VUV Synchrotron Radiation Research Articles

Dose and wavelength dependent study of graphene oxide photoreduction with VUV Synchrotron radiation

Photoreduction has so far received less attention than other approaches used to reduce graphene oxide (GO) like thermal and chemical reduction, although its potential is huge. The mechanism of deoxygenation is still questioned. In photolithographic applications, this means that one cannot predict whether the minimum feature size is close to the diffraction limit or larger. In this work we have studied GO photoreduction with vacuum ultraviolet Synchrotron radiation. GO has been exposed to extreme-UV and soft-X-ray radiation and the ratio between C–O and sp2 C–C bonds has been measured by photoemission spectroscopy as a function of dose and number of photons. The deoxygenation rate has been demonstrated to be proportional to the imaginary part f2 of the oxygen scattering factor. Experiments at the wavelengths where the f2 of oxygen is close to its maximum (namely close to 2nm and between 20nm and 60nm) have demonstrated that GO can be rapidly reduced (even few minutes exposure time) with doses well below those needed for photothermal heating (the corresponding temperature rise is ∼1μK) and without damaging the basal plane (the in-plane sp2 C–C bonds are not broken). Efficient nanometer scale GO photopatterning is therefore demonstrated to be possible.

Mechanism of Fast Pyrolysis of Lignin: Studying Model Compounds

Fast pyrolysis of lignin is one of the most promising methods to convert the complex and irregular structure of lignin into renewable chemicals and fuel. During pyrolysis the complex set of radical reactions, rearrangements, and eliminations is influenced by temperature, pressure, and the lignin origin and structure. This model compound study aims to understand reaction pathways and how primary intermediates lead to the observed product selectivity. The pyrolysis microreactor directly connected to the gas chromatograph with a mass spectrometer (py-GC/MS) detects the final products, while imaging photoelectron photoion coincidence (iPEPICO) with VUV synchrotron radiation shows primary decomposition radicals. The tested model compounds, diphenylether (DPE) and ortho-methoxyphenol (guaiacol), represent a common lignin linkage and the most present subunit in lignin, respectively. Radical fragments, such as the hydroxycyclopentadienyl radical in guaiacol decomposition, are identified by mass-selected threshold photoelectron spectra (ms-TPES) in excellent agreement with the Franck-Condon simulation. While homolysis produces phenoxy-, phenyl-, and hydroxyphenoxy radicals, which are observed in high vacuum, radically initiated reactions are dominant in ambient conditions and produce recombination and rearrangement products, such as 2-hydroxybenzaldehyde in the case of guaiacol. The degree of substitution plays a dominant role in both the stabilization of the intermediate radical and the following degree of recombination. The recombination of phenoxy radicals is enhanced compared to hydroxy-phenoxy radicals.

Isomer-Specific Product Detection of Gas-Phase Xylyl Radical Rearrangement and Decomposition Using VUV Synchrotron Photoionization

Xylyl radicals are intermediates in combustion processes since their parent molecules, xylenes, are present as fuel additives. In this study we report on the photoelectron spectra of the three isomeric xylyl radicals and the subsequent decomposition reactions of the o-xylyl radical, generated in a tubular reactor and probed by mass selected threshold photoelectron spectroscopy and VUV synchrotron radiation. Franck-Condon simulations are applied to augment the assignment of elusive species. Below 1000 K, o-xylyl radicals decompose by hydrogen atom loss to form closed-shell o-xylylene, which equilibrates with benzocyclobutene. At higher temperatures relevant to combustion engines, o-xylylene generates styrene in a multistep rearrangement, whereas the p-xylylene isomer is thermally stable, a key point of difference in the combustion of these two isomeric fuels. Another striking result is that all three xylyl isomers can generate p-xylylene upon decomposition. In addition to C8H8 isomers, phenylacetylene and traces of benzocyclobutadiene are observed and identified as further reaction products of o-xylylene, while there is also some preliminary evidence for benzene and benzyne formation. The experimental results reported here are complemented by a comprehensive theoretical C8H8 potential energy surface, which together with the spectroscopic assignments can explain the complex high-temperature chemistry of o-xylyl radicals.

