Vacuum Ultraviolet Spectroscopy Research Articles

Time resolved plasma diagnostics for pulsed 2.45GHz hydrogen discharges.

A review of the last ten years of advances in temporal resolved plasma diagnostics for pulsed operated 2.45GHz microwave-driven hydrogen discharges is presented. Special attention has been paid to the study of breakdown and decay processes where some particular phenomena are explored for application in ion sources. Measurements of plasma temperature and density using vacuum ultraviolet spectroscopy, ultra-fast photography, and ion mass spectroscopy are performed in detail, including examples and results.

Advanced data preprocessing for comprehensive two-dimensional gas chromatography with vacuum ultraviolet spectroscopy detection.

Comprehensive two-dimensional gas chromatography with vacuum ultraviolet detection results in sizable data for which noise and baseline drift ought to be corrected. As the data is acquired from multiple channels, preprocessing steps have to be applied to the data from all channels while being robust and rather fast with respect to the significant size of the data. In this study, we have described advanced data preprocessing techniques for such data which were not available in the existing commercial software solutions and which were dedicated primarily to noise and baseline correction. Noise reduction was performed on both the spectral and the time dimension. For the baseline correction, a morphological approach based on iterated convolutions and rectifier operations was proposed. On the spectral dimension, much less noisy and reliable spectra were obtained. From a quantitative point of view, mentioned preprocessing steps significantly improved the signal-to-noise ratio for the analyte detection (circa six times in this study). These preprocessing methods were integrated into the plugim! platform (https://www.plugim.fr/).

Infrared–vacuum ultraviolet spectroscopy of the CH stretching vibrations of jet‐cooled aromatic azine molecules and the anharmonic analysis

AbstractVibrational spectra of the CH stretching vibration of diazine molecules, pyrimidine and pyrazine, were measured by infrared (IR)–vacuum ultraviolet (VUV) spectroscopy under the jet‐cooled gas‐phase condition. The observed IR spectra were analyzed by three anharmonic algorithms to account for the Fermi‐resonance (F‐R). The anharmonic analysis of the F‐R pattern was performed with second‐order vibrational perturbation theory (VPT2) with quartic potentials (QPs) at the DFT level of B2PLYP/6–311++G(d,p), followed by vibrational configuration interaction (VCI) method with the same QP. The VPT2 + QP method reasonably reproduced most of the bands in the observed spectra for all the species, especially for pyrimidine, a decent agreement is obtained with respect to the band positions and relative intensities. The analyses of the spectra show that all the observed spectra can be well interpreted by the F‐R between the CH stretching fundamentals and the first overtone and 1 + 1 combination bands involving the in‐plane CH bending vibrations and the contribution of the higher‐order anharmonic coupling to the observed spectra seems to lead to red‐shift of the F‐R patterns. Discrete variable representation based methods with potential energy surfaces at CCSD/aug‐cc‐pVDZ theory with 10 degrees of freedom were also carried out to assess the quality of QP at DFT level.

Effect of the modification of alumina supports with chloride on the structure and catalytic performance of Ag/Al2O3 catalysts for the selective catalytic reduction of NOx with propene and H2/propene

The effect of the modification of an alumina support with chloride on the structure and the catalytic performance of Ag/Al2O3 catalysts (SA) was investigated for the selective catalytic reduction (SCR) of NO using C3H6 or H2/C3H6 as reductants. The Ag/Al2O3 catalyst and Cl−-modified Ag/Al2O3 catalysts (SA-Cl) were prepared by a conventional impregnation method and characterized by X-ray diffraction, Brunauer-Emmett-Teller isotherm analysis, electron probe microanalysis, transmission electron microscopy, UV-Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and hydrogen temperature-programmed reduction. The catalytic activities in the C3H6-SCR and H2/C3H6-SCR reactions were evaluated, and the reaction mechanism was studied using in situ diffuse reflectance infrared Fourier transform spectroscopy and synchrotron vacuum ultraviolet photoionization mass spectroscopy (SVUV-PIMS). We found that Cl− modification of the alumina-supported Ag/Al2O3 catalysts facilitated the formation of oxidized silver species (Agnδ+) that catalyze the moderate-temperature oxidation of hydrocarbons into partial oxidation products (mainly acetate species) capable of participating in the SCR reaction. The low-temperature promoting effect of H2 on the C3H6-SCR (“hydrogen effect”) was found to originate from the enhanced decomposition of strongly adsorbed nitrates on the catalyst surface and the conversion of these adsorbed species to –NCO and –CN species. This “H2 effect” occurs in the presence of Agnδ+ species rather than the metallic Ag0 species. A gaseous intermediate, acrylonitrile (CH2CHCN), was also identified in the H2/C3H6-SCR reaction using SVUV-PIMS. These findings provide novel insights in the structure-activity relationship and reaction mechanisms of the SA-catalyzed HC-SCR reaction of NO.

