OH- and C=O-functionalized surfaces might be used for e.g. secondary functionalization, enhanced aggregation or printing, processes leading to morphological changes open new possibilities of microstructurization. Series of experiments made under different experimental conditions brought evidence of two different reaction pathways: introduction of OH- and C=O-groups at the polystyrene pathways is mainly due to the reaction of reactive oxygen species (hydroxyl radicals, atomic oxygen, ozone) produced in the gas phase between the VUV-radiation source and the substrate. However, oxidative fragmentation leading to morphological changes, oxidation products of low molecular weight and eventually to mineralization of the organic substrate is initiated by electronic excitation of the polymer leading to C-C-bond homolysis and to a complex oxidation manifold after trapping of the C-centred radicals by molecular oxygen. The pathways of oxidative functionalization or fragmentation could be differentiated by FTIR-ATR analysis of irradiated polystyrene surfaces before and after washing with acetonitrile and microscopic fluorescence analysis of the surfaces secondarily functionalized with the N,N,N-tridodecyl-triaza-triangulenium (TATA) cation. Ozonization of the polystyrene leads to oxidative functionalization of the polymer surface but cannot initiate the fragmentation of the polymer backbone. Oxidative fragmentation is initiated by electronic excitation of the polymer (contact-mode AFM analysis), and evidence of the generation of intermediate C-centred radicals is given e.g. by experiments in the absence of oxygen leading to cross-linking (solubility effects, optical microscopy, friction-mode AFM) and disproportionation (fluorescence).

The SPARC/X SASE-FEL Projects

SPARC and SPARX are two different initiatives toward an ItalianFreeElectronLaser (FEL) source operating in theSelfAmplifiedSpontaneousEmission (SASE) mode, in which several national research institutions are involved. SPARC is a high gain FEL project devoted to provide a source of visible and VUV radiation while exploiting the SASE mechanism. An advanced Photo-Injector system, emittance compensating RF-gun plus a 150 MeV Linac, will inject a high quality e-beam into the undulator to generate high brilliance FEL radiation in the visible region at the fundamental wavelength, (∼500 nm). The production of flat top drive laser beams, high peak current bunches, and emittance compensation scheme will be investigated together with the generation of higher harmonic radiation in the VUV region. SPARX is the direct evolution of such a high gain SASE FEL toward the 13.5 and 1.5 nm operating wavelengths, at 2.5 GeV. To get the required value for the bunch peak current, Ipeak≈ 2.5 kA, the “hybrid” scheme, RF-compression stage plus magnetic chicane, is analyzed and compared with the more standard double stage of magnetic compression. The two options are reviewed considering the tolerance to the drive laser pulse phase jitter.