Vacuum Ultraviolet Excitation Research Articles

Temperature behavior of Ce3+ emission in (Lu,Y)2SiO5 single crystals excited by vacuum ultraviolet synchrotron light

Cerium doped (Lu,Y)2SiO5 (or LYSO) single crystals have been studied by means of luminescence excitation spectroscopy in the temperature range from 7 up to 300 K. Vacuum ultraviolet excitations in 4.5–8 eV energy range from synchrotron radiation of 1.5 GeV storage ring of MAX IV synchrotron facility. It is shown that both type of Ce3+ emission centers (seven and six coordinated centers) are excited under any excitation energy used. It was concluded that the same energy transfer processes from host lattice to impurity ions are involved independently on coordination of Ce3+. It is also demonstrated that excitonic mechanism of energy transfer is dominant under chosen excitation and intrinsic and bound excitons are included in excitation of Ce3+ luminescence in LYSO.

Luminescence Properties of an Orthorhombic KLaF4 Phosphor Doped with Pr3+ Ions under Vacuum Ultraviolet and Visible Excitation.

Fluorides have a wide bandgap and therefore, when doped with the appropriate ions, exhibit emissions in the ultraviolet C (UVC) region. Some of them can emit two photons in the visible region for one excitation photon, having a quantum efficiency greater than 100%. In a novel exploration, praseodymium (Pr3+) ions were introduced into KLaF4 crystals for the first time. The samples were obtained according to a high-temperature solid-state reaction. They exhibited an orthorhombic crystal structure, which has not been observed for this lattice yet. The optical properties of the material were investigated in the ultraviolet (UV) and visible ranges. The spectroscopic results were used to analyze the Pr3+ electronic-level structure, including the 4f5d configuration. It has been found that KLaF4:Pr3+ crystals exhibit intense luminescence in the UVC range, corresponding to multiple 4f → 4f transitions. Additionally, under vacuum ultraviolet (VUV) excitation, distinct transitions, specifically 1S0 → 1I6 and 3P0 → 3H4, were observed, which signifies the occurrence of photon cascade emission (PCE). The thermal behavior of the luminescence and the thermometric performance of the material were also analyzed. This study not only sheds light on the optical behavior of Pr3+ ions within a KLaF4 lattice but also highlights its potential for efficient photon management and quantum-based technologies.

On the Energy-specific Photodissociation Pathways of 14N2 and 14N15N Isotopomers to N Atoms of Different Reactivity: A Quantum Dynamical Perspective

Photodissociation of the nitrogen molecule in the vacuum ultraviolet (VUV) is a major source of reactive nitrogen atoms in the upper atmosphere of Earth and throughout the solar system. Recent experimental studies have revealed strong energy dependence of the VUV photodissociation branching ratios to the N(4S3/2)+N(2D J ) and N(4S3/2)+N(2P J ) product channels, the primary dissociation pathways in the 108,000–116,000 cm−1 energy region. This produces N(2D J ) and N(2P J ) excited atoms that differ significantly in their chemical reactivity. The branching ratios oscillate with increase in the VUV excitation energy. We use high-level ab initio quantum chemistry to compute the potential curves of 17 electronic excited states and their nonadiabatic and spin–orbit couplings. The dynamics follow the sequential evolution from the optically excited but bound singlets. Spin–orbit coupling enables transfer to the dissociative triplet and quintet states. We compute the photodissociation yields through the dense manifold of electronic states leading to both exit channels. The dynamical simulations accurately capture the branching oscillations and enable a detailed look into the photodissociation mechanism. The major contribution to the dissociation is through the two lowest 3Π u states. However, for both isotopomers, at about 110,000 cm−1 there is an abnormally low dissociation rate into the N(4S3/2)+N(2P J ) channel that enables comparable participation of triplet and quintet 5Π u electronic states. This leads to the first peak in the branching ratio. At higher energies, trapping of the population in the 33Π u bound triplet state occurs. This favors dissociation to the lower-energy N(4S3/2)+N(2D J ) channel and results in the observed second switch in branching ratios.