Determination of vacuum ultraviolet detector response factors by hyphenation with two-dimensional comprehensive gas chromatography with flame ionization detection.

Two-dimensional comprehensive gas chromatography is an established technique, employed for the characterization of complex samples. Broadband vacuum ultraviolet absorption spectroscopy detection has recently attracted a lot of attention as it is a universal detection technique characterized by good selectivity but also ease of use and amenability to coupling with two-dimensional comprehensive gas chromatography. Vacuum ultraviolet spectroscopy is particularly interesting due to the possibility of performing spectral decomposition for species that coelute in gas chromatography analysis. This detector has quantitative capabilities, however not all species absorb vacuum ultraviolet radiation the same. Unfortunately, vacuum ultraviolet relative response factors for compounds are not always available. Methods to rapidly measure vacuum ultraviolet relative response factors and generate a large database that would allow calibration free quantitative analysis of complex mixtures are therefore of great interest. In this work, a universal methodology that permits rapid measurement of vacuum ultraviolet relative response factors is reported. It involves flow modulated two-dimensional comprehensive gas chromatography with dual vacuum ultraviolet and flame ionization detection. In this set-up, flame ionization detection is employed as a quantitative reference allowing to scale vacuum ultraviolet responses of investigated compounds. This approach was validated by flow measurements and by comparing relative response factors obtained for model compounds with literature data.

Structures of Pyridine–Water Clusters Studied with Infrared–Vacuum Ultraviolet Spectroscopy

The infrared (IR) spectra of the O-H stretching vibrations of pyridine-water clusters (Pyd)m(H2O)n, with m, n = 1-4, have been investigated with infrared-vacuum ultraviolet (VUV) spectroscopy under a jet-cooled condition. The time-of-flight mass spectrum of (Pyd)m(H2O)n+ by VUV ionization at ∼9 eV showed an unusual intensity pattern with very weak ion signals for m = 1 and 2 and stronger signals for m ≥ 3. This unusual mass pattern was explained by a drastic structural change of (Pyd)m(H2O)n upon the VUV ionization, which was followed by the elimination of water molecules. Among the recorded IR spectra, only one spectrum monitored, (Pyd)2+ cation, showed a well-resolved structure. The spectrum was analyzed by comparing with the simulated ones of possible stable isomers of (Pyd)2(H2O)n, which were obtained with quantum-chemical calculations. Most of the calculated (Pyd)2(H2O)n clusters had the characteristic structure in which H2O or (H2O)2 forms a hydrogen-bonded bridge between two pyridines to form the π-stacked (Pyd)2, and an additional H2O molecule(s) extends the H-bonded network. The π-stacked (Pyd)2(H2O)n moiety is very stable and is thought to exist as a local structure in a pyridine/water mixed solution. The Fermi resonance between the O-H stretch fundamentals and the overtones of the O-H bending vibrations in (Pyd)m(H2O)n was found to be less pronounced in the case of (Pyd)m(NH3)n studied previously.

Unmasking the cis-Stilbene Phantom State via Vacuum Ultraviolet Time-Resolved Photoelectron Spectroscopy and Ab Initio Multiple Spawning.

We present the first vacuum ultraviolet time-resolved photoelectron spectroscopy (VUV-TRPES) study of photoisomerization dynamics in the paradigmatic molecule cis-stilbene. A key reaction intermediate in its dynamics, known as the phantom state, has often been invoked but never directly detected in the gas phase. We report direct spectral signatures of the phantom state in isolated cis-stilbene, observed and characterized through a combination of VUV-TRPES and ab initio multiple spawning (AIMS) nonadiabatic dynamics simulations of the channel-resolved observable. The high VUV probe photon energy tracks the complete excited-state dynamics via multiple photoionization channels, from initial excitation to its return to the "hot" ground state. The TRPES was compared with AIMS simulations of the dynamics from initial excitation, to the phantom-state intermediate (an S1 minimum), through to the ultimate electronic decay to the ground state. This combination revealed the unique spectral signatures and time-dependent dynamics of the phantom-state intermediate, permitting us to report here its direct observation.

Upgrade of vacuum ultraviolet spectroscopy system on J-TEXT tokamak.

