VUV Beamline Research Articles

VUV absorption spectra of water and nitrous oxide by a double-duty differentially pumped gas filter.

The differentially pumped rare-gas filter at the end of the VUV beamline of the Swiss Light Source has been adapted to house a windowless absorption cell for gases. Absorption spectra can be recorded from 7 eV to up to 21 eV photon energies routinely, as shown by a new water and nitrous oxide absorption spectrum. By and large, the spectra agree with previously published ones both in terms of resonance energies and absorption cross sections, but that of N2O exhibits a small shift in the {\tilde{\bf D}} band and tentative fine structures that have not yet been fully described. This setup will facilitate the measurement of absorption spectra in the VUV above the absorption edge of LiF and MgF2 windows. It will also allow us to carry out condensed-phase measurements on thin liquid sheets and solid films. Further development options are discussed, including the recording of temperature-dependent absorption spectra, a stationary gas cell for calibration measurements, and the improvement of the photon energy resolution.

Performance of the merged APPLE-Knot undulator for soft x-ray beamline in medium energy ring

APPLE-Knot undulator can effectively solve the on-axis heat load problem and is proven to perform well in VUV beamline and soft x-ray beamline in high energy storage ring. However, for soft x-ray beamline in a medium energy ring, whether the APPLE-Knot undulator excels the APPLE undulator is still a question. Here, a merged APPLE-Knot undulator is studied to generate soft x-ray in a medium energy ring. Its advantages and problems are discussed. Though the on-axis heat load of the APPLE-Knot undulator is lower in linear polarization modes compared to the APPLE undulator, its flux is lower. The APPLE-Knot undulator shows no advantage when only fundamental harmonic is needed. However, in circular polarization mode, the APPLE-Knot undulator shows the ability to cover a broader energy range which can remedy the notable shortcoming of the APPLE undulator.

Threshold Photoelectron Spectrum of the Phenoxy Radical.

The high-resolution photoion mass-selected threshold photoelectron spectrum (ms-TPES) of the phenoxy radical (C6H5O•), produced by pyrolysis of anisole, was investigated at the VUV beamline of the Swiss Light Source. Adiabatic ionization energies have been determined to be 8.56, 9.42, 9.76, and 9.94 eV to the X̃+1A1, ã+3A2, Ã+1A2, and b̃+3B2 cation states, respectively, supported by DFT, WFT, and composite-method calculations. A ring deformation mode was found to be active upon ionization by Franck-Condon analysis and responsible for the vibrational structure of the TPES in all four ion states. While the X̃+1A1 and ã+3A2 states' assignment agrees with the literature, we revise the energetic order of the Ã+1A2 and b̃+3B2 cation states in the ms-TPES, based on a pronounced lifetime broadening of the excited triplet state. This is rationalized by strong coupling between the triplet states as confirmed by EOM-EE-CCSD calculations indicating a conical intersection with a low-lying seam to the ã+3A2 state. Our study provides a well-resolved spectrum, to be used for the isomer-selective assignment of reactive species in combustion and catalysis and also serves as benchmark to evaluate theoretical methods to address closed- and open-shell singlet and triplet cation intermediates.

Threshold Photoelectron Spectroscopy of Quinoxaline, Quinazoline, and Cinnoline

The threshold photoelectron spectra of cinnoline, quinazoline, and quinoxaline, three small naphthalene-analogue polycyclic nitrogen-containing hydrocarbons of C8H6N2 composition, were recorded. The spectra are assigned to understand their electronic structure and the role of isomerism. Furthermore, this work provides reference data for the selective identification of such species as gas-phase reaction products at low number densities. Imaging photoelectron photoion coincidence spectroscopy was used at the VUV beamline of the Swiss Light Source to record the spectra from the ionization onset to 12 eV. To assign and interpret the spectral features, we relied on (time-dependent) density functional theory and EOM-IP-CCSD calculations and computed vertical and adiabatic ionization energies as well as Franck-Condon factors to simulate ground- and excited-state spectra. Vibrational progressions belonging to four electronic states could be simulated in each of the samples, and we report a total of 12 adiabatic ionization energies, including the ones to the ground and excited cation states. Such a wealth of spectral information, as well as the reliable ab initio modeling, is promising with regards to analytical applications. While cinnoline can be easily distinguished by its lowest adiabatic ionization energy, quinazoline and quinoxaline show different vibrational fingerprints, which can be used to distinguish the three isomers even in complex reaction mixtures. Finally, we also relate the cation electronic states to the neutral molecular orbitals and note that Koopmans' approximation fails in these N2-containing species very much like it does in N2.