Elemental Fluctuation in Gd3Al2Ga3O12:Ce Crystals Imposed by Li+ and Mg2+ Co-Doping: The Impact on Defects, Luminescence, and Scintillation Properties

This research revealed the response of Ga and Al sublattices to the incorporation of mismatching substituents in Gd3Al2Ga3O12:Ce single crystals. Incompatible in size and charge, Li+ and Mg2+ substituents violated configurational entropy. This led to lattice distortion and triggered structural rearrangements. The radial fluctuation of the Ga and Al elements was proven by multi-elemental energy-dispersive X-ray spectroscopy mapping and elemental composition analysis. Further evidence was observed by the shift of the exciton creation energy toward higher energy in the vacuum ultraviolet excitation spectra recorded with synchrotron radiation. In the Li+ and Mg2+ co-doped samples, the crystal core was depleted with Ga atoms and enriched with Al elements. The crystal rim showed the opposite behavior. The change in thermoluminescence peak positions revealed a different mechanism for the formation of localized traps. As a result, Li+ co-doping slightly improved the light yield value, but at the same time decelerated the scintillation decay time. On the contrary, Mg2+ co-doping markedly diminished scintillation parameters.

Two-body dissociation of isoxazole following double photoionization - an experimental PEPIPICO and theoretical DFT and MP2 study.

The dissociative double photoionization of isoxazole molecules has been investigated experimentally and theoretically. The experiment has been carried out in the 27.5-36 eV photon energy range using vacuum ultraviolet (VUV) synchrotron radiation excitation combined with ion time-of-flight (TOF) spectrometry and photoelectron-photoion-photoion coincidence (PEPIPICO) technique. Five well-resolved two-body dissociation channels have been identified in the isoxazole's coincidence maps, and their appearance energies have been determined. The coincidence yield curves of these dissociation channels have been obtained in the photon energy ranges from their appearance energies up to 36 eV. The double photoionization of isoxazole produces a C3H3NO2+ transient dication, which decomposes into fragments differing from previously reported photofragmentation products of isoxazole. We have found no evidence of pathways leading to the C3H2NO+, HCN+, C2H2O+, C3HN+, or C2H2+ fragments or their neutral counterparts that have been observed in previous neutral photodissociation and single photoionization studies. Instead, the dissociation of isoxazole after the ejection of two electrons is bond-selective and is governed by two reactions, HCO+ + H2CCN+ and H2CO+ + HCCN+, whose appearance energies are 28.6 (±0.3) and 29.4 (±0.3) eV, respectively. A third dissociation channel turns out to be a variant of the most intense channel (HCO+ + H2CCN+), where one of the fragment ions contains a heavy isotope. Two minor dissociation channels occurring at higher energies, CO+ + CH3CN+ and CN+ + H3CCO+, are also identified. The density functional and ab initio quantum chemical calculations have been performed to elucidate the dissociative charge-separating mechanisms and determine the energies of the observed photoproducts. The present work unravels hitherto unexplored photodissociation mechanisms of isoxazole and thus provides deeper insight into the photophysics of five-membered heterocyclic molecules containing two heteroatoms.

Defect‐Induced Effects in Nanomaterials

Defect‐Induced Effects in Nanomaterials

Spectroscopic studies on Pr3+ doped YPO4 and LuPO4 upon vacuum ultraviolet (VUV) and synchrotron radiation excitation

This work concerns the spectroscopic properties of micro- and nanoscale LuPO4:Pr3+ and microscale YPO4:Pr3+ upon excitation by VUV synchrotron radiation at a wide range of energies (5–45 eV) and two different temperatures (7 and 300 K).Both Pr3+ doped host materials show more intense UV-C emission than other Pr3+ phosphors studied earlier at FinEstBeAMS beamline under the same measurement conditions. The various relaxation processes of 5d14f1 → 4f2 interconfigurational transitions of Pr3+ and the difference between both hosts are discussed in detail.

A study of the impact of oxygen vacancies on the VUV efficiency of rare-earth doped YBO3 via Ca2+ co-doping

Vacuum ultraviolet excitation and reflectance spectroscopy were used to study doped yttrium borate phosphors with composition Y1-x-yBO3:REy, Cax up to x = 0.10 and up to y = 0.06 (Eu3+ or Tb3+). These data were used to estimate host-to-activator energy transfer efficiencies. The introduction of Ca2+ leads to the formation of oxygen vacancies, VO●●, that act as energy traps. We find that the introduction of these defects lowers the transfer efficiency to both dopants, with Eu3+ being impacted more than Tb3+. The results are discussed in terms of the energies of dopant states relative to band states, and the difference in trapping mechanisms between the two rare earth ions.

