VUV Spectral Range Research Articles

LUMINESCENCE CHARACTERISTICS OF CASO4:TB,M (M=NA,K,RB) PHOSPHORS

In the present study, the investigation of complex luminescence terbium centres as well as the analysis of technological features of the synthesis of small-grained wide-gap CaSO :Tb3+ anhydrite phosphors containing 1% of Tb3+ and some amount of additional impurity ions (Na+, K+, Rb+) have been continued using the methods of optical and thermoactivation spectroscopy. Of particular interest was a contribution of calcium ions into formation of complex luminescence centres. Ca causes efficient dynamic hybridization of electronic states and influence on the formation of terbium and near-terbium electronic excitations. The excitation spectra of Tb3+-centre emission have been measured in the VUV spectral range at 77 or 300 K. The excitation band related to the lowest f–d transition of Tb3+ ions at ≈5.9 eV in CaSO4 undergoes broadening toward the long- wavelength side, when the radius of a charge compensator increases (K+→ Rb+). The band at 7.5–9 eV ascribed to the excitation of oxyanions has a different shape in CaSO4:Tb,K and CaSO4:Tb,Rb. Thermally stimulated luminescence (80–700 K) have been analyzed for a set of anhydrite phosphors previously irradiated by X-rays at 80 and 300 K. Keywords: CaSO , Tb3+, photoluminescence, thermoluminescence, vacuum ultraviolet spectroscopy.

Band Gap Engineering and Trap Depths of Intrinsic Point Defects in RAlO3 (R = Y, La, Gd, Yb, Lu) Perovskites

The possibility of band gap engineering (BGE) in RAlO3 (R = Y, La, Gd, Yb, Lu) perovskites in the context of trap depths of intrinsic point defects was investigated comprehensively using experimental and theoretical approaches. The optical band gap of the materials, Eg, was determined via both the absorption measurements in the VUV spectral range and the spectra of recombination luminescence excitation by synchrotron radiation. The experimentally observed effect of Eg reduction from ∼8.5 to ∼5.5 eV in RAlO3 perovskites with increasing R3+ ionic radius was confirmed by the DFT electronic structure calculations performed for RMIIIO3 crystals (R = Lu, Y, La; MIII = Al, Ga, In). The possibility of BGE was also proved by the analysis of thermally stimulated luminescence (TSL) measured above room temperature for the far-red emitting (Y/Gd/La)AlO3:Mn4+ phosphors, which confirmed decreasing of the trap depths in the cation sequence Y → Gd → La. Calculations of the trap depths performed within the super cell approach for a number of intrinsic point defects and their complexes allowed recognizing specific trapping centers that can be responsible for the observed TSL. In particular, the electron traps of 1.33 and 1.43 eV (in YAlO3) were considered to be formed by the energy level of oxygen vacancy (VO) with different arrangement of neighboring YAl and VY, while shallower electron traps of 0.9–1.0 eV were related to the energy level of YAl antisite complexes with neighboring VO or (VO + VY). The effect of the lowering of electron trap depths in RAlO3 was demonstrated for the VO-related level of the (YAl + VO + VY) complex defect for the particular case of La substituting Y.

Luminescence and vacuum ultraviolet excitation spectroscopy of cerium doped Gd3Ga3Al2O12 single crystalline scintillators under synchrotron radiation excitations

Cerium doped Gd3Ga3Al2O12 (GGAG) single crystals as well as GGAG:Ce single crystals co-doped by divalent (Mg2+, Ca2+), trivalent (Sc3+) or tetravalent (Zr4+, Ti4+) ions have been studied by means of the excitation luminescence spectroscopy in vacuum ultraviolet spectral range. Synchrotron radiation from the undulator beam was utilized for the luminescence excitation in the energy range from 4.5 to 800 eV. The influence of the co-dopant ions on the excitonic transitions as well as on the intrinsic defects in GGAG was revealed examining the luminescence emission and excitation spectra of both Gd3+ and Ce3+ ions in all single crystals studied. Special attention was paid to the analysis of Ce3+ excitation spectra in VUV spectral range (4.5–45 eV) where multiplication of electronic excitation (MEE) processes occur. It was obtained that GGAG:Ce single crystals having different co-dopant ions reveal distinguished efficiency of MEE. The role of intrinsic defects in MEE processes in the co-doped GGAG:Ce single crystals was elucidated.

