UV VUV Research Articles

The role of torsional modes in the electronic absorption spectrum of acetone

The electronic absorption spectrum of acetone is revisited to evaluate the role of hot bands due to low lying torsional modes in the assignment of vibronic transitions. The UV–VUV photoabsorption spectrum of acetone is recorded in the energy region 3.5–11.8eV at a resolution of ∼4meV at 4eV and ∼10meV at 10eV using synchrotron radiation. The absorption spectrum is dominated by richly structured Rydberg series (ns, np and nd) converging to the first ionization potential of acetone at 9.708eV. Careful consideration of hot band contributions from torsional modes and symmetry selection rules have resulted in an improved set of vibronic assignments as compared to earlier room temperature work. Revised quantum defect values for some of the Rydberg transitions and a few new assignments in the nd series are also reported in this paper.

Photo- and radioluminescence of lithium hafnate Li2HfO3

Photoluminescence, excitation and pulse cathode luminescence spectra, and decay kinetics of lithium hafnate Li2HfO3 were investigated at temperatures 300 and 7.5K. Time-resolved luminescence spectra were excited with synchrotron radiation in UV–VUV range. Strong emission with the peak position at 4.42eV and with the decay time in μs range was revealed at T=7.5K. The emission was attributed to radiative decay of self-trapped excitons (STE). The features observed in the excitation spectra at 6.6eV were likely to take place because of the exciton formation, with inter-band transitions dominating at 6.7eV. Crystal structure defects underpin the luminescence spectra formation at room temperature in the frame of vacancy model.

Sonophotolytic diethyl phthalate (DEP) degradation with UVC or VUV irradiation

This study investigated the degradation of diethyl phthalate (DEP) by sonolytic, photolytic and sonophotolytic processes. Two types of UV lamps, UVC (254nm) and VUV (185nm+254nm), were combined with ultrasound (283kHz). The pseudo-first order degradation rate constants were in the order of 10−1–10−3min−1 depending on the processes. The sonolytic DEP degradation rate increased with increasing applied power. Photolytic or sonophotolytic degradation of DEP when using a VUV lamp appeared to be effective because the photo ІІ (UVC/VUV) resulted in a significantly faster degradation than the photo І (UVC) processes due to the higher photon energy and higher hydroxyl radical generation by homolysis of water by VUV. Significant degradation and mineralization (TOC) of DEP were observed with the combined sonophotolytic processes. Moreover, synergistic effects of 1.68 and 1.23 were exhibited at DEP degradation of the sonophoto I and sonophoto II processes, respectively. This was attributed to the UV-induced dissociation of hydrogen peroxide (H2O2) generated by the application of US to hydroxyl radicals. Therefore, US in sonophotolytic processes can play an important role in enhancing DEP degradation. Moreover, the sonophoto ІІ process is more effective on the mineralization and biodegradability of DEP.

Luminescence of Tb-doped Ca 3Y 2(Si 3O 9) 2 oxide upon UV and VUV synchrotron radiation excitation

Powders of calcium yttrium silicate, Ca 3Y 2(Si 3O 9) 2, containing 0.1–3% Tb 3+ were prepared using a sol-gel method and characterized with XRD, IR, UV–vis and UV–VUV spectroscopies at room temperature and 10 K. Structural analysis revealed pure monoclinic phase of Ca 3Y 2(Si 3O 9) 2 after heat-treatment at 1000 °C. Infrared spectroscopy showed that between 800 and 900 °C a short-range structural organization of the components proceeded, yet without crystallization. A strong emission of Tb 3+ had been observed both in the green part of the spectrum due to the 5 D 4→ 7 F J transitions and in the blue-violet region owing to the 5 D 3→ 7 F J radiative relaxation. The color of the light could be tuned from yellowish-green to bluish-white both by means of the dopant content and the temperature of synthesis. Efficient luminescence of Tb 3+-doped Ca 3Y 2(Si 3O 9) 2 phosphors could also be obtained upon stimulation with vacuum ultraviolet synchrotron radiation demonstrating that an energy transfer from the host to the Tb 3+ ions takes place.

Crystal structure and luminescence properties ofLiYP4O12:Ce3 + phosphor

LiYP4O12 polyphosphatedoped with Ce3 + ions was prepared by the melt solution technique. The crystal structure,interatomic distances, and atom coordination numbers were determined usingx-ray powder diffraction. A study of the spectral–kinetic luminescent propertieswas performed employing excitation with pulsed radiation from a synchrotron(UV–VUV range) and a laboratory x-ray source. The characteristics ofCe3 + luminescence, namely the emission doublet maxima at 3.97 and 3.72 eVand the 4f–5d excitation maxima at 4.20, 5.11, 5.40, 5.65 and 6.55 eV, arediscussed in terms of crystal field splitting in a low-symmetry site of theLiYP4O12 host lattice. Thelocation of the Ce3 + energy levels with respect to the valence and conduction bands of theLiYP4O12 host is estimated from the temperature dependence of the decay time measured forCe3 + 5d–4f luminescence.