Luminescence spectroscopy and electronic structure of the PbMoO4 and Pb2MoO5 single crystals

Intrinsic photoluminescence (PL) and electronic structure of Pb2MoO5 single crystal are first described and compared with corresponding properties of PbMoO4 crystal. The PL properties of PbMoO4 and Pb2MoO5 crystals grown by Czochralski method are studied under the VUV synchrotron radiation excitations (3.7–14eV region of excitation photon energies) at T=8K. The electronic band structures of perfect PbMoO4 and Pb2MoO5 are calculated by the Full-Potential Linear Augmented Plane Wave method. The Pb2MoO5 crystals reveal intensive PL band in the yellow–red spectral region peaking at ∼600nm. All studied PbMoO4 samples reveal intensive PL band in the green spectral region and this band is accompanied by additional fast-decaying (τ≈3.5ns) component in the violet when the samples are grown with 30° orientation of seed with respect to the c axis. The excitation spectra of the violet component of PbMoO4 reveal complex excitation band with main peak at 6.2eV. It is assumed that the violet PL component of PbMoO4 is generated by intra-center excitations from the ground to singlet excited states 1A1–1T1,2 of the MoO42− molybdate groups located near point defects in subsurface layer of the crystal. Strong anisotropy of reflectivity spectra of PbMoO4 below 6eV is a consequence of peculiarities of the electronic structure of the crystal and reflectivity bands at ∼3.6 and ∼4.3eV are formed by band-to-band electronic transitions.

Synchrotron Radiation Study of Interconfigurational 5d-4f Luminescence of Pr3+ in KLuP2O7

The double phosphate KLuP2O7 doped with Pr3+ ions was prepared by solid-state reaction. The material obtained was characterized by powder X-ray diffraction. The luminescence spectroscopy and the excited state dynamics of this material were studied upon excitation with VUV synchrotron radiation. The 5d-4 f emission of Pr3+ upon both direct and band gap excitation was detected and assigned. The decay kinetics of the Pr3+ 5d-4 f emission is characterized by a decay time of about 20 ns and is nearly temperature independent within the range 8 - 300 K. Both dynamics and energy transfer peculiarities revealed in the study suggested good potentials for application of KLuP2O7:Pr3+ as a fast scintillator material.

A photoionization investigation of small, homochiral clusters of glycidol using circularly polarized radiation and velocity map electron–ion coincidence imaging

A detailed study of the valence photoionization of small homochiral glycidol (C3O2H6) clusters is carried out with the help of circularly-polarized VUV synchrotron radiation by recording photoionization-based spectroscopic data detected by velocity map electron imaging with coincidence ion selection. We show that information on the stability of cationic as well as neutral chiral clusters can be obtained with enhanced sensitivity by examining the chiral fingerprint encapsulated in Photoelectron Circular Dichroism (PECD) spectra. In particular, by varying the clustering conditions we demonstrate that the PECD signal effectively carries the signature of the neutral precursor species, prior to any fragmentation of the ion, as may be inferred from the below-threshold monomer measurements (including ion imaging). Here the monomer's direct ionization channel is closed and the monomer ion hence must result exclusively as a fragment from dissociative ionization of the dimer (or higher) clusters. At higher photon energies, the mass-selection on the electron spectroscopy data, achieved through filtering the electron images in coincidence with selected ion masses, evidently succeeds in providing a degree of size-selection on the neutral clusters being ionized with, in particular, a clear differentiation of monomer and dimer PECD, showing the strong sensitivity of this chiroptical effect to the non-local long-range molecular potential.

Direct Observation of para-Xylylene as the Decomposition Product of the meta-Xylyl Radical Using VUV Synchrotron Radiation

Xylyl (methylbenzyl) radicals are important combustion intermediates, formed in the pyrolysis and oxidation of xylenes and other substituted aromatic fuel additives. We have used VUV synchrotron radiation and imaging photoelectron photoion coincidence (iPEPICO) spectroscopy techniques to identify para-xylylene as the dominant stable C8H8 product arising from thermal decomposition of the meta-xylyl radical. A complex rearrangement from a meta- to a para-substituted aromatic, supported by quantum chemical calculations, can rationalize the observed reaction products. This work provides the first experimental evidence for the pyrolysis products of the meta-xylyl radical and can explain why the decomposition of this radical is considerably slower than that of the ortho and para isomers. This study emphasizes the utility of VUV synchrotron radiation and iPEPICO spectroscopy to tackle the reaction mechanism of combustion-relevant processes.