Validation of thin film TiO2 optical constants by reflectometry and ellipsometry in the VUV spectral range

Optical constants of thin films in the vacuum ultraviolet spectral range are often not precisely known, since not only the experimental determination is difficult, but also the results depend on the sample preparation. We compared measurements in the wavelength interval between 130 nm and 226 nm using reflectometry and ellipsometry with particular regard to their respective uncertainty budgets. Since for both methods a numerical analysis is required to obtain the optical constants from the measured data, particular attention was paid to the error propagation through the calculation routines, and the uncertainties introduced by the numerical methods and models themselves. The measurement methods were applied to a thin layer of 6 nm titania (TiO2) on a silicon substrate. The result is used to validate our method, and to identify further analytical requirements.

Optical characteristics of UV–VUV lamps on the electronic‐vibrational transitions of the hydroxyl radical pumped by a nanosecond capacitive discharge

This paper presents the results of a study of UV and VUV low-pressure lamp pumped by nanosecond capacitive discharge. This lamp works on the mixtures of helium and argon with water vapor (H2O) or ‘heavy’ water (D2O). At the capacitive low-pressure lamp emission spectra are mainly narrow band 308 nm OH (A–X) and large band in the VUV spectral range: 140–200 nm OH (C, B, A). Found, that the increasing the partial pressure of H2O and D2O (>130–150 Pa) led to a decrease in the intensity of VUV radiation of hydroxyl radicals. In the transition from He/H2O mixture to He/D2O mixture, the optimal pressure of helium is decreased. Reduced partial pressure of argon led to increased intensity of radiation of hydroxyl in the VUV range. The study of emission characteristics of OD* – lamps on a mixture of He/D2O and the OH* – lamps on a mixture of He/H2O showed that the intensity of the bands OD (X, C, A) is 1.5 times larger than the intensity of the bands OH (X, C, A). The maximum absolute intensity of UV–VUV radiation of the lamp (at a frequency of 1000 Hz) reached 1 W.

The high-resolution absorption spectroscopy branch on the VUV beamline DESIRS at SOLEIL.

A VUV absorption spectroscopy facility designed for ultra-high spectral resolution is in operation as a dedicated branch on the DESIRS beamline at Synchrotron SOLEIL. This branch includes a unique VUV Fourier transform spectrometer (FTS) and a dedicated versatile gas sample chamber. The FTS instrument can cover a large UV-VUV spectral range from 4 to 30 eV, with an ultimate line width of 0.08 cm(-1) on a large spectral window, ΔE/E = 7%, over which all spectral features can be acquired in a multiplex way. The performance can be considered to be a middle ground between broadband moderate-resolution spectrometers based on gratings and ultra-high-spectral-resolution VUV tunable-laser-based techniques over very narrow spectral windows. The various available gaseous-sample-handling setups, which function over a wide range of pressures and temperatures, and the acquisition methodology are described. A selection of experimental results illustrates the performance and limitations of the FTS-based facility.

VUV spectroscopy of complex fluoride systems Na0.4(Y1−xREx)0.6F2.2 (RE3+ = Nd3+, Tm3+)

