Vacuum Ultraviolet Beam Research Articles

Development of dual-beamline photoelectron momentum microscopy for valence orbital analysis.

The soft X-ray photoelectron momentum microscopy (PMM) experimental station at the UVSOR Synchrotron Facility has been recently upgraded by additionally guiding vacuum ultraviolet (VUV) light in a normal-incidence configuration. PMM offers a very powerful tool for comprehensive electronic structure analyses in real and momentum spaces. In this work, a VUV beam with variable polarization in the normal-incidence geometry was obtained at the same sample position as the soft X-ray beam from BL6U by branching the VUV beamline BL7U. The valence electronic structure of the Au(111) surface was measured using horizontal and vertical linearly polarized (s-polarized) light excitations from BL7U in addition to horizontal linearly polarized (p-polarized) light excitations from BL6U. Such highly symmetric photoemission geometry with normal incidence offers direct access to atomic orbital information via photon polarization-dependent transition-matrix-element analysis.

New insights into the oxidation chemistry of pyrrole, an N-containing biomass tar component

Pyrrole, the smallest molecule with a nitrogen atom in the heterocycle ring, is an important tar component from coal and nitrogen-rich biomass devolatilization. Understanding the combustion chemistry of pyrrole can help to elucidate the pollutant formation chemistry from fuel nitrogen, thus enabling cleaner biomass energy utilization technologies. Experimental measurements were performed in a jet stirred reactor coupled with time of flight molecular beam mass spectrometry using synchrotron vacuum ultraviolet beam as photon ionization source, and gas chromatography-mass spectrometry to provide comprehensive measurements of 31 species including nine C4 and C5 N-containing compounds. Based on the evidence from the experiments and aiming to improve the kinetic model performance, possible formation routes are proposed with OH addition as the entrance reaction. Reaction rate coefficients for the OH addition channel as well as those for key H-atom abstraction reactions (H, OH, CH3, and HO2) were calculated by quantum chemical methods and updated in the model. The updated model can qualitatively predict the identified C4 N-containing species and perform reasonably well for a large set of experimental data considered for validation, overall improving the performance of the previous model. The influence of the investigated reactions on the predictions of fuel reactivity and pollutant formation motivates further investigations of N-containing fuel chemistry.

Characterization of the high harmonics source for the VUV ellipsometer at ELI Beamlines

In this paper, the authors present the characterization experiments of a 20 fs vacuum ultraviolet beam from a high harmonic generation source. The beam hits a silicon sample and passes a triple reflection gold polarizer located inside an ultrahigh vacuum chamber. The polarizer’s Malus curve was obtained; the total acquisition time for each point of the curve was 30 s. This aims to be the first vacuum ultraviolet time-resolved user station dedicated to ellipsometry. The high harmonic beam is generated by a 12 mJ, 1 kHz, 20 fs, in-house-developed laser and detected by a back-illuminated charge-coupled device.

Photo-processing of astro-PAHs

Polycyclic aromatic hydrocarbons (PAHs) are key species in astrophysical environments in which vacuum ultraviolet (VUV) photons are present, such as star-forming regions. The interaction with these VUV photons governs the physical and chemical evolution of PAHs. Models show that only large species can survive. However, the actual molecular properties of large PAHs are poorly characterized and the ones included in models are only an extrapolation of the properties of small and medium-sized species. We discuss here experiments performed on trapped ions including some at the SOLEIL VUV beam line DESIRS. We focus on the case of the large dicoronylene cation, , and compare its behavior under VUV processing with that of smaller species. We suggest that C2 H2 is not a relevant channel in the fragmentation of large PAHs. Ionization is found to largely dominate fragmentation. In addition, we report evidence for a hydrogen dissociation channel through excited electronic states. Although this channel is minor, it is already effective below 13.6 eV and can significantly influence the stability of astro-PAHs. We emphasize that the competition between ionization and dissociation in large PAHs should be further evaluated for their use in astrophysical models.

Bright multicolor emitting phosphors Ba2Gd(BO3)2Cl: RE3+ (RE = Dy, Sm, Tb, Eu) for multifunctional application

Single RE3+ (RE = Eu3+, Tb3+, Dy3+, Sm3+) doped Ba2Gd(BO3)2Cl phosphors were synthesized by the high temperature solid state reaction. Spectroscopic properties of Ba2Gd(BO3)2Cl: RE3+ under vacuum ultraviolet (VUV), ultraviolet (UV) light and low-voltage electron beam excitations were investigated. The Ba2Gd(BO3)2Cl: Dy3+ shows a white light emission under VUV, UV light and electron beam excitation, respectively. Upon different excited source excitation, Ba2Gd(BO3)2Cl: Tb3+ shows similar yellowish-green emissions. Ba2Gd(BO3)2Cl: Sm3+ and Ba2Gd(BO3)2Cl: Eu3+ phosphors show bright orange–red and red emitting under VUV, UV light and electron beam excitation. These results indicated that the Ba2Gd(BO3)2Cl: RE3+ phosphors can be considered as a bright multicolor phosphors for multifunctional application, such as Hg-free lamps, PDPs, W-LEDs and FEDs.