UV/VUV photoprocessing of protonatedN-hetero(poly)acenes

ABSTRACTN-heterocycles are suspected to play an important role in the chemical origin of life. Despite their detection in meteorites and in Titan’s atmosphere, their extra-terrestrial chemical formation networks remain elusive. Furthermore N-heterocyclics are undetected in the interstellar medium. This paper assesses the photostability of protonated N-hetero(poly)acenes after ultraviolet (UV) and vacuum ultraviolet (VUV) excitation. It provides information on their ability to retain the N atom into the cycle to generate larger N-containing species or functionalized N-heterocyles. Protonated N-hetero(poly)acenes were generated using electrospray ionization and injected into a linear ion trap where they were irradiated by radiation of 4.5 to 10 eV using the DESIRS beamline at the synchrotron SOLEIL. The photodissociation action spectra of protonated pyridine, quinoline, isoquinoline, and acridine were measured by recording the photofragment yields as a function of photon energy. The four systems exhibit dissociation channels associated with H2 and HCN/HNC loss but with different branching ratios. The results indicate that increasing the size of the N-hetero(poly)acenes increases the chance of retaining the N atom in the larger fragment ion after photodissociation but it remains that all the protonated N-hetero(poly)acenes studied lose their N atom at part of a small neutral photofragment, with high propensity. Therefore, protonated N-hetero(poly)acenes in interstellar space are unlikely precursors to form larger N-containing species. However, protonated pyridine, quinoline, isoquinoline, and acridine are most likely to retain their N atoms in planetary atmospheres where UV radiation at the planet’s surface is typically restricted to wavelengths greater than 200 nm – suggesting such environments are possible substrates for prebiotic chemistry.

Photodissociation dynamics of CO2 + hv → CO(X1Σ+) + O(1D2) via the 3P1Πu state.

The vacuum ultraviolet (VUV) photodissociation of CO2 is important to understand the primary photochemical processes of CO2 induced by solar VUV excitation in the Earth's atmosphere. Here, we report a detailed study of vibrational-state-specific photodissociation dynamics of the CO(X1Σ+) + O(1D2) channel via the 3P1Πu state by using the time-sliced velocity-mapped ion imaging apparatus combined with the single VUV photoionization detection scheme. By recording the sliced images of the O(1D2) photoproducts formed by VUV photoexcitation of CO2 to the individual vibrational structure of the 3P1Πu state, both the vibrational state distributions of the counterpart CO(X1Σ+) photoproducts and the vibrational-state-specific product anisotropy parameters (β) are determined. The experimental results show that photodissociation of CO2 at 108.22, 107.50, 106.10, and 104.76nm yields less anisotropic (β > 0) and inverted distributed CO(X1Σ+, v) photoproducts. The possible dissociation mechanism for the CO(X1Σ+) + O(1D2) channel may involve the non-adiabatic transition of excited CO2* from the initially prepared state to the 31A' state with potential energy barriers. While at 108.82 and 107.35nm, the vibrational distributions are found to have the population peaked at a low vibrational state, and the anisotropy parameters turn out to be negative. Such variation indicates the possibility of another non-adiabatic dissociation pathway that may involve Coriolis-type coupling to the low-lying dissociative state. These observations show sclear evidence of the influence of the initially vibrational excitations on the photodissociation dynamics of CO2 via the 3P1Πu state.

Luminescence and Vacuum Ultraviolet Excitation Spectroscopy of Nanophosphors under Synchrotron Irradiation

Vacuum ultraviolet (VUV) excitation spectroscopy under synchrotron irradiation is a powerful tool for the investigation of luminescence properties of wide bandgap nanocrystalline phosphor materials. The advantages of selective excitation in a wide spectral range present the possibility to explore the different aspects of luminescence processes in nanophosphors separately. It is shown that direct excitations of emission centers in wide bandgap materials need excitations in VUV spectral range and such excitations provide valuable information about f–f, f–d transitions, defects, and excitonic transitions in wide bandgap nanophosphors. It is also demonstrated that size‐dependent luminescence properties of nanophosphors are significant if the electron thermalization length becomes larger than the size of the nanoparticles. In this case, multiplication of electronic excitations processes well‐known in bulk materials are suppressed strongly in nanophosphors.

Vacuum Ultraviolet Excited State Dynamics of the Smallest Ketone: Acetone.