Emission and excitation spectra as well as luminescence decay kinetics of complex non-stoichiometric fluoride crystals Na0.4(Y1−xNdx)0.6F2.2 (x=0.005, 0.05, 0.2, 1) and Na0.4(Y1−xTmx)0.6F2.2 (x=0.0005, 0.01, 0.05, 0.1) have been studied in the VUV spectral range at liquid-helium (T∼10K) temperatures. It has been shown that these crystals show intense broad-band VUV luminescence due to the interconfiguration 5d-4f transitions in Nd3+ and Tm3+ ions. Remarkable concentration quenching is observed for Nd3+ 5d-4f luminescence whereas fast (spin-allowed) 5d-4f luminescence of Tm3+ shows no concentration quenching for the studied doping level up to 10%. The spin-allowed 5d-4f luminescence of Tm3+ in these crystals was found to be rather weak compared to spin-forbidden 5d-4f luminescence because of efficient nonradiative relaxation from higher-energy 5d states of Tm3+ to the lowest-energy 5d level responsible for spin-forbidden 5d-4f luminescence. The studied fluoride systems can be considered as promising active media for the development of VUV solid state lasers with optical pumping.

On amplitude beam splitting of tender X-rays (2-8 keV photon energy) using conical diffraction from reflection gratings with laminar profile.

Conical diffraction is obtained when a radiation beam impinges onto a periodically ruled surface structure parallel or almost parallel to the ruling. In this condition the incident intensity is diffracted through an arc, away from the plane of incidence. The diffracted intensity thus lies on a cone, which leads to the name `conical diffraction'. In this configuration almost no part of the ruled structure will produce any shadowing effect for the incident or the diffracted beam. Then, compared with a grating in the classical orientation, relatively higher diffraction efficiencies will be observed for fewer diffraction orders. When the incident beam is perfectly parallel to the grooves of a rectangular grating profile, the symmetry of the setup causes diffraction of the intensity symmetrically around the plane of incidence. This situation was previously tested experimentally in the VUV spectral range for the amplitude beam splitting of a radiation beam with a photon energy of 25 eV. In this case the ideally expected beam splitting efficiency of about 80% for the diffraction into the two first orders was confirmed for the optimum combination of groove depth and angle of grazing incidence. The feasibility of the amplitude beam splitting for hard X-rays with 12 keV photon energy by use of the same concept was theoretically confirmed. However, no related experimental data are presented yet, not even for lower energy soft X-rays. The present study reports the first experimental data for the conical diffraction from a rectangular grating profile in the tender X-ray range for photon energies of 4 keV and 6 keV. The expected symmetries are observed. The maximum absolute efficiency for beam splitting was measured to be only about 30%. As the reflectivity of the grating coating at the corresponding angle of grazing incidence was found to be only of the order of 50%, the relative beam splitting efficiency was thus 60%. This is to be compared also here with an ideally expected relative efficiency of 80%. It is predicted that a beam splitting efficiency exceeding 50% should be possible by use of more appropriate materials.

Dispersed fluorescence spectrometry from the VIS to VUV spectral range for experiments at heavy-ion storage facilities

For the electronic- and charge-state specific determination of VUV–VIS fluorescence emission cross sections after collisions between heavy ions and neutral gases or electrons a fluorescence spectrometer for the VUV–VIS spectral range is planned. Tentative experiments showed that signal rates after collisions between Xe atoms and ions are high enough to allow efficient experiments.

Efficient generation of below-threshold harmonics for high-fidelity multi-photon physics in the VUV spectral range.

We demonstrate the generation of microjoule level, sub-20-fs, Ti:Sa fifth-harmonic pulses utilizing a loose-focusing geometry in a long Ar gas cell. The VUV pulses centered at 161.8 nm reach pulse energies of 1.1 μJ per pulse, while the corresponding pulse duration is measured with a second-order, fringe-resolved autocorrelation scheme to be 18±1 fs. Nonresonant, two-photon ionization of Kr and three-photon ionization of Ne verify the fifth-harmonic pulse high-intensity content and indicate the feasibility of multi-photon VUV pump-VUV probe studies of ultrafast atomic and molecular dynamics.

Photon–photon coincidence apparatus with position sensitive detectors

An apparatus for the coincident detection of two photons in the visible and VUV spectral range is presented equipped with two position- and time resolving detectors. The equipment enables angular resolution for the detected photons and thus allows an angular correlation between the two detected photons without changing the target cell geometry. Two different configurations of this apparatus are presented and compared in terms of solid angle coverage, imaging properties and suitability for their use in gas phase experiments at synchrotron radiation facilities.