The vacuum ultraviolet (VUV) spectroscopy system on the Joint Texas Experimental Tokamak has been upgraded to achieve fast acquisition for the study of impurity transport in transient modulated experiments. In this upgrade, the previous high-energy charge-coupled device detector was replaced by a microchannel plate with a CsI-coated photocathode and P43 phosphor to transform the VUV light to visible light, which is then acquired by a high-speed electron-multiplying charge-coupled device. Two-stage focusing was achieved using a reference slit plate illuminated successively by a green light source and the Lyman series hydrogen spectral lines from the vacuum-conditioning plasma. The spatial resolution was evaluated as ∼4mm based on the level of image blurring from the alignment plate. A response time of ∼2ms was obtained with the ten-vertical-track setup.

The nature of intrinsic recombination emission in SrSO4 irradiated with ultraviolet photons

The synthesized SrSO4 crystalline powders were irradiated at photon energies from 6 ÷ 12 eV, in the temperature range from 15 ÷ 300 K. The investigation was carried out by the methods of atomic absorption, luminescence and vacuum ultraviolet spectroscopy. Intrinsic emissions were found at 3.7-3.8 eV, 4.2-4.6 eV and 4.9-5 eV, and long-wavelength recombination emissions at 2.6-2.7 eV and 2.9-3.0 eV.The synthesized SrSO4 crystalline powders were irradiated at photon energies from 6 ÷ 12 eV, in the temperature range from 15 ÷ 300 K. The investigation was carried out by the methods of atomic absorption, luminescence and vacuum ultraviolet spectroscopy. Intrinsic emissions were found at 3.7-3.8 eV, 4.2-4.6 eV and 4.9-5 eV, and long-wavelength recombination emissions at 2.6-2.7 eV and 2.9-3.0 eV.

Probing the Spin State of Spin-Crossover Complexes on Surfaces with Vacuum Ultraviolet Angle-Resolved Photoemission Spectroscopy

Spin-crossover complexes in direct contact with substrates have sparked considerable interest, in particular, in view of their potential applications in molecular electronics. While a huge number of spin-crossover complexes is available, many of them are not robust enough to withstand the sample preparation and/or the interaction with the substrate. The techniques usually employed for these investigations, namely, near-edge X-ray absorption fine structure spectroscopy and low-temperature scanning tunneling microscopy, are not adapted for systematic studies because of the limited access to synchrotron-radiation facilities and complexity of the (indirect) spin determination, respectively. Here, we detail a methodology using a (more) commonly available technique, namely, vacuum ultraviolet (angle-resolved) photoemission spectroscopy, to determine the spin state of layers of three different spin-crossover complexes with thicknesses down to the submonolayer regime. We present an approach to determine the thicknesses of the investigated layers, relying on the inelastic mean free path of electrons determined from combined photoemission and X-ray absorption measurements. We report on the high-spin to low-spin relaxation dynamics of spin-crossover (SCO) layers and the influence of the ultraviolet light on these dynamics. While the observed relaxation processes occur on a timescale on the order of minutes, probing spin-state dynamics on the picosecond timescale is foreseeable with pump–probe photoemission spectroscopy.

Graphene Oxide Layer-by-Layer Films for Sensors and Devices.

Layer-by-layer films of poly (allylamine hydrochloride) (PAH) and graphene oxide (GO) were characterized, looking at growth with the number of bilayers, morphology, and electrical properties. The PAH/GO films revealed a linear increase in absorbance with the increase in the number of deposited bilayers, allowing the determination that 10.7 ± 0.1 mg m−2 of GO is adsorbed per unit of area of each bilayer. GO absorption bands at 146, 210, 247 and 299 nm, assigned to π-π* and n-π* transitions in the aromatic ring (phenol) and of the carboxylic group, respectively, were characterized by vacuum ultraviolet spectroscopy. The morphological characterization of these films demonstrated that they are not completely uniform, with a bilayer thickness of 10.5 ± 0.7 nm. This study also revealed that the films are composed of GO and/or PAH/GO fibers and that GO is completely adsorbed on top of PAH. The electrical properties of the films reveal that PAH/GO films present a semiconductor behavior. In addition, a slight decrease in conduction was observed when films were prepared in the presence of visible light, likely due to the presence of oxygen and moisture that contributes to the damage of GO molecules.