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.

Resonant inelastic x-ray scattering and UV–VUV luminescence at the Be 1s edge inBeO

We carried out a combined study of UV–VUV luminescence and resonant x-ray emissionfrom BeO single crystals with incident photon energies in the vicinity of the Be 1sabsorption edge. The x-ray emission spectra show that at the Be 1s photoabsorption edgethe lattice relaxation processes in the excitation site take place already on the timescale ofthe radiative decay of the core excitation. Comparison of the x-ray emission andthe luminescence spectra indicates that the maximum energy loss of the processof lattice relaxation during the decay of inner-shell holes is similar to the lossthat occurs in the self-trapping process of valence excitons. The possible decaychannels of core excitations have been discussed and the mechanism for the creationof 5.2 eV luminescence at the photoabsorption resonances has been suggested.

Thermoluminescence and synchrotron radiation studies on the persistent luminescence of BaAl 2O 4:Eu 2+,Dy 3+

The persistent luminescence materials, barium aluminates doped with Eu 2+ and Dy 3+ (BaAl 2O 4:Eu 2+,Dy 3+), were prepared with the combustion synthesis at temperatures between 400 and 600 °C as well as with the solid state reaction at 1500 °C. The concentrations of Eu 2+/Dy 3+ (in mol% of the Ba amount) ranged from 0.1/0.1 to 1.0/3.0. The electronic and defect energy level structures were studied with thermoluminescence (TL) and synchrotron radiation (SR) spectroscopies: UV–VUV excitation and emission, as well as with X-ray absorption near-edge structure (XANES) methods. Theoretical calculations using the density functional theory (DFT) were carried out in order to compare with the experimental data.

Luminescent properties of Tb 3+ activated GdTaO 4 with M and M′ type structure under UV–VUV excitation

The photoluminescence of Tb 3+ doped M and M′ type gadolinium orthotantalate Gd 1− x Tb x TaO 4 (0.01≤ x≤0.20) was investigated under ultraviolet and vacuum ultraviolet excitation. For the samples of Gd 1− x Tb x TaO 4 with different crystallographic structures, emission spectra were the same in addition to intensity; the optimal concentration for Tb 3+ was about 10 mol % in M type Gd 1− x Tb x TaO 4 but 5 mol % in M′ type Gd 1− x Tb x TaO 4. These differences could be corresponding with the difference in structures. In addition, compared to commercial Zn 2SiO 4: Mn 2+, the integrated intensity of M and M′ type GdTaO 4: Tb 3+ could reach 67% and 85%, respectively, of that at 147 nm excitation, which indicates that GdTaO 4: Tb 3+ would be a promising vacuum ultraviolet phosphor for application in PDP and Hg-free lamp.

Luminescent properties of BaCl 2 under VUV excitation

Abstracts We observed the luminescence and excitation spectra of a BaCl2 single crystal, which is a promising fast scintillating material, under the UV–VUV excitation. A prominent band was observed at 410 nm under the excitation of almost the entire range of wavelengths region investigated, and it was revealed that this band is extrinsic, because this band was excited with sub-band-gap radiation. A dramatic increase in the intensity of this band was observed at wavelengths less than 90 nm, which was a manifestation of electron excitation multiplication. In addition, we observed a band at 310 nm under the excitation of only 170–190 nm, and this band was possibly a result of self-trapped excitons. The observation of these two bands was quite similar to the scintillation spectra obtained under X-ray or high-energy proton irradiation in previous reports. The knowledge of these luminescence origins are highly important from the viewpoint of the improvement in the scintillation properties of BaCl2 and its application to scintillating materials operative at a high counting rate.

