Vacuum Ultraviolet Spectroscopy Research Articles

Mechanistic Insights into Radical-Induced Selective Oxidation of Methane over Nonmetallic Boron Nitride Catalysts

Boron-based nonmetallic materials (such as B2O3 and BN) emerge as promising catalysts for selective oxidation of light alkanes by O2 to form value-added products, resulting from their unique advantage in suppressing CO2 formation. However, the site requirements and reaction mechanism of these boron-based catalysts are still in vigorous debate, especially for methane (the most stable and abundant alkane). Here, we show that hexagonal BN (h-BN) exhibits high selectivities to formaldehyde and CO in catalyzing aerobic oxidation of methane, similar to Al2O3-supported B2O3 catalysts, while h-BN requires an extra induction period to reach a steady state. According to various structural characterizations, we find that active boron oxide species are gradually formed in situ on the surface of h-BN, which accounts for the observed induction period. Unexpectedly, kinetic studies on the effects of void space, catalyst loading, and methane conversion all indicate that h-BN merely acts as a radical generator to induce gas-phase radical reactions of methane oxidation, in contrast to the predominant surface reactions on B2O3/Al2O3 catalysts. Consequently, a revised kinetic model is developed to accurately describe the gas-phase radical feature of methane oxidation over h-BN. With the aid of in situ synchrotron vacuum ultraviolet photoionization mass spectroscopy, the methyl radical (CH3•) is further verified as the primary reactive species that triggers the gas-phase methane oxidation network. Theoretical calculations elucidate that the moderate H-abstraction ability of predominant CH3• and CH3OO• radicals renders an easier control of the methane oxidation selectivity compared to other oxygen-containing radicals generally proposed for such processes, bringing deeper understanding of the excellent anti-overoxidation ability of boron-based catalysts.

Water Plays Multifunctional Roles in the Intervening Formation of Secondary Organic Aerosols in Ozonolysis of Limonene: A Valence Photoelectron Spectroscopy and Density Functional Theory Study

Although water may affect aqueous aerosol chemistry, how it intervenes in the formation of secondary organic aerosols (SOAs) at the molecular level remains elusive. Ozonolysis of limonene is one of the most important sources of indoor SOAs. Here, we report the valence electronic properties of limonene aerosols and SOAs derived from limonene ozonolysis (Lim-SOAs) via aerosol vacuum ultraviolet photoelectron spectroscopy, with a focus on the effects of water on Lim-SOAs. The first vertical ionization energy of limonene aerosols is measured to be 8.79 ± 0.07 eV. While water significantly increases the total photoelectron yield of Lim-SOAs, three photoelectron features attributable to Lim-SOAs each exhibit distinct dependence on the fraction of water in aerosols, implying that different formation pathways and molecular origins are involved in the formation of Lim-SOAs. Combined with density functional theory calculation and mass spectrometry measurements, this study reveals that water, particularly the water dimer, enhances the formation of Lim-SOAs by altering the ozonolysis energetics and pathways by intervening in its Criegee chemistry, acting as both a catalyst and a reactant. The atmospheric implication is discussed.

A portable diagnostic system for the quantification of VUV fluxes emitted from low-temperature plasmas

Vacuum-ultraviolet (VUV) spectroscopy is linked to a huge effort, in particular if absolute numbers are required. To overcome this, a flexible device is developed based on a photodiode and optical filters for wavelength selection. Characterization of the diagnostic is performed against a VUV spectrometer that is radiometrically calibrated down to 46 nm. In the first instance, the latter is used for an analysis of VUV spectra of inductively coupled low-temperature plasmas for a variety of discharge gases and mixtures. The measured photon fluxes can easily have comparable magnitude as the occurring ion flux, and it is demonstrated that the photonic energy distribution can change drastically with varying external parameters (pressure, RF power). This highlights the need for energy resolution for VUV flux detection and a set of ten optical filters was selected for the VUV diode system according to the respective prominent emission ranges of typical discharge gases for plasma processing. Recommended filter combinations for each of the discharge gases and mixtures are given. The developed diagnostic is calibrated in-house against the VUV spectrometer, which makes energy-resolved absolute VUV flux measurements up to photon energies of 27 eV possible. The calibration is performed individually for all the investigated discharge gases and the corresponding filter combinations, and an accuracy of better than 25% compared to the VUV spectrometer is attested for the investigated parameter space (0.3–10 Pa, 200–1100 W RF power). Its applicability is demonstrated by measuring VUV fluxes at two further low-temperature plasma setups.

Conformational diversity and environmental implications of trans-2-pentenal.