We combined tunable vacuum-ultraviolet time-resolved photoelectron spectroscopy (VUV-TRPES) with high-level quantum dynamics simulations to disentangle multistate Rydberg-valence dynamics in acetone. A femtosecond 8.09 eV pump pulse was tuned to the sharp origin of the A1(n3dyz) band. The ensuing dynamics were tracked with a femtosecond 6.18 eV probe pulse, permitting TRPES of multiple excited Rydberg and valence states. Quantum dynamics simulations reveal coherent multistate Rydberg-valence dynamics, precluding simple kinetic modeling of the TRPES spectrum. Unambiguous assignment of all involved Rydberg states was enabled via the simulation of their photoelectron spectra. The A1(ππ*) state, although strongly participating, is likely undetectable with probe photon energies ≤8 eV and a key intermediate, the A2(nπ*) state, is detected here for the first time. Our dynamics modeling rationalizes the temporal behavior of all photoelectron transients, allowing us to propose a mechanism for VUV-excited dynamics in acetone which confers a key role to the A2(nπ*) state.

Green-emitting Tb3+ doped LaP3O9 phosphors for plasma display panel

La1−xP3O9:xTb3+ (x = 0.005 ≤x ≤ 0.11) phosphors were prepared via the co-precipitation method. The crystal phase was analyzed by X-ray diffraction and the various functional groups in the host were investigated using Fourier transform infra-red (FT-IR) spectroscopy. The luminescence properties of the Tb3+ ions were studied using photoluminescence (PL) excitation and emission spectra. The PL analyses under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation both gave almost similar results. The green emission was observed at 546 nm, corresponding to the transition 5D4→7F5 under 251, 147 and 172 nm excitation. The optimum concentration was 0.09 mol. The calculated CIE coordinates were shifted towards the intense green region from the pale white region with increasing Tb3+ ion concentration. The obtained results demonstrated the potential for LaP3O9 as a candidate in lighting and plasma display panel (PDP) applications.

Tb3+ activated Al2Y4O9 green emitting phosphors under ultraviolet and vacuum ultraviolet excitation

Al2Y4−xO9:xTb3+ luminescent powder phosphors were synthesized by the sol-gel method. The prepared samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy, photoluminescence (PL), and CIE techniques. The microstructural and structural analyses of samples confirmed the formation of phosphors. In the PL studies, an intense peak was observed in the green region and its intensity increased with increasing Tb3+ ions concentration. Concentration quenching was observed for 0.11 mol of Tb3+ ions concentration. These results were confirmed using CIE analysis. UV-VUV studies at two excitation wavelengths confirmed the suppression of the green peak transitions with decreasing excitation wavelength (or increasing radiation energy). The results support the potential for this phosphor to be used as a single source for green light emission in display devices.

Improvement of quantum efficiency through Gd3+ to Eu3+ energy transfer in YF3 phosphor.

This paper reports the energy transfer from Gd3+ to Eu3+ in YF3 and the consequent downconversion luminescence for the YF3 :Gd3+ , Eu3+ fluoride phosphor. The phosphor was synthesized using a soft chemical route, followed by a reactive atmosphere process. Because of the wide band gap in YF3 and the correct energy site for 8 S7/2 -6 GJ transitions of Gd3+ ions, fluoride YF3 doped with Gd3+ -Eu3+ were studied in their vacuum-ultraviolet (VUV) spectral regions. Powder X-ray diffraction (XRD) analysis showed the structural purity of YF3 . VUV excitation and emission properties were explored using a VUV synchrotron radiation beam line. Downconversion of energy from VUV (157 nm) to visible light with quantum efficiency c. 189% was seen. This YF3 :Gd3+ , Eu3+ phosphor would be an option for mercury-free fluorescence lamps.

SYNTHESIS, CHARACTERIZATION AND PHOTOLUMINESCENCE PROPERTIES OF EU3+DOPED CALCIUM SILICATE PHOSPHOR BY CONVENTIONAL SOLID STATE REACTION METHOD

Calcium Silicate phosphor acquires a higher luminous efficiency when it is doped with rare earth activated ions. Silicate phosphors are used for a fluorescent, a cathode-ray tube, a luminous body, a vacuum ultraviolet excitation light emitting element etc. The silicates of calcium are known for their thermal stability, high temperature strength, low thermal expansion, cheep residence and chemical inertness. The present paper reports on the synthesis, characterization and photoluminescence properties of Eu3+ doped calcium silicate phosphor prepared by conventional solid state reaction method heating at 12000C for 3 hrs. The received cakes are grounded for 30 minutes each. The phosphors are prepared and the received powder is subjected to PL, XRD, SEM, EDAX and CIE analysis. The following section discusses and the experimental results are mentioned in these phosphors. The present Phosphor can act as a host for red light emission in many display devices and technological applications.