Photoionization of Two Potential Biofuel Additives: γ-Valerolactone and Methyl Butyrate.

The photoionization of two potential biofuel additives, γ-valerolactone (GVL, C5H8O2) and methyl butyrate (MB, C5H10O2) has been studied by imaging photoelectron photoion coincidence spectroscopy (iPEPICO) at the VUV beamline of the Swiss Light Source (SLS). The vibrational fine structure in the photoelectron spectrum is compared with a Franck-Condon simulation for the electronic ground-state band of the GVL cation. In the lowest energy dissociative photoionization channel of GVL, CO2 is lost, resulting in a 1-butene fragment ion with a 0 K appearance energy of E0 = 10.35 ± 0.01 eV. A newly calculated 1-butene ionization energy of 9.595 ± 0.015 eV establishes the reverse barrier height to CO2 loss as 66.6 ± 4.3 kJ mol-1. Methyl butyrate cations undergo McLafferty rearrangement, which explains the missing ion signal at the computed adiabatic ionization energy of 9.25 eV. After H transfer, ethylene is lost in the lowest energy dissociation channel to yield the methyl acetate enol ion at E0 = 10.24 ± 0.04 eV. This value connects the energetics of methyl butyrate with that of methyl acetate enol ion, which is established at ΔfHo0K[CH2C(OH)OCH3+] = 502 ± 6 kJ mol-1. Parallel to ethylene loss, methyl loss is also observed from the enol tautomer of the parent ion. Both samples exhibit low-energy nonstatistical dissociative ionization channels. In GVL, the methyl-loss abundance rises quickly but levels off suddenly in the energy range of the first electronically excited states, indicating nonstatistical competition between CH3 and CO2 loss. In MB, the major parallel dissociation channel is the loss of a methoxy radical. Calculations indicate that McLafferty rearrangement is inhibited on the excited-state surface. Indeed, breakdown curve modeling of this and a sequential CO-loss channel confirms a second statistical regime in dissociative photoionization, decoupled from ethylene loss.

A pressurized flow reactor combustion experiment interfaced with synchrotron double imaging photoelectron photoion coincidence spectroscopy

A new pressurized low-temperature combustion experiment has been commissioned at the Swiss Light Source, Paul Scherrer Institute. The experiment uses photoionization with tunable synchrotron radiation and double imaging photoelectron photoion coincidence (i2PEPICO) detection at the vacuum ultraviolet beamline. The experimental setup is described, including the high-pressure reactor experiment, sampling interface, and reactant delivery system. The CRF-PEPICO (Combustion Reactions Followed by Photoelectron Photoion Coincidence) endstation and VUV beamline are briefly elaborated. The novel aspects of the apparatus and the new components are elucidated in detail, such as the fluid supply system to the reactor and the reactor integration into the endstation. We also present a system overview of the experimental setup. The technical details are followed by a description of the experimental procedure used to operate the pressurized flow reactor setup. Finally, first experimental results demonstrating the capability of the setup are provided and analyzed. A major advantage of this new experiment is that the excellent isomer resolution capabilities of the i2PEPICO technique can be transferred to the investigation of reactions at elevated pressures of several bars. This enables the investigation of pressure effects on the reactivity of fuel mixtures and covers more realistic conditions found in technical combustors. The capability to obtain quantitative oxidation data is confirmed, and the main and certain intermediate species are quantified for a selected condition. The results show excellent agreement with a chemical kinetics model and previously published reference measurements performed with a gas chromatography setup.

Fast beam monitor diamond-based devices for VUV and X-ray synchrotron radiation applications.