This study investigates the conformational intricacies of trans-2-pentenal (trans-2PA), a significant biogenic volatile organic compound. To unveil its potential implications in atmospheric chemistry and environmental pollution, we employ advanced infrared resonant vacuum ultraviolet mass-analysed threshold ionisation spectroscopy. Through this method, we identify the major conformers within trans-2PA, encompassing trans-s-trans (tt-) and trans-s-cis (tc-) structures with planar (cis) and non-planar (gauche) configurations introduced by a methyl group. In a pioneering spectroscopic examination, we analyze trans-2PA in both the neutral and cationic states. This approach allows us to gain a comprehensive understanding of its molecular behavior. Our conformer-specific vibrational spectra not only reveal the relative populations of the main conformers, notably tt-cis and tt-gauche conformers, but also shed light on atmospheric oxidation processes and lower tropospheric organic aerosol formation mechanisms. Our findings expand the understanding of the role of trans-2PA in environmental and biological contexts. Additionally, they contribute to a broader understanding of its influence on air quality, climate, and atmospheric dynamics. The collaboration between advanced experimental techniques and computational methods fortifies the scientific underpinning of this study, opening doors to further exploration in the realms of atmospheric chemistry and environmental science.

In-situ near-ambient-pressure photoelectron spectroscopy investigations of high-work-function MoO3 on 4H-SiC(0001)

How to realize effective Ohmic contact to SiC represents one of the major challenges for high-power and high-frequency SiC electronics. Forming a high-work-function interfacial layer between metal electrode and semiconductor has been considered as a promising approach to realize effective Ohmic contact with wide bandgap semiconductors. In this work, a high-work-function (∼7.2 eV) interfacial molybdenum trioxide (MoO3) layer was grown on 4H-SiC(0001) using conventional vacuum thermal deposition process. We have also systematically investigated the evolution of surface properties of MoO3/4H-SiC(0001) via annealing under high vacuum and various gas environments (H2 and O2). The combination of in-situ near-ambient-pressure x-ray photoelectron spectroscopy (NAP-XPS) and ultra-high vacuum (UHV) ultraviolet photoelectron spectroscopy (UPS) investigations of the MoO3/4H-SiC(0001) reveal that the surface MoO3 tends to be reduced into Mo5+ and Mo4+ during annealing under UHV and 0.5 mbar H2. Most importantly, the surface work function of MoO3/4H-SiC(0001) was found to decrease accordingly together with the defect states originated from the transition from Mo+6 to Mo5+ and Mo4+. In contrast, the oxygen sites as well as its high work function are retained even after 400 °C annealing under 0.5 mbar O2, which indicates that the MoO3/4H-SiC(0001) interface is more thermally stable under oxygen treatment.

Plug im ! software for comprehensive two-dimensional gas chromatography with vacuum ultraviolet detection – A tutorial

The use of two-dimensional gas chromatography hyphenated with vacuum ultraviolet absorbance spectroscopy (GC × GC − VUV) is constantly finding new applications. To qualitatively and quantitively exploit the information obtained with this type of analysis, suitable softwares are needed. However, to date, existing multidimensional chromatography softwares are not fully adapted to this type of data and offer only limited processing capabilities. Moreover, other solutions often rely on homemade programs that are not accessible to new users. For these reasons, several data processing modules for GC × GC − VUV data were developed and were integrated into the free access plug im ! software (https://www.plugim.fr/plugin/107). The usage guidelines and functionalities of the proposed modules are described, and several video tutorials are supplied. The proposed modules allow to visualize and preprocess GC × GC − VUV data. The extraction of spectra for user-designed template zones is also made possible which enables their further use for both quantification and spectral decomposition. These tools also result in more sensitive methods thanks to the increase of the signal-to-noise ratio through application of adapted data preprocessing.

Product Identification in the Low-Temperature Oxidation of Cyclohexane Using a Jet-Stirred Reactor in Combination with SVUV-PEPICO Analysis and Theoretical Quantum Calculations.

Cyclohexane oxidation chemistry was investigated using a near-atmospheric pressure jet-stirred reactor at T = 570 K and equivalence ratio ϕ = 0.8. Numerous intermediates including hydroperoxides and highly oxygenated molecules were detected using synchrotron vacuum ultraviolet photoelectron photoion coincidence spectroscopy. Supported by high-level quantum calculations, the analysis of photoelectron spectra allowed the firm identification of molecular species formed during the oxidation of cyclohexane. Besides, this work validates recently published gas chromatography and synchrotron vacuum ultraviolet photoionization mass spectrometry data. Unambiguous detection of characteristic hydroperoxides (e.g., γ-ketohydroperoxides) and their respective decomposition products provides support for the conventional O2 addition channels up to the third addition and their relative contribution to the cyclohexane oxidation. The results were also compared with the predictions of a recently proposed new detailed kinetic model of cyclohexane oxidation. Most of the predictions are in line with the current experimental findings, highlighting the robustness of the kinetic model. However, the analysis of the recorded slow photoelectron spectra indicating the possible presence of C5 species in the kinetic model provides hints that the substituted cyclopentyl radicals from cyclohexyl ring opening might play a minor role in cyclohexane oxidation. Potentially important missing reactions are also discussed.