Comparative study of the luminescence properties of macro- and nanocrystalline MgO using synchrotron radiation

MgO nano-powder with an average crystallite size of nanoparticles ranging 10–15nm was synthesized using the extractive-pyrolytic method and was studied by room temperature VUV spectroscopy under synchrotron radiation excitation. Comparative analysis of their luminescent properties with that of macrocrystalline powder analogues and an MgO single crystal, grown by the arc-fusion method, has been performed under excitation by pulsed VUV synchrotron radiation. Special attention was paid to VUV spectral range, which is not reachable with commonly used lamp and laser sources. A considerable blue shift of about ∼0.3eV in the excitation spectra of 2.95eV emission band, was revealed in nanocrystalline MgO samples.

Specific features of Yb3+ ions in electronic band energy structure and optical functions of RbNd(WO4)2 crystals: Synchrotron ellipsometry measurements and DFT simulations

Abstract The influence of 5% and 10% (in molar units) of ytterbium doping of RbNd(WO 4 ) 2 single crystals on the dielectric permittivity e in the spectral range of electronic excitations was studied. The corresponding differences of the pseudo-dielectric functions 〈 e ( E )〉 for pure and Yb-doped RbNd(WO 4 ) 2 were studied by spectroscopic ellipsometry method including VUV spectral range using synchrotron radiation light source. This method opens an opportunity for spectroscopic diagnostics of the rare earth dopants in crystals using information concerning the electronic inter-band optical transitions. To better understand the experimental results, for the first time the ab initio calculations of band structure, related properties and optical spectra of the centrosymmetric RbNd(WO 4 ) 2 single crystal were performed using the VASP code. The calculated real and imaginary parts of dielectric function dispersions e 1 ( E ) and e 2 ( E ) agree satisfactorily with our experimental results. It was established that differences of optical functions for pure and doped crystals may serve as a powerful tool for spectroscopic diagnostic of localized rare earth ions with respect to the strong inter-band transitions in the wide band dielectrics.

Spectroscopic evaluation of electron density and temperature of a laser plasma source in the vacuum ultraviolet

A time-integrated space-resolved emission spectroscopic technique was developed in the VUV spectral range for the measurement of the electron number density and excitation temperature of laser-produced steel plasmas. 400 mJ pulses of Nd:YAG laser (7 ns duration) emitted at the fundamental wavelength of 1.06 μm are used to create the steel plasmas under moderate vacuum conditions (∼ 0.2 mbar). The electron number density was estimated from measurements of the FWHM of the Stark-broadened C II 68.73nm carbon spectral line. The excitation temperature was evaluated by the Boltzmann’s plot method for five selected Fe III iron spectral lines in the wavelength range 89-93nm.

Atomic oxygen in a cold argon plasma jet: TALIF spectroscopy in ambient air with modelling and measurements of ambient species diffusion

By investigating the atomic oxygen density in its effluent, two-photon absorption laser-induced fluorescence (TALIF) spectroscopy measurements are for the first time performed in a cold argon/oxygen atmospheric pressure plasma jet. The measurements are carried out in ambient air and quenching by inflowing air species is considered. We propose a novel absorption technique in the VUV spectral range, where emission originating from within the discharge is used as light source to determine the inflow of atmospheric oxygen into the effluent. Furthermore, we propose a modelling solution for the on-axis density of inflowing ambient air based on the stationary convection–diffusion equation.