The improved performance of third-generation light sources and the advent of next-generation synchrotron radiation facilities require the use of extremely precise monitoring of the main photon-beam parameters, such as position, absolute and relative intensity, and temporal structure. These parameters, and associated real-time feedbacks, are fundamental at the beamline control level and at the machine control level, to improve the stability of the photon beams and to provide bunch-to-bunch quantitative information. Fast response time, high radiation hardness and visible-blind response are main features of photon-beam monitors for VUV and X-ray synchrotron radiation beamlines; hence diamond-based detectors are outstanding candidates. Here, results are presented of an extensive measurement campaign aiming at optimizing the capabilities of diamond detectors to discern time structures below the 100 ps timescale. A custom-built device has been fabricated and tested at the Italian Synchrotron Radiation Laboratory Elettra in Trieste. The results obtained show that diamond is an excellent material for ultra-fast photon pulses with picosecond time resolution; finally the possibilities for application at free-electron laser sources are discussed.

Dissociative photoionization of chromium hexacarbonyl: A round-trip ticket to non-statisticality and a detective story in thermochemistry

The fragmentation processes of internal energy selected chromium hexacarbonyl cations, Cr(CO)6+, were investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy at the VUV beamline of the Swiss Light Source. In the 9.3–21.5 eV photon energy range, Cr(CO)6+ dissociates by six sequential carbonyl ligand losses. The fragment ion fractional abundances, plotted in the breakdown diagram, along with the time-of-flight mass spectra for the first three metastable CO-loss channels were modeled using a statistical approach. Between 12 and 16 eV, the statistical model overestimates the degree of fragmentation, which is explained by enhanced kinetic energy release in impulsive CO loss on repulsive electronic states of the parent ion Cr(CO)6+, as confirmed by TD-DFT calculations. This is the first reported example for an embedded non-statistical unimolecular dissociation regime, bracketed by statistical regimes at low and at high energies. The statistical model was employed to derive 0 K appearance energies for Cr(CO)n+ (n = 0–5). The Cr(CO)5+ appearance energy and the literature COCr(CO)5+ bond dissociation energy yield an adiabatic Cr(CO)6+ ionization energy of 8.195 ± 0.120 eV, independent of photoelectron spectroscopy measurements. The 0 K appearance energy of Cr+ is 61 kJ mol–1 higher than predicted based on literature enthalpies of formation, which we suggest is most likely due to an error in the enthalpy of formation of Cr(CO)6, and propose a revised ΔfH[Cr(CO)6, g] = –972.1 ± 4.1 and –968.9 ± 4.1 kJ mol–1 at 0 and 298 K, respectively, and ΔfH298K[Cr(CO)6, c] = –1040.6 ± 4.2 kJ mol–1 for the crystalline state, based on the known enthalpy of sublimation. The measured CrCO bond dissociation energies in [(CO)nCrCO]+ (n = 0–5), and the enthalpies of formation of the chromium carbonyl ion series are also reported.

Development of optical choppers for time-resolved measurements at soft X-ray synchrotron radiation beamlines.

Two types of optical choppers for time-resolved measurements at synchrotron radiation soft X-ray beamlines have been developed. One type uses an air-spindle-type rotation mechanism with a two-stage differential pumping system to maintain the ultra-high vacuum of the X-ray beamline, and the other uses a magnetic bearing. Both can be installed at the soft X-ray beamlines at SPring-8, greatly improving the accessibility of pump-and-probe spectroscopy. The combination of X-ray chopper and pump-and-probe photoemission electron microscope at SPring-8 provides drastic improvements in signal-to-noise ratio and resolution compared with techniques using high-voltage gating of channel plate detectors. The choppers have the capability to be used not only at synchrotron radiation facilities but also at other types of soft X-ray and VUV beamlines.

Single-Photon, Double Photodetachment of Nickel Phthalocyanine Tetrasulfonic Acid 4- Anions.

Single-photon, two-electron photodetachment from nickel phthalocyanine tetrasulfonic acid tetra anions, [NiPc](4-), was examined in the gas-phase using a linear ion trap coupled to the DESIRS VUV beamline of the SOLEIL Synchrotron. This system was chosen since it has a low detachment energy, known charge localization, and well-defined geometrical and electronic structures. A threshold for two-electron loss is observed at 10.2 eV, around 1 eV lower than previously observed double detachment thresholds on multiple charged protein anions. The photodetachment energy of [NiPc](4-) has been previously determined to be 3.5 eV and the photodetachment energy of [NiPc](3-•) is determined in this work to be 4.3 eV. The observed single photon double electron detachment threshold is hence 5.9 eV higher than the energy required for sequential single electron loss. Possible mechanisms are for double photodetachment are discussed. These observations pave the way toward new, exciting experiments for probing double photodetachment at relatively low energies, including correlation measurements on emitted photoelectrons.