Final State Distributions of O2 Photodesorbed from TiO2(110)

The UV photodesorption of molecular oxygen from a reduced TiO2(110) single-crystal surface was investigated as a function of photon excitation energy, substrate temperature, and preannealing conditions. A pump-delayed-probe method using pulsed lasers for UV excitation (pump) and VUV ionization (probe) were used in conjunction with time-of-flight mass spectrometry to measure velocity distributions of the desorbing O2 molecules. The measured velocity distributions exhibit three distinct features, two of which are attributed to prompt desorption resulting in “fast” velocity distributions and one “slow” channel whose average kinetic energy tracks the surface temperature. The latter is assigned to trapping-desorption of photoexcited O2* which are trapped in the physisorption well prior to thermal desorption. The velocity distributions show no dependence on photon energy over the range studied (3.45−4.16 eV), consistent with a substrate-mediated, hole-capture desorption mechanism. The observed prompt desorption channels have mean translational energies of ∼0.14 and ∼0.50 eV and are attributed to the photodesorption of two distinct initial states of chemisorbed oxygen. The identities of the chemisorbed initial states associated with oxygen vacancy or interstitial defect sites are discussed in light of previous experimental and theoretical studies of oxygen on reduced TiO2(110) surfaces.

Synchrotron radiation spectroscopy of rare earth doped persistent luminescence materials

The electronic structure of the polycrystalline CaAl 2O 4:Eu 2+,Ce 3+ persistent luminescence materials were studied with X-ray absorption (XANES) and UV–VUV emission and excitation spectroscopy by using synchrotron radiation. Theoretical calculations using the density functional theory (DFT) were carried out simultaneously with the experimental work. The experimental band gap energy ( E g) value of 6.7 eV agrees very well with the DFT value of 6.4 eV. From the 4f 7→4f 65d 1 excitation bands of Eu 2+, the positions of the 4f 7 ground as well as the 4f 65d 1 excited levels were established. The excitonic fine structure which could act as trap levels close to the bottom of the conduction band could not be observed, however. The different processes contributing to the mechanism of persistent luminescence from CaAl 2O 4:Eu 2+ were constructed and discussed.

Probing the excitonic emission of ZnO nanoparticles using UV–VUV excitations

This study deals with the influence of the excitation (UV-lamp, UV-laser and VUV synchrotron radiation) on the 3.31 eV band of ZnO microcrystals and of variously treated nanoparticles. The nanoparticles are synthesized in ultra high vacuum condition and their stoichiometry and crystallinity can be controlled. This provides an efficient way to probe the influence of these factors on the excitonic emission. The energy and intensity of the excitation have a strong influence on the excitonic luminescence and particularly on the 3.31 eV emission band. The result of these experiments are used to probe the origins of this band which is found to be not linked to any surface phenomena. Indeed, the only way to fully explain our results is to consider that the 3.31 eV band involve the superposition of two emissions features: the first due to acceptor defects and the other originates form the LO phononic repliqua of the free exciton.

UV–VUV spectroscopy of rare earth doped persistent luminescence materials

The electronic and defect energy level structure of polycrystalline SrAl 2O 4:Eu 2+,R 3+ persistent luminescence materials were studied with thermoluminescence and UV–VUV synchrotron radiation emission and excitation spectroscopy. Theoretical calculations using the density functional theory (DFT) were carried out simultaneously with the experimental work. The experimental band gap energy ( E g) value of 6.6 eV agrees very well with the DFT value of 6.4 eV. The 4f 7 → 4f 65d 1 excitation bands of Eu 2+ were found rather similar irrespective of the R 3+ co-dopant. The trap level energy distribution depended strongly on the R 3+ co-dopant except for the shallowest trap energy above the room temperature remaining the same, however. The different processes in the mechanism of persistent luminescence from SrAl 2O 4:Eu 2+,R 3+ was constructed and discussed.

Electronic structure of the Sr 2MgSi 2O 7:Eu 2+ persistent luminescence material

The electronic structures of the distrontium magnesium disilicate (Sr 2MgSi 2O 7(:Eu 2+)) materials were studied by a combined experimental and theoretical approach. The UV–VUV synchrotron radiation was applied in the experimental study while the electronic structures were investigated theoretically by using the density functional theory. The structure of the valence and conduction bands and the band gap energy of the material as well as the position of the Eu 2+ 4f ground state were calculated. The calculated band gap energy (6.7 eV) agrees well with the experimental value of 7.1 eV. The valence band consists mainly of the oxygen states and the bottom of the conduction band of the Sr states. The calculated occupied 4f ground state of Eu 2+ lies in the energy gap of the host though the position depends strongly on the Coulomb repulsion strength. The position of the 4f ground state with respect to the valence and conduction bands is discussed using the theoretical and experimental evidence available.