Threshold Photoionization of Fluorenyl, Benzhydryl, Diphenylmethylene, and Their Dimers

Two π-conjugated radicals, fluorenyl (C13H9) and benzhydryl (C13H11), as well as the carbene diphenylmethylene (C13H10) were studied by imaging photoelectron-photoion coincidence spectroscopy using VUV synchrotron radiation. The reactive intermediates were generated by flash pyrolysis from 9-bromofluorene and α-aminodiphenylmethane (adpm), respectively. Adiabatic ionization energies (IEad) for all three species were extracted. Values of 7.01 ± 0.02 eV for fluorenyl and 6.7 ± 0.1 eV for benzhydryl are reported. For the triplet diphenylmethylene, an IEad of 6.8 ± 0.1 eV is found. The dissociative photoionization of 9-bromofluorene, the precursor for fluorenyl, was also studied and modeled with an SSACM approach, yielding an appearance energy AE0K(C13H9(+)/C13H9Br) of 9.4 eV. All experimental values are in very good agreement with computations. For fluorenyl, the IEad agrees well with earlier values, while for the benzhydryl radical, we report a value that is more than 0.6 eV lower than the one previously reported. The geometry change upon ionization is small for all three species. Although individual vibrational bands cannot be resolved, some vibrational transitions in the threshold photoelectron spectrum of fluorenyl are tentatively assigned based on a Franck-Condon simulation. In addition, the dimerization products of fluorenyl and the benzhydryl radical were detected. Ionization energies of (7.69 ± 0.04) and (8.11 ± 0.04) eV were determined for C26H18 and C26H22, respectively. On the basis of the ionization energies, we identified both molecules to be the direct dimerization products, formed in the pyrolysis without further rearrangement. Both dimers might be expected to play a role in soot formation because the radical monomers do appear in flames.

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

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

Valence photodissociation of trifluoroethyl iodide investigated by photoelectron photoion coincidence spectroscopy

Dissociative photoionization of trifluoroethyl iodide (C2H2F3I) molecule has been investigated by electron-ion coincidence technique (PEPICO) and VUV synchrotron radiation. Mass spectra have been recorded for this molecule in the photon energy range of 10–22eV. The molecular ion as well as cationic fragments have been detected and analyzed as a function of photon energy and the main dissociation pathways are proposed. We also performed ab initio calculations for the neutral molecule, its cation and the ion fragments in order to determine electronic and structural parameters.

Application of VUV synchrotron radiation to proteomic and analytical mass spectrometry

Recently, the coupling of ion traps with VUV beamlines has allowed performing activation of large biomolecular ions formed by electrospray. We discuss the potential of this new field along with the set up of an ion trap at the DESIRS beamline at SOLEIL. Study of primary structure of proteins becomes now possible at synchrotron facilities.

Slow Photoelectron Spectroscopy of 3-Hydroxyisoquinoline

We studied the single photon ionization of gas phase 3-hydroxyisoquinoline by means of VUV synchrotron radiation coupled to a velocity map imaging electron/ion coincidence spectrometer. Near the ionization thresholds of 3-hydroxyisoquinoline, the photoionization is found to occur mainly via a direct process. The spectra are assigned with the help of theoretical calculations on the equilibrium geometries, electronic states patterns, harmonic and anharmonic wavenumbers of the lactim and lactam forms of 3-hydroxyisoquinoline and their cations. The slow photoelectron spectrum (SPES) of this lactim is dominated by vibrational transitions to the X̃ state of the cation. In addition, several weaker and complex bands are observed, corresponding to the population of the vibrational bands (pure or combination) of the à electronically excited state of the cation. The adiabatic ionization energy of 3-hydroxyisoquinoline and the lowest electronic state energetics of the lactim and lactam cationic forms are determined.

Comparative study of the luminescence of Al2O3:Ti and Al2O3 crystals under VUV synchrotron radiation excitation

The comparative study of the luminescent properties of Al2O3:Ti crystal in comparison with those for undoped Al2O3 crystal counterpart is performed under synchrotron radiation excitation with an energy of 3.7–25eV. Apart from the main emission band peaked at 725nm related to the 2E→2T2 radiative transitions of Ti3+ ions, the luminescence of excitons localized around Ti ions in the band peaked at 290nm and the luminescence of F+–Ti and F–Ti centers in the bands peaked at 325 and 434nm are also found in the emission spectra of Al2O3:Ti crystal. We show also that the luminescence of Ti3+ ions in Al2O3:Ti crystal can be effectively excited by the luminescence of excitons localized around Ti dopant as well as by the luminescence of F–Ti centers.

Theoretical and experimental study of VUV ionization and fragmentation of size-selected selenium microclusters

Ab initio and mass-resolved fragmentation study of thermally evaporated selenium clusters Sen induced by VUV synchrotron radiation.