Effects of plasma and vacuum-ultraviolet exposure on the mechanical properties of low-k porous organosilicate glass

The effects of plasma exposure and vacuum-ultraviolet (VUV) irradiation on the mechanical properties of low-k porous organosilicate glass (SiCOH) dielectric films were investigated. Nanoindentation measurements were made on SiCOH films before and after exposure to an electron-cyclotron-resonance plasma or a monochromatic synchrotron VUV beam, to determine the changes of film hardness, elastic modulus, and crack threshold due to these exposures. This permits the effects of ion bombardment and photon bombardment to be analyzed separately. The role of energetic ions was examined with a variety of inert plasma-exposure conditions. The role of VUV photons was analyzed as a function of synchrotron photon energy. It was found that both energetic ions and VUV photons with energies larger than the bond energy of the Si-O bond cause a significant increase in film hardness along with a smaller increase in elastic modulus and crack threshold. Differential Fourier transform infrared spectra and x-ray photoemission spectroscopy results show that the energetic ions affect the SiCOH properties mainly through physical bombardment, during which the ions transfer their momentum to the Si-O-Si backbone and transform them into more energetically stable Si-O-Si network structures. This results in the Si-O-Si network structures becoming densified. VUV photons assist reaction that increase the number of bridging O3≡Si-O-Si≡O3 bonds and deplete nonbridging O3≡Si-O and C-Si≡O3 bonds. This increased degree of cross linking in porous organosilicate dielectrics can substantially enhance their hardness and elastic modulus while showing no significant film shrinkage or densification.

Absolute Photoionization Cross Section of the Ethyl Radical in the Range 8–11.5 eV: Synchrotron and Vacuum Ultraviolet Laser Measurements

The absolute photoionization cross section of C(2)H(5) has been measured at 10.54 eV using vacuum ultraviolet (VUV) laser photoionization. The C(2)H(5) radical was produced in situ using the rapid C(2)H(6) + F → C(2)H(5) + HF reaction. Its absolute photoionization cross section has been determined in two different ways: first using the C(2)H(5) + NO(2) → C(2)H(5)O + NO reaction in a fast flow reactor, and the known absolute photoionization cross section of NO. In a second experiment, it has been measured relative to the known absolute photoionization cross section of CH(3) as a reference by using the CH(4) + F → CH(3) + HF and C(2)H(6) + F → C(2)H(5) + HF reactions successively. Both methods gave similar results, the second one being more precise and yielding the value: σ(C(2)H(5))(ion) = (5.6 ± 1.4) Mb at 10.54 eV. This value is used to calibrate on an absolute scale the photoionization curve of C(2)H(5) produced in a pyrolytic source from the C(2)H(5)NO(2) precursor, and ionized by the VUV beam of the DESIRS beamline at SOLEIL synchrotron facility. In this latter experiment, a recently developed ion imaging technique is used to discriminate the direct photoionization process from dissociative ionization contributions to the C(2)H(5)(+) signal. The imaging technique applied on the photoelectron signal also allows a slow photoelectron spectrum with a 40 meV resolution to be extracted, indicating that photoionization around the adiabatic ionization threshold involves a complex vibrational overlap between the neutral and cationic ground states, as was previously observed in the literature. Comparison with earlier photoionization studies, in particular with the photoionization yield recorded by Ruscic et al. is also discussed.

Nanograting-based compact vacuum ultraviolet spectrometer and beam profiler for in situ characterization of high-order harmonic generation light sources

A compact, versatile device for vacuum ultraviolet (VUV) beam characterization is presented. It combines the functionalities of a VUV spectrometer and a VUV beam profiler in one unit and is entirely supported by a standard DN200 CF flange. The spectrometer employs a silicon nitride transmission nanograting in combination with a microchannel plate-based imaging detector. This enables the simultaneous recording of wavelengths ranging from 10 to 80 nm with a resolution of 0.25-0.13 nm. Spatial beam profiles with diameters up to 10 mm are imaged with 0.1 mm resolution. The setup is equipped with an in-vacuum translation stage that allows for in situ switching between the spectrometer and beam profiler modes and for moving the setup out of the beam. The simple, robust design of the device is well suited for nonintrusive routine characterization of emerging laboratory- and accelerator-based VUV light sources. Operation of the device is demonstrated by characterizing the output of a femtosecond high-order harmonic generation light source.