Effect of Mg 2+ and Ti IV doping on the luminescence of Y 2O 3:Eu 3+

The Y 2O 3:Eu 3+,Mg 2+,Ti IV materials ( x Eu: 0.02, x Mg: 0.08, x Ti: 0.04) were prepared by solid state reaction. The purity and crystal structure of the material was studied with the X-ray powder diffraction. Luminescence properties were studied in the UV–VUV range with the aid of synchrotron radiation. The emission of Y 2O 3:Eu 3+,Mg 2+,Ti IV had a maximum at 612 nm ( λ exc: 250 nm) due to the 5D 0→ 7F 2 transition of Eu 3+. The excitation spectra ( λ em: 612 nm) showed a broad band at 233 nm, due to the charge transfer transition between O 2− and Eu 3+, and at 297 nm due to the Ti→Eu 3+ energy transfer. Only very weak persistent luminescence was discovered. In the room and 10 K temperature excitation spectra, the line at 208 nm is due to the formation of a free exciton (FE) and a broad band at 199 nm was related to the valence-to-conduction band absorption of the Y 2O 3 host lattice. The absorption edge was ca. 205 nm giving 6.1 eV as the energy gap of Y 2O 3.

Electronic properties and rare-earth ions photoluminescence behaviors in borosilicate: SrB 2Si 2O 8

Undoped and RE ions doped SrB 2Si 2O 8 were successfully synthesized. After the application of UV and VUV spectroscopy measurements, we made a novel discovery that the emission of SrB 2Si 2O 8:Eu prepared in air can be switched between red and blue by the different excitations. The information is that quite a part of Eu 3+ was spontaneously reduced to Eu 2+ in air. The PL properties of Eu 2+ in VUV and Eu 3+, Ce 3+ and Tb 3+ in UV–VUV region in SrB 2Si 2O 8 were evaluated for the first time. The excitation mechanisms of the O 2−–Eu 3+ CT, Ce 3+ f– d and Tb 3+ f–d transitions in UV region as well as the Eu 3+ f–d, O 2−–Ce 3+ CT, O 2−–Tb 3+ CT transitions and the host lattice absorption in VUV region were established. In addition, first principles calculation within the LDA of the DFT was applied to calculate the electronic structure and linear optical properties of SrB 2Si 2O 8 and the results were compared with the experimental data.

Luminescence of Ce doped LaCl3 microcrystals incorporated into a single-crystalline NaCl host

Formation of LaCl3:Ce microcrystals embedded in an NaCl host has been revealed forNaCl–LaCl3 (1 mol%)–CeCl3 (0.05 mol%) composition by means of scanning electron microscopy (SEM) andluminescence spectroscopy techniques. Scanning electron microscopy has shown the size ofLaCl3:Ce microcrystals to be in the10–100 µm range. Spectral-kineticcharacteristics of NaCl–LaCl3 (1 mol%)–CeCl3 (0.05 mol%) luminescence have been studied upon excitation in the ultraviolet–visible ultraviolet (UV–VUV) spectral range. Luminescent properties ofLaCl3:Ce microcrystalsin NaCl–LaCl3 (1 mol%)–CeCl3 (0.05 mol%) are shown to be substantially similar to those of bulkLaCl3:Ce (1 mol%) crystal. The energy transfer mechanism from the NaCl host andLaCl3 microcrystal to cerium centres is discussed.

Spectral-kinetic characteristics of Pr 3+ luminescence in LiLuF 4 host upon excitation in the UV–VUV range

Spectral-kinetic study of Pr 3+ luminescence has been performed for LiLuF 4:Pr(0.1 mol%) single crystal upon the excitation within 5–12 eV range at T=8 K. The fine-structure of Pr 3+ 4f 2→4f 5d excitation spectra is shown for LiLuF 4:Pr(0.1 mol%) to be affected by the efficient absorption transitions of Pr 3+ ions into 4f 5d involving 4f 1 core in the ground state. Favourable conditions have been revealed in LiLuF 4:Pr(0.1 mol%) for the transformation of UV–VUV excitation quanta into the visible range. Lightly doped LiLuF 4:Pr crystals are considered as the promising luminescent materials possessing the efficient Pr 3+ 3P 0 visible emission upon UV–VUV excitation. The mechanism of energy transfer between Lu 3+ host ion and Pr 3+ impurity is discussed.

Luminescence of YbP3O9 upon excitation in the UV–VUV range

X-ray powder diffraction and luminescence spectral-kineticstudies have been performed for ytterbium metaphosphate(YbP3O9) in the 12–290 K temperature range. The diffraction investigation has shownYbP3O9 to be ofmonoclinic P 21/c structure at T = 12–290 K. Charge transfer luminescence originating fromYb3+ ion has been revealed. The carrier confinement within theYb3+ charge transfer stateis pronounced at T = 12 K. A tendency has been revealed for charge carriers inYbP3O9 to be localized in a trapped exciton state at room temperature. The quenching mechanisms forYb3+ charge transfer luminescence and processes competing with the formation of theYb3+ charge transfer state are discussed.