Effect of terbium and silver co-doping on the enhancement of photoluminescence in CaSO4 phosphors

Abstract In this work, the photoluminescence properties of CaSO4 crystals co-doped with terbium and silver from silver oxide or silver nanoparticles (NPs) in different concentrations are investigated. Phosphors are obtained by means of a slow evaporation route and calcined at 600 °C for 1 h. X-ray diffraction analysis shows the formation of a single-phase anhydrite structure with orthorhombic symmetry. The photoluminescence properties are investigated using vacuum ultraviolet, ultraviolet and X-ray excitation. All luminescent measurements reveal the characteristic emissions of Tb3+. The analyses using excitation with ultraviolet light show that the Ag NPs are able to generate structural defects more satisfactorily compared to CaSO4:Tb with silver oxide. We also observe that higher concentrations of Ag increase the Tb3+ emission intensity, while higher concentrations of Ag NPs decrease it. The good stability of CaSO4 results in an efficient energy transfer from the host lattice to the activator under X-ray excitation, with the NPs contributing to the increase in luminescent intensity. Investigation of the terbium valence in the CaSO4 host shows a complete reduction of the Tb4+ present in Tb4O7 to Tb3+, as indicated by the X-ray absorption near edge structure. The vacuum ultraviolet excitation spectra reveal three broad bands, attributed to the charge-transfer excitations within SO42− complexes and to the 4f8 → 4f75d1 transitions on Tb3+. Our results also reveal that silver particles are responsible for generating deeper capture centers.

Defect-related photoluminescence and photoluminescence excitation as a method to study the excitonic bandgap of AlN epitaxial layers: Experimental and ab initio analysis

We report defect-related photoluminescence (PL) and its vacuum ultraviolet photoluminescence excitation (PLE) spectra of aluminum nitride layers with various layer thicknesses and dislocation densities grown on two different substrates: sapphire and silicon. The defect-related transitions have been distinguished and examined in the emission and excitation spectra investigated under synchrotron radiation. The broad PL bands of two defect levels in the AlN were detected at around 3 eV and 4 eV. In the PLE spectra of these bands, a sharp excitonic peak originating most probably from the A-exciton of AlN was clearly visible. Taking into account the exciton binding energy, the measurements allow determination of the bandgaps of the investigated AlN samples and their temperature dependencies. Next, they are compared with the literature data obtained by other experimental techniques for bulk AlN crystals and layers grown on different substrates. The obtained results revealed that the AlN bandgap depends on the substrate. The theoretical analysis using density functional theory calculations showed that the effect is induced by the tetragonal strain related to the lattice mismatch between the substrate and the AlN layer, which has a strong influence on the spectral positions of the intrinsic excitons, and consequently on the bandgap of AlN layers.

Low-temperature luminescence of ScF3 single crystals under excitation by VUV synchrotron radiation

Photoluminescence and excitation spectra of ScF3 single crystals have been measured under vacuum ultraviolet excitations utilizing undulator synchrotron radiation from 1.5 GeV storage ring of MAX IV synchrotron. The emission peak at 280 nm is explained as emission band of self-trapped excitons in ScF3. This emission is quenched at 50 K and activation energy of thermal quenching was obtained. The excitation spectrum in vacuum ultraviolet spectral range exhibits that the luminescence of self-trapped excitons effectively occurs under direct excitation in the excitonic absorption band, whereas under higher energies this excitation is strongly suppressed, however, multiplication of electronic excitation processes have been successfully identified.

Low-temperature luminescence of catangasite single crystals under excitation by vacuum ultraviolet synchrotron radiation

The luminescent properties of Ca3TaGa3Si2O14 (CTGS, catangasite) single crystals have been studied by means of the vacuum ultraviolet excitation spectroscopy utilizing synchrotron radiation from 1.5 GeV storage ring of MAX IV synchrotron facility. Two emission bands at 320 nm (3.87 eV) and 445 nm (2.78 eV) have been detected. Examining excitation spectra in vacuum ultraviolet spectral range, the 320 nm emission band was explained as the emission band of self-trapped exciton in CTGS single crystal. Its atomic structure is discussed. It is also proposed that the 445 nm (2.78 eV) emission in the CTGS is due to the F centers, which have shown a well-resolved excitation (absorption) band at 5.1 eV (243 nm).