The role of photon self-absorption on the H (n = 2) density determination by means of VUV emission spectroscopy and TDLAS in low pressure plasmas

The H (n = 2) atomic density determination by means of vacuum ultra-violet (VUV) emission spectroscopy of the Lyman-α line in laboratory low pressure plasmas is strongly affected by self-absorption of emitted photons inside the plasma leading to an underestimation of this density. A correction of the densities obtained from VUV emission spectroscopy measurements is performed by using the escape factor method. The corrections applied can reach orders of magnitude even in low pressure plasmas. Assumptions on the spatial distribution of emitting and absorbing particles as well as on the corresponding line profiles have to be made. Consequently, additional measurements are performed, which raises the requirement of a benchmark of the applied correction procedure. In contrast, tunable diode laser absorption spectroscopy (TDLAS) on the Balmer-α line is a direct measurement of the H (n = 2) density and is thus suitable for a benchmark. For the ICP under consideration, H (n = 2) densities obtained via TDLAS are near 3 × 1015 m−3 whereas uncorrected VUV emission spectroscopy gives values in the range of roughly 1013 m−3 depending on pressure and applied RF power. The calculated escape factors are on the order of 2–8 × 10−3. An excellent agreement with TDLAS is observed by applying them to the results of the VUV emission spectroscopy.

An atomic hydrogen etching sensor for H2 plasma diagnostics.

A simple and selective new technique for atomic hydrogen flux measurements in a hydrogen plasma environment is introduced and demonstrated in this work. This technique works by measuring the etching rate of an amorphous carbon film and translating this to an incoming hydrogen radical flux through a well-defined carbon etch yield per radical. Ions present in the plasma environment have a much higher etch yield than radicals do. For that reason, suppression of the ion flux toward the carbon film is crucial to ensure that the observed carbon etch rate is dominated by atomic hydrogen etching. It is demonstrated that this can be achieved using a simple cylindrical pipe (hereinafter "chimney") in which a bend is introduced to enforce ion-wall collisions, neutralizing the ions. The chimney is made out of Macor, a material with low catalytic surface activity, to preserve the incoming atomic hydrogen flux while effectively suppressing ions. Ultimately, the etching sensor is deployed in a radio frequency inductively coupled hydrogen plasma operated at low pressure (1-10Pa). Atomic hydrogen fluxes are measured and compared with heat flux sensor and vacuum ultraviolet absorption spectroscopy measurements in the same setup. All sensors agreed within a factor 4 in the atomic hydrogen flux range 1019 to 1021m-2s-1.

Ultraviolet spectroscopy of pressurized and supercritical carbon dioxide

Carbon dioxide (CO2) is prevalent in planetary atmospheres and sees use in a variety of industrial applications. Despite its ubiquitous nature, its photochemistry remains poorly understood. In this work we explore the density dependence of pressurized and supercritical CO2 electronic absorption spectra by vacuum ultraviolet spectroscopy over the wavelength range 1455-2000 Å. We show that the lowest absorption band transition energy is unaffected by a density increase up to and beyond the thermodynamic critical point (137 bar, 308 K). However, the diffuse vibrational structure inherent to the spectrum gradually decreases in magnitude. This effect cannot be explained solely by collisional broadening and/or dimerization. We suggest that at high densities close proximity of neighboring CO2 molecules with a variety of orientations perturbs the multiple monomer electronic state potential energy surfaces, facilitating coupling between binding and dissociative states. We estimate a critical radius of ~4.1 Å necessary to cause such perturbations.

Development and preliminary test of a space-resolved vacuum-ultraviolet spectroscopy in EAST.

The impurity radiation from the divertor region of the EAST tokamak is dominantly in the wavelength range of vacuum ultraviolet (VUV) due to the elevated edge electron temperature. A space-resolved VUV spectroscopy is developed to measure impurity radiation in the divertor region. An eagle-type VUV spectrometer with a focal length of 1m is adopted in this system, equipped with a spherical grating and a charged-coupled device (CCD) detector. The performance of the VUV spectrometer is preliminarily tested on a synchrotron radiation facility. The wavelength calibration is conducted near 65nm. It is found that the wavelength range observed by the CCD detector is about 11.07nm around the central wavelength of about 65nm. With a linear dispersion of 0.0053 nm/pixel, it is possible to measure the ion temperature lower than 20eV at the edge region by analyzing the Doppler broadening of a carbon line. These test results show that the performance of the VUV spectrometer is capable of measuring divertor radiation and analyzing the ion temperature of edge impurity ions.

Energy transfer of intrinsic electronic excitation to impurities in the CaSO4 − Mn crystal

The nature of the impurity emission center and the energy transfer mechanisms from intrinsic electronic excitations to the Mn2+ impurity centers in the CaSO4 − Mn crystal have been investigated. The methods of vacuum ultraviolet, atomic absorption and luminescence spectroscopy are used in a wide temperature range from 5 K to 300 K. The energy transfer of intrinsic electronic excitations to impurity emission centers in CaSO4 − Mn has been detected.