Tunable VUV photochemistry using Rydberg H-atom time-of-flight spectroscopy

In this article, we report an experimental method for studying tunable vacuum ultraviolet (VUV) photochemistry using the H-atom Rydberg tagging technique. In this method, two VUV laser light beams were generated using nonlinear four-wave mixing scheme in a single Kr gas cell: one VUV beam is fixed at the 121.6 nm wavelength to probe the H-atom product through the Lyman alpha transition, the other beam can be tunable for photodissociating molecules in the wavelength range lambda(VUV)=121-190 nm. Preliminary results on the H(2)O photodissociation in the B state are reported here. These results suggest that the experimental method is a powerful tool for investigating photodissociation dynamics in the VUV region for molecules involving H-atom processes.

Photodissociation dynamics of fluorobenzene (C6H5F) at 157 and 193nm: Branching ratios and distributions of kinetic energy

Following photodissociation of fluorobenzene (C6H5F) at 193 and 157 nm, we detected the products with fragmentation-translational spectroscopy by utilizing a tunable vacuum ultraviolet beam from a synchrotron for ionization. Between two primary dissociation channels observed upon irradiation at 193 (157) nm, the HF-elimination channel C6H5F --> HF + C6H4 dominates, with a branching ratio of 0.94+/-0.02 (0.61+/-0.05) and an average release of kinetic energy of 103 (108) kJ mol(-1); the H-elimination channel C6H5F --> H + C6H4F has a branching ratio of 0.06+/-0.02 (0.39+/-0.05) and an average release of kinetic energy of 18.6 (26.8) kJ mol(-1). Photofragments H, HF, C6H4, and C6H4F produced via the one-photon process have nearly isotropic angular distributions. Both the HF-elimination and the H-elimination channels likely proceed via the ground-state electronic surface following internal conversion of C6H5F; these channels exhibit small fractions of kinetic energy release from the available energy, indicating that the molecular fragments are highly internally excited. We also determined the ionization energy of C6H4F to be 8.6+/-0.2 eV.

Scalable multichannel micromachining with pseudo-nondiffracting vacuum ultraviolet beam arrays generated by thin-film axicons

Scalable multichannel microstructuring was demonstrated by multiplexing a focused molecular fluorine laser beam with cylindrical nanolayer microaxicons into an array of converging pseudo-nondiffracting subbeams. The axicons were fabricated by shadow-mask vapor deposition of magnesium fluoride onto substrates of identical material. Long-period surface gratings of variable pitch were generated on poly(methyl methacrylate) by varying the target position within the converging periodic focal lines.

Photodissociation dynamics of formyl fluoride (HFCO) at 193 nm: Branching ratios and distributions of kinetic energy

Following photodissociation of formyl fluoride (HFCO) at 193 nm, we detected products with fragmentation translational spectroscopy utilizing a tunable vacuum ultraviolet beam from a synchrotron for ionization. Among three primary dissociation channels observed in this work, the F-elimination channel HFCO-->HCO+F dominates, with a branching ratio approximately 0.66 and an average release of kinetic energy approximately 55 kJ mol(-1); about 17% of HCO further decomposes to H+CO. The H-elimination channel HFCO-->FCO+H has a branching ratio approximately 0.28 and an average release of kinetic energy approximately 99 kJ mol(-1); about 21% of FCO further decomposes to F+CO. The F-elimination channel likely proceeds via the S1 surface whereas the H-elimination channel proceeds via the T1 surface; both channels exhibit moderate barriers for dissociation. The molecular HF-elimination channel HFCO-->HF+CO, correlating with the ground electronic surface, has a branching ratio of only approximately 0.06; the average translational release of 93 kJ mol(-1), approximately 15% of available energy, implies that the fragments are highly internally excited. Detailed mechanisms of photodissociation are discussed.

Tracking ground state Ba+ions in an expanding laser–plasma plume using time-resolved vacuum ultraviolet photoionization imaging

We report results from a study of the integrated column density and expansion dynamics of ground-state-selected Ba+ions in a laser–plasma plume using a new experimental system—VPIF (vacuum-ultraviolet photoabsorption imaging facility). The ions are tracked by recording the attenuation of a pulsed and collimated vacuum ultraviolet beam, tuned to the 5p–6dinner-shell resonance of singly ionized barium, as the expanding plasma plume moves across it. The attenuated beam is allowed to fall on a CCD array where the spatial distribution of the absorption is recorded. Time-resolved ion velocity and integrated column density maps are readily extracted from the photoionization images.

Vacuum-ultraviolet beam array generation by flat micro-optical structures.

Micro-optical structures for VUV laser beam shaping and wave-front sensing were manufactured by thin-film deposition onto CaF2 and transfer by etching. Arrays of Bessel-like F2 laser beams at a wavelength of 157 nm with extremely small conical angles were generated by microaxicon lenses. Beam propagation was studied in simulations and experiments. Apodization by absorbing layers is proposed for beam cleaning.