Progress Towards an FEL Oscillator Operating in the VUV to Soft X-ray Spectral Range

From the early days of free-electron lasers (FELs), the idea of lasing at wavelengths where gain media had not previously existed was one of the most attractive aspects of these devices. The spectral region in the soft X-ray was both inviting, due to the exciting physics one could do with such a source, and challenging, due to the need for a very bright electron beam (high charge density), very high gain to overcome cavity losses, and the cost of providing all this at very high electron beam energies with high-duty-cycle electron beams. Electron beam brightness is defined as the ratio of the peak current to the product of the horizontal and vertical normalized emittances. The normalized electron beam emittance is the product of the rms transverse size and momentum spread. High gain is usually obtained using a very long undulator, which requires a very small energy spread in addition to the small transverse emittance. Electron beams in the 1970s generally provided either low peak current or large energy spread and very low charge density. Even the VUV spectral range was considered too difficult for the accelerators and optics available then. Early FELs therefore operated in the infrared range. FEL oscillators were eventually pushed into the visible and ultraviolet but could not push into the VUV due to either low gain or poor mirror reflectivity.

Coherent Light with Tunable Polarization from Single-Pass Free-Electron Lasers

Tunable polarization over a wide spectral range is a required feature of light sources employed to investigate the properties of local symmetry in matter. In this Letter, we provide the first experimental characterization of the polarization of the harmonic light produced by a free-electron laser and demonstrate a method to obtain free-electron laser harmonics with tunable polarization. Experimental results are successfully compared with theory. Our findings can be expected to have a deep impact on the design and realization of experiments requiring full control of light polarization.

Photon cascade luminescence from Pr3+ ions in LiPrP4O12 polyphosphate

The spectral-kinetic properties of luminescence from LiY1−xPrxP4O12 (x = 1, 0.1) phosphors are studied in the 10–300 K temperature range upon excitation within the UV–VUV spectral range. The luminescence observed for LiPrP4O12 at low temperatures is attributed to both intra-configurational 4f2 → 4f2 and inter-configurational 4f5d → 4f2 radiative transitions of Pr3+. The temperature dependence of emission and the decay kinetics of Pr3+ luminescence from LiY1−xPrxP4O12 (x = 1, 0.1) phosphors are explained by temperature stimulated transitions from the 1S0 to the Pr3+ 4f5d state. At low temperatures, the energy potential barrier between the 1S0 and 4f5d states is 0.014 eV for LiPrP4O12. The spectral overlap of the 1S0 and 4f5d states of Pr3+ in LiY0.9Pr0.1P4O12 determines the luminescence properties in the entire temperature range studied.

Kinetic model for low-density non-stationary gas discharge in water vapour

A capacitive discharge in water vapour in the pressure range 133–150 Pa was studied. The discharge is an effective ecologically pure source of OH* hydroxyl emission in the VUV and UV spectral range (A 2Σ+ → X 2Π transition). The time profiles of densities of plasma species at different values of reduced electric field (E/N) were calculated using a kinetic model of the discharge. It is shown that E/N ∼ 190–200 Td is a breaking field for water vapour plasma, and with an increase in E/N a transition from electronegative to electropositive plasma takes place. The primary positive ions in the water vapour plasma are H3O+ ions, and the primary negative ions are OH− ions.

Detection of Yb impurities in the VUV spectral range of NdGaO 3 crystals

Substantial influence of the 1 at.% Yb doping of NdGaO 3 single crystals on the optical functions ε 1( E) and ε 2( E) in the spectral range of electronic excitations is established. The corresponding differences of the optical functions for pure and doped NdGaO 3 have been monitored using spectroscopic ellipsometry method and synchrotron radiation light source. This opens an opportunity for the spectroscopic diagnostics of the rare earth dopants in crystals using the spectroscopic information concerning the electronic inter-band optical transitions. To understand better the experimental results obtained, the ab-initio calculations of band structure and optical spectra of the centrosymmetric single crystal NdGaO 3 have been performed using the VASP code (Vienna Ab-initio Simulation Package). The calculated dielectric functions ε 1( E) and ε 2( E) agree satisfactorily with our experimental results obtained with using the spectroscopic ellipsometry method and synchrotron radiation. It was demonstrated that differences of the optical functions for pure and doped crystals using stable synchrotron source may serve as a powerful tool for spectroscopic diagnostic of localised rare earth ions with respect to the strong inter-band transitions.