The high-resolution absorption spectroscopy branch on the VUV beamline DESIRS at SOLEIL.

A VUV absorption spectroscopy facility designed for ultra-high spectral resolution is in operation as a dedicated branch on the DESIRS beamline at Synchrotron SOLEIL. This branch includes a unique VUV Fourier transform spectrometer (FTS) and a dedicated versatile gas sample chamber. The FTS instrument can cover a large UV-VUV spectral range from 4 to 30 eV, with an ultimate line width of 0.08 cm(-1) on a large spectral window, ΔE/E = 7%, over which all spectral features can be acquired in a multiplex way. The performance can be considered to be a middle ground between broadband moderate-resolution spectrometers based on gratings and ultra-high-spectral-resolution VUV tunable-laser-based techniques over very narrow spectral windows. The various available gaseous-sample-handling setups, which function over a wide range of pressures and temperatures, and the acquisition methodology are described. A selection of experimental results illustrates the performance and limitations of the FTS-based facility.

The plane grating monochromator beamline U49-2 PGM-1 at BESSY II

U49/2 PGM1 is one of HZB open-port VUV beamlines. Therefore and due to the fact that it delivers highest flux with very acceptable energy resolution it is the most heavily booked BESSY II beamline. Earlier work has largely focused on surface science and catalysis. After shut down of the former U41 PGM an increasing number of experiments on liquids and solutions are carried out.

Dissociative Ionization Mechanism and Appearance Energies in Adipic Acid Revealed by Imaging Photoelectron Photoion Coincidence, Selective Deuteration, and Calculations.

Adipic acid, a model compound for oxygenated organic aerosol, has been studied at the VUV beamline of the Swiss Light Source. Internal energy selected cations were prepared by threshold photoionization using vacuum ultraviolet synchrotron radiation and imaging photoelectron photoion coincidence spectroscopy (iPEPICO). The threshold photoelectron spectrum yields a vertical ionization energy (IE) of 10.5 eV, significantly above the calculated adiabatic IE of 8.6 eV. The cationic minimum is accessible after vertical ionization by H-transfer from one of the γ-carbons to a carbonyl oxygen and is sufficiently energetic to decay by water loss at the ionization onset. The slope of the breakdown curves, quantum chemical calculations, and selective deuteration of the carboxylic hydrogens establish the dissociative photoionization mechanism. After ionization, one γ-methylene hydrogen and the two carboxylic hydrogens are randomized prior to H2O loss. On the basis of the deuteration degree in the H2O + CO-loss product at higher energies, a direct water-loss channel without complete randomization also exists. The breakdown diagram and center of gravity of the H2O + CO-loss peak were modeled to obtain 0 K appearance energies of 10.77, 10.32, and 11.53 eV for H2O + CO loss, CH2COOH loss, and H2O + CH2COOH loss from adipic acid. These agree well with the CBS-QB3 calculated values of 10.68, 10.45, and 11.57 eV, respectively, which shows that threshold photoionization can yield energetics data as long as the dissociation is statistical, even when the parent ion cannot be observed. The results can be used as a starting point for a deeper understanding of the ionization and low-energy fragmentation of organic aerosol components.

Photodissociation of protonated Leucine-Enkephalin peptide in the VUV range

SynopsisWe present results of VUV action spectroscopy of gas-phase protonated Leucin-Enkephalin peptide. The experiment has been performed by coupling a linear quadrupole ion trap with a synchrotron radiation VUV beamline.

Design of a high flux VUV beamline for low energy photons**Supported by National Natural Science Foundation of China (10979021, 11027401, 11174054, 11304338, 11227902), the Ministry of Science and Technology of China (2011CB921800) and “Strategic Priority Research Program(B)” of Chinese Academy of Sciences (XDB04010100)

A VUV beamline at the Shanghai Synchrotron Radiation Facility (SSRF) for angle-resolved photoelectron spectroscopy (ARPES) measurements is designed. To increase the resolution and bulk sensitivity, a photon energy as low as 7 eV is desired. Because the reflectivity for p-polarized photons strongly decreases when the photon energy is below 30 eV, the design of a high flux beamline for low energy VUV photons is a challenge. This work shows a variable including angle Varied Line-space Plane-Grating Monochromator (VLPGM) with varied grating depth (VGD) which can achieve both high resolution and high flux with broad energy coverage.