Defect‐Induced Effects in Nanomaterials

Defect‐Induced Effects in Nanomaterials

Synthetic aromatic kerosene property prediction improvements with isomer specific characterization via GCxGC and vacuum ultraviolet spectroscopy

This research explores an advanced method of fuel composition determination and builds upon typical hydrocarbon group type analyses performed with two-dimensional gas chromatography (GCxGC). In this study, structural information of individual species within Virent’s Synthetic Aromatic Kerosene (SAK) is identified by vacuum ultraviolet (VUV) spectroscopy. By mass, 71.3% of the components elute within six peaks of the chromatogram, from which 12 unique species are identified through a novel deconvolution method. Overall, the identification of 93.6%m across 26 structural isomers is made by the methods described in this work. With 93.6%m ascribed to specific isomers, the precision of fuel property predictions improves dramatically. For example, the absolute error of the viscosity prediction is reduced by 90% because of this advancement in diagnostic capability, and its 95-percentile confidence interval (precision only) is reduced by 93%. Additionally, the properties of SAK, blended with hydro processed esters fatty acids (HEFA), are demonstrated to have blended properties consistent with conventional jet fuel.

Wide range applications of process plasma diagnostics using vacuum ultraviolet absorption spectroscopy

Wide range applications of process plasma diagnostics using vacuum ultraviolet absorption spectroscopy

High-speed hydrophilic and ashing treatments of polyimide using Ar/O2 atmospheric-pressure microwave line plasma

The performance of a newly-developed 1D-long atmospheric-pressure microwave plasma, i.e. atmospheric-pressure microwave line plasma (APMLP), is demonstrated. Using vacuum ultraviolet atomic absorption spectroscopy, spatially uniform oxygen radical density along a 50 cm length is observed. With the APMLP using Ar/O2 gas, very fast hydrophilic treatment of polyimide (PI) film within 20 ms is obtained. Using looped-waveguide-type APMLP, fast ashing treatment of PI film at an ashing rate of 20 μm min−1 is demonstrated.

Space-resolved vacuum-ultraviolet spectroscopy for measuring impurity emission from divertor region of EAST tokamak

The measurement of impurity distribution in the divertor region of tokamaks is key to studying edge impurity transport. Therefore, a space-resolved vacuum-ultraviolet (VUV) spectrometer is designed to measure impurity emission in the divertor region on EAST. For good spectral resolution, an eagle-type VUV spectrometer with 1 m long focal length with spherical holograph grating is used in the system. For light collection, a collimating mirror is installed between the EAST plasma and the VUV spectrometer to extend the observing range to cover the upper divertor region. Two types of detectors, i.e. a back-illuminated charge-coupled device detector and a photomultiplier-tube detector, are adopted for the spectral measurement and high-frequency intensity measurement for feedback control, respectively. The angle between the entrance and exit optical axis is fixed at 15°. The detector can be moved along the exit axis to maintain a good focusing position when the wavelength is scanned by rotating the grating. The profile of impurity emissions is projected through the space-resolved slit, which is set horizontally. The spectrometer is equipped with two gratings with 2400 grooves/mm and 2160 grooves/mm, respectively. The overall aberration of the system is reduced by accurate detector positioning. As a result, the total spectral broadening can be reduced to about 0.013 nm. The simulated performance of the system is found to satisfy the requirement of measurement of impurity emissions from the divertor area of the EAST tokamak.

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.

Quantitative measurement model for sulfate in the temperature range of 298.15–343.15 K based on vacuum ultraviolet absorption spectroscopy

In order for SO3 measurement without impurity gas interference in coal-fired power plants, a quantitative measurement model for sulfate in the temperature range of 298.15–343.15 K based on vacuum ultraviolet absorption spectroscopy was created. A vacuum ultraviolet absorption spectrum measurement system that eliminates oxygen interference was set up, and the vacuum ultraviolet absorption of sulfate was ascertained at 1 atm pressure by means of vacuum combined with nitrogen purging. The law of the vacuum ultraviolet absorption of sulfate with temperature was revealed, that the full width at half maximum (FWHM) and peak absorbance (Amax) both increased linearly with temperature. The quadratic function relationship between the integrated area (OP) of the vacuum ultraviolet absorption spectrum of sulfate and temperature was found, and a universal sulfate concentration measurement model was established in the temperature range of 298.15–343.15 K. The average relative calculation error of this model for 0.0025 mol L−1 sulfate was as low as 0.6%, which supports the measurement of trace sulfate.