Vacuum ultraviolet photoabsorption spectroscopy of space-related ices: formation and destruction of solid carbonic acid upon 1 keV electron irradiation

Context. Carbonic acid (H2CO3) is a weak acid relevant to astrobiology which, to date, remains undetected in space. Experimental work has shown that the β-polymorph of H2CO3 forms under space relevant conditions through energetic (UV photon, electron, and cosmic ray) processing of CO2- and H2O-rich ices. Although its α-polymorph ice has been recently reassigned to the monomethyl ester of carbonic acid, a different form of H2CO3 ice may exist and is synthesized without irradiation through surface reactions involving CO molecules and OH radicals, that is to say γ-H2CO3. Aims. We aim to provide a systematic set of vacuum ultraviolet (VUV) photoabsorption spectroscopic data of pure carbonic acid that formed and was destroyed under conditions relevant to space in support of its future identification on the surface of icy objects in the Solar System by the upcoming Jupiter ICy moons Explorer mission and on interstellar dust by the James Webb Space Telescope spacecraft. Methods. We present VUV photoabsorption spectra of pure and mixed CO2 and H2O ices exposed to 1 keV electrons at 20 and 80 K to simulate different interstellar and Solar System environments. Ices were then annealed to obtain a layer of pure H2CO3 which was further exposed to 1 keV electrons at 20 and 80 K to monitor its destruction pathway. Fourier-transform infrared (FT-IR) spectroscopy was used as a secondary probe providing complementary information on the physicochemical changes within an ice. Results. Our laboratory work shows that the formation of solid H2CO3, CO, and O3 upon the energetic processing of CO2:H2O ice mixtures is temperature-dependent in the range between 20 and 80 K. The amorphous to crystalline phase transition of H2CO3 ice is investigated for the first time in the VUV spectral range by annealing the ice at 200 and 225 K. We have detected two photoabsorption bands at 139 and 200 nm, and we assigned them to β-H2CO3 and γ-H2CO3, respectively. We present VUV spectra of the electron irradiation of annealed H2CO3 ice at different temperatures leading to its decomposition into CO2, H2O, and CO ice. Laboratory results are compared to Cassini UltraViolet Imaging Spectrograph observations of the 70−90 K ice surface of Saturn’s satellites Enceladus, Dione, and Rhea.

Observation of Carbon-Carbon Coupling Reaction in Neutral Transition-Metal Carbonyls.

Neutral titanium-metal carbonyl complexes with the chemical formula Ti(CO)n (n = 4-7) are produced in the gas phase by the reactions of titanium atoms with carbon monoxide in a pulsed laser vaporization-supersonic expansion source. Their infrared absorption spectra in the carbonyl stretching frequency region are measured by infrared plus vacuum ultraviolet (IR+VUV) two-color ionization spectroscopy based on a tunable VUV free electron laser. Infrared spectroscopy in conjunction with quantum chemical calculations confirm that all of these complexes have unexpected titanium ketenylidene OTiCCO(CO)n-2 structures. Bonding analysis indicates that the OTiCCO core structure can be described by the bonding interactions between a TiO+ cation in the doublet ground state and a doublet ground state of CCO-. The results reveal that the C-O bond breaking and C-C bond formation proceed efficiently in the reactions between laser-vaporized titanium atoms and carbon monoxide.

Vacuum Ultraviolet Absorption Spectroscopy Analysis of Breath Acetone Using a Hollow Optical Fiber Gas Cell.

Human breath is a biomarker of body fat metabolism and can be used to diagnose various diseases, such as diabetes. As such, in this paper, a vacuum ultraviolet (VUV) spectroscopy system is proposed to measure the acetone in exhaled human breath. A strong absorption acetone peak at 195 nm is detected using a simple system consisting of a deuterium lamp source, a hollow-core fiber gas cell, and a fiber-coupled compact spectrometer corresponding to the VUV region. The hollow-core fiber functions both as a long-path and an extremely small-volume gas cell; it enables us to sensitively measure the trace components of exhaled breath. For breath analysis, we apply multiple regression analysis using the absorption spectra of oxygen, water, and acetone standard gas as explanatory variables to quantitate the concentration of acetone in breath. Based on human breath, we apply the standard addition method to obtain the measurement accuracy. The results suggest that the standard deviation is 0.074 ppm for healthy human breath with an acetone concentration of around 0.8 ppm and a precision of 0.026 ppm. We also monitor body fat burn based on breath acetone and confirm that breath acetone increases after exercise because it is a volatile byproduct of lipolysis.

Various applications of process plasma measurement with vacuum ultraviolet absorption spectroscopy

Various applications of process plasma measurement with vacuum ultraviolet absorption spectroscopy