Slow muon experiment by laser resonant ionization method at RIKEN-RAL muon facility

We report first results of the slow muon generation from pulsed surface muon beam using a method of resonant laser ionization of muonium. The muonium is produced from the muon beam by electron capture near the surface of thin tungsten foil heated to 2100 K. The ionization is resonantly enhanced by using a vacuum-ultraviolet (VUV) radiation resonant with 2P state of muonium (λ 1S–2P=122.09 nm). A pulsed beam at 355 nm is used for an efficient ionization from the 2P state. The VUV beam with pulse duration of 4 ns and with bandwidth of over 100 GHz to match the Doppler broadening of muonium (200 GHz) is generated by resonant third-order sum-difference frequency conversion (ω VUV=2ω R−ω T) in krypton gas phase-matched with argon buffer gas. The ω R beam has been tuned to the two-photon resonance of 4p 55p[1/2,0] state in krypton to enhance the conversion process. Yield of the VUV radiation has been investigated for different energies and confocal parameters of the incident beams. The whole apparatus, including a beamline for transport and detection of ionized particles, has been tested by ionizing residual hydrogen atoms at an ultra-high vacuum level of 1.1×10 −9 hPa with efficiency better than 10 −3. The yield of slow muons obtained during the first beamtime was 0.03 μ +/s.

Test results on the silicon pixel detector for the TTF-FEL beam trajectory monitor

Test measurements on the silicon pixel detector for the beam trajectory monitor at the free-electron laser of the TESLA test facility are presented. To determine the electronic noise of the detector and the read-out electronics and to calibrate the signal amplitude of different pixels, the 6 keV photons of the manganese K α/K β line are used. Two different methods determine the spatial accuracy of the detector: in one setup a laser beam is focused to a straight line and moves across the pixel structure. In the other, the detector is scanned using a low-intensity electron beam of an electron microscope. Both methods show that the symmetry axis of the detector defines a straight line within 0.4 μm . The sensitivity of the detector to low-energy X-rays is measured using a vacuum ultraviolet beam at the synchrotron light source HASYLAB. Additionally, the electron microscope is used to study the radiation hardness of the detector.

Vacuum-ultraviolet resonant photoabsorption imaging of laser produced plasmas

We present results from a vacuum-ultraviolet (VUV) “photoabsorption imaging” technique based on the measurement of the time and space resolved absorption of a quasimonochromatic VUV beam from a laser plasma light source. The use of VUV radiation as a probe beam permits direct access to resonance lines of (singly and more highly charged) ions and also to the resonant and nonresonant continua of atoms and ions. In this experiment we have confined ourselves to measurements using the 3p–3d resonances of Ca, Ca+, and Ca2+ as markers of the temporal and spatial distribution of ground state atoms and ions in an expanding laser plasma plume. We show how time resolved column density maps may be extracted from such images. In addition we have extracted plasma plume velocities from the data, which compare well with an analytical laser ablation model.

Vacuum-ultraviolet laser-induced refractive index change of fused silica

Increases as large as 0.036–0.038 in the refractive index of fused silica induced by multi-wavelength irradiation of substrates by vacuum-ultraviolet (VUV) light emitted from a VUV Raman laser are reported. The index change is ascribed to photoinduced decomposition of Si–O bonds in fused silica by the VUV beam irradiation. The modified areas are practically invisible in optical microscope and atomic force microscope observations. The modified depth is estimated to be about 0.53–0.62 μm. Efficient phase gratings are created by illuminating the substrates through periodically patterned contact masks.

High-performance beamline for vacuum-ultraviolet-excited material processing

A high-performance beamline dedicated to vacuum-ultraviolet (VUV)-excited semiconductor processing has been constructed on the compact electron storage ring “Super ALIS”. The first toroidal mirror accepts photons (emitted at a bending magnet) for 70 mrad in the horizontal direction, and the reflected parallel beam is focused onto the specimen surface by the second toroidal mirror. A brilliant VUV beam (with a photon density of 1.3×1016 s−1 mm−2 when the ring current is 300 mA) illuminates a circular field (10 mm in diameter) on the target surface. The eight 5-m-long differential pumping stages allow the reactant gas to be introduced into the reaction chamber at pressures up to 0.1 Torr. The processing is monitored in real time by using ultraviolet phase-modulated spectroscopic ellipsometry, and VUV-stimulated evaporation of SiO2 film has been demonstrated.

Some Aspects of VUV Beam Line Design

Some aspects of the design and usage of vacuum ultra violet beam lines are discussed. Fermat's principle for imaging is introduced and applied to grating monochromators. Some typical vacuum ultraviolet beam lines are presented, and some further topics believed to be of importance today and in the future are mentioned.