Combined Crossed Beam and Theoretical Studies of the N(2D) + C2H4Reaction and Implications for Atmospheric Models of Titan

The dynamics of the H displacement channels in the reaction N((2)D) + C(2)H(4) have been investigated by the crossed molecular beam technique with mass spectrometric detection and time-of-flight analysis at two different collision energies (17.2 and 28.2 kJ/mol). The interpretation of the scattering results is assisted by new electronic structure calculations of stationary points and product energetics for the C(2)H(4)N ground state doublet potential energy surface. RRKM statistical calculations have been performed to derive the product branching ratio under the conditions of the present experiments and of the atmosphere of Titan. Similarities and differences with respect to a recent study performed in crossed beam experiments coupled to ionization via tunable VUV synchrotron radiation are discussed (Lee, S.-H.; et al. Phys. Chem. Chem. Phys.2011, 13, 8515-8525). Implications for the atmospheric chemistry of Titan are presented.

Structure and Charge‐State Dependence of the Gas‐Phase Ionization Energy of Proteins

Photoionization of protein ions: The ionization energy of polyprotonated protein cations in the gas phase measured using VUV synchrotron radiation appears to be correlated with the charge state z of the protein and its tertiary structure. A simple electrostatic model accounts for the results and also shows predictive capabilities to derive a mean radius Rm of the protein ion from the ionization energy, and vice versa.

Numerical simulation of energy relaxation processes in a ZnMoO4 single crystal

We present the results of our experimental investigation and numerical simulation of excitation-energy relaxation processes in zinc molybdate crystals. We show that our kinetic model of the energy relaxation makes it possible to describe basic features of experimental results. Using this model, we estimate the trap concentration in ZnMoO4 upon irradiation of the crystal by VUV synchrotron radiation and X-ray radiation. We conclude that prolonged phosphorescence of ZnMoO4 that is observed after irradiation of the crystal by X-ray radiation can be caused by the occurrence of additional traps with a low activation energy.

Fast in vacuo photon shutter for synchrotron radiation quadrupole ion trap tandem mass spectrometry

A specific mechanical beam shutter has been designed as a part of a novel experimental setup for tandem mass spectrometry and ion spectroscopy of electrosprayed ions in a linear quadrupole ion trap using VUV and soft X-ray synchrotron radiation. This low-cost and robust shutter controls the photon irradiation of the ions in the trap, starting at a desired moment and during a well-defined period of time, with fast cut-off (on) of the photon beam (<1ms rise time). At the same time, it is characterized by a stable and a reproducible operation over day-long non-stop beamtime duration, under high vacuum conditions (10−7mbar).

Thermal Desorption/Tunable Vacuum–Ultraviolet Time-of-Flight Photoionization Aerosol Mass Spectrometry for Investigating Secondary Organic Aerosols in Chamber Experiments

This paper describes thermal desorption/tunable vacuum-ultraviolet photoionization time-of-flight aerosol mass spectrometry (TD-VUV-TOF-PIAMS) for the real-time analysis of secondary organic aerosols (SOAs) in smog chamber experiments. SOAs are sampled directly from atmospheric pressure and are focused through an aerodynamic lens assembly into the mass spectrometer. Once the particles have entered the source region, they impact on a heater and are vaporized. The nascent vapor is then softly ionized by tunable VUV synchrotron radiation. TD-VUV-TOF-PIAMS was used in conjunction with the smog chamber to study SOA formation from the photooxidation of toluene with hydroxyl radicals. The ionization energies (IEs) of these SOA products are sometimes very different with each other. As the ideal photon source is tunable, its energy can be adjusted for each molecular to be ionized. The mass spectra obtained at different photon energies are then to be useful for molecular identification. Real-time analysis of the mass spectra of SOAs is compared with previous off-line measurements. These results illustrate the potential of TD-VUV-TOF-PIAMS for direct molecular characterization of SOAs in smog chamber experiments.

Chemical synthesis in acetonitrile containing discharges. Insights from photoionization experiments with synchrotron radiation

The photoinduced ion chemistry of acetonitrile is investigated by using tuneable VUV synchrotron radiation in an octupolar ion guide with mass spectrometric detection. The appearance energies and yields of primary photoions and secondary ionic products deriving from self-reactions at thermal collision energies are measured. Comparison of the appearance energies of the different product ions provides insights into the reaction mechanisms. Experimental data are interpreted with the help of theoretical calculations (energies and structures at the G3 level of theory) for the various isomers of the most relevant ionic products. We find that the ion C2H2N+ is formed at the threshold with a cyclic structure, but only its linear isomer (lying 0.47eV higher in energy) can react with acetonitrile to form CH2CNCH2+ plus HCN via a cyclic transition state.