Temperature-dependent characteristics of ZnO phosphors from synchrotron-based vacuum ultraviolet photoluminescence spectroscopy

Temperature-dependent characteristics of ZnO phosphors from synchrotron-based vacuum ultraviolet photoluminescence spectroscopy

Helically aligned fused carbon hollow nanospheres with chiral discrimination ability.

While the functions of carbon materials with precisely controlled nanostructures have been reported in many studies, their chiral discriminating abilities have not been reported yet. Herein, chiral discrimination is achieved using helical carbon materials devoid of chiral attachments. A Fe3O4 nanoparticle template with ethyl cellulose (carbon source) is self-assembled on dispersed multiwalled carbon nanotubes (MWCNTs) fixed in a lamellar structure, with helical nanoparticle alignment induced by the addition of a binaphthyl derivative. Carbonization followed by template removal produces helically aligned fused carbon hollow nanospheres (CHNSs) with no chiral molecules left. Helicity is confirmed using vacuum-ultraviolet circular dichroism spectroscopy. Chiral discrimination, as revealed by the electrochemical reactions of binaphthol and a chiral ferrocene derivative in aqueous and nonaqueous electrolytes, respectively, is attributable to the chiral space formed between the CHNS and MWCNT surfaces.

The rise of an exciton in solid ammonia.

We trace a polymorphic phase change in solid ammonia films through the emergence of a Frenkel exciton at 194.4 nm, for deposition temperatures of 48 K, 50 K and 52 K. Observations on a timescale of hours give unparalleled access to the individual processes of nucleation and the phase change itself. The excitonic transition is forbidden in the low temperature phase, but greater flexing of the solid state structure in the higher temperature phase makes the transition allowed, as the nano-crystals approach ∼30 unit cells through nucleation. We find activation energies of 21.7 ± 0.6 kJ mol-1 for nucleation and 22.8 ± 0.6 kJ mol-1 for the phase change, corresponding to the breaking of two to three hydrogen bonds.

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.

Towards fuel composition and properties from Two-dimensional gas chromatography with flame ionization and vacuum ultraviolet spectroscopy

Already low volume (<1mL) test methods facilitate the development of sustainable aviation fuel platforms and higher fidelity computational methods. Here a novel technique with two-dimensional gas chromatography (GCxGC) and Vacuum Ultraviolet (VUV) identification is used to characterize fuel composition and determine properties compared to previous work. Ten properties are predicted, including the temperature dependence of density, viscosity, thermal conductivity, and heat capacity. Property predictions incorporate uncertainty quantification (UQ) from analyte quantification (UQ1), root property uncertainty (UQ2), and the uncertainty associated with isomeric variance (UQ3), when an analyte is not identified via VUV. Comparisons to a previous method illustrate the ability of VUV identification to increase the fidelity of property predictions and decrease uncertainties. This method is applied to a surrogate intended to mimic the first-order properties and composition of a representative Jet A/A-1. In addition to nominal and temperature-dependent properties, the derived cetane number (DCN) of the surrogate is calculated for the distillation fraction evolved. The DCN there is shown to vary across the fraction of fuel distilled. Collectively, this method documents a process to prescreen novel sustainable aviation fuel candidates, facilitate the development of chemical process models, and automate property determinations for computational fluid dynamics.

Vacuum Ultraviolet Emission Properties of Rocksalt‐Structured MgZnO Microcrystals Prepared on Quartz Glass Substrates

Rocksalt‐structured (RS) MgZnO crystals are grown on quartz glass substrates by the mist chemical vapor deposition (mist CVD) method. Cathodoluminescence (CL) emission properties are comparatively studied with those for the RS‐MgZnO epitaxial film prepared on MgO substrate to explore their potential for vacuum ultraviolet (VUV) emitter. In the experiment, the newly developed VUV spectroscopy system is employed, where the CL and optical transmittance spectra are simultaneously measured. Growths of RS‐MgZnO on quartz glass substrate indicate formation of microcrystals (MCs). The MCs dominantly exhibit a near‐band‐edge VUV emission at 6.20 eV (200 nm) at 6 K and 6.07 eV (204 nm) at 300 K. Moreover, the MCs show a noticeable higher energy shoulder at 6.6 eV (188 nm) at 6 K and 6.4 eV (194 nm) at 300 K. The results ensure potential of RS‐MgZnO crystals for active element in VUV emitter regardless of the substrate material.