Dissociative photoionization of quinoline and isoquinoline.

Two nitrogen-containing polycyclic aromatic hydrocarbon isomers of C9H7N composition, quinoline, and isoquinoline have been studied by imaging photoelectron photoion coincidence spectroscopy at the VUV beamline of the Swiss Light Source. High resolution threshold photoelectron spectra have been recorded and are interpreted applying a Franck-Condon model. Dissociative ionization mass spectra as a function of the parent ion internal energy are analyzed with the use of breakdown diagrams. HCN loss and H loss are the dominant dissociation paths for both C9H7N(•+) isomers at photon energies below 15.5 eV. Computed C9H7N(•+) potential energy surfaces suggest that the lowest energy path leading to HCN-loss yields the benzocyclobutadiene cation. A statistical model is used to fit the breakdown diagram and-to account for the kinetic shift-the time-of-flight mass spectra that reveal the dissociation rates. We have derived appearance energies of 11.9 ± 0.1 (HCN loss) and 12.0 ± 0.1 (H loss), as well as 11.6 ± 0.2 (HCN loss) and 12.1 ± 0.2 (H loss) eV, for the dissociative ionization of quinoline and isoquinoline, respectively. The results are compared to a recent study on the dissociative ionization of naphthalene. Implications for the formation and destruction of nitrogenated PAHs in the interstellar medium and in Titan's atmosphere are highlighted.

A new extra-focus monochromator designed for high-performance VUV beamlines.

A new monochromator called an extra-focus constant-included-angle varied-line-spacing (VLS) cylindrical-grating monochromator (extra-focus CIA-VCGM) is described. This monochromator is based on the Hettrick-Underwood scheme where the plane VLS grating is replaced by a cylindrical one in order to zero the defocus at three reference photon energies in the vacuum-ultraviolet range. It has a simple mechanical structure and a fixed focus spot with high performance over a wide energy range. Furthermore, its mechanical compatibility with a standard VLS plane-grating monochromator allows convenient extension into the soft-X-ray range.

Photostimulated phosphor based image plate detection system for HRVUV beamline at Indus-1 synchrotron radiation source

A high resolution vacuum ultraviolet (HRVUV) beamline based on a 6.65m off-plane Eagle spectrometer is in operation at the Indus-1 synchrotron radiation source, RRCAT, Indore, India. To facilitate position sensitive detection and fast spectral recording, a new BaFBr:Eu2+ phosphor based image plate (IP) detection system interchangeable with the existing photomultiplier (PMT) scanning system has been installed on this beamline. VUV photoabsorption studies on Xe, O2, N2O and SO2 are carried out to evaluate the performance of the IP detection system. An FWHM of ~0.5Å is achieved for the Xe atomic line at 1469.6Å. Reproducibility of spectra is found to be within the experimental resolution. Compared to the PMT scanning system, the IP shows several advantages in terms of sensitivity, recording time and S/N ratio, which are highlighted in the paper. This is the first report of incorporation of an IP detection system in a VUV beamline using synchrotron radiation. Commissioning of the new detection system is expected to greatly enhance the utilization of the HRVUV beamline as a number of spectroscopic experiments which require fast recording times combined with a good signal to noise ratio are now feasible.

In situ flame chemistry tracing by imaging photoelectron photoion coincidence spectroscopy

Adaptation of a low-pressure flat flame burner with a flame-sampling interface to the imaging photoelectron photoion coincidence spectrometer (iPEPICO) of the VUV beamline at the Swiss Light Source is presented. The combination of molecular-beam mass spectrometry and iPEPICO provides a new powerful analytical tool for the detailed investigation of reaction networks in flames. First results demonstrate the applicability of the new instrument to comprehensive flame diagnostics and the potentially high impact for reaction mechanism development for conventional and alternative fuels. Isomer specific identification of stable and radical flame species is demonstrated with unrivaled precision. Radical detection and identification is achieved for the initial H-abstraction products of fuel molecules as well as for the reaction controlling H, O, and OH radicals. Furthermore, quantitative evaluation of changing species concentrations during the combustion process and the applicability of respective results for kinetic model validation are demonstrated. Utilization of mass-selected threshold photoelectron spectra is shown to ensure precise signal assignment and highly reliable spatial profiles.

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.