Renfei Feng

VESPERS: A Beamline for Combined XRF and XRD Measurements

VESPERS (VEry Sensitive Elemental and Structural Probe Employing Radiation from a Synchrotron) is a bending magnet beamline that is just beginning construction at the Canadian Light Source. The beamline has several novel design elements that are intended to increase its operating flexibility and availability to users. First, there is a requirement to deliver a microscopic beam with a variable bandwidth, thus enabling the generation of Laue diffraction patterns and high yield X‐ray fluorescence spectra from the same region preferably simultaneously. Thus, the bandpass of the VESPERS monochromator can be readily changed to focus radiation into the same 2–4 micron diameter area that is either polychromatic or having a bandwidth of 10%, 1.6% or 0.01%. This allows the user to change the diffraction pattern to suit the complexity of the crystal and the spectral signal to noise ratio to suit the detection sensitivity required. Second, the beamline is designed to have two branches capable of operating simultaneou...

Absolute photoabsorption oscillator strengths by electron energy loss methods: the valence and S 2p and 2s inner shells of sulphur dioxide in the discrete and continuum regions (3.5–260 eV)

Abstract Absolute photoabsorption oscillator strengths (cross-sections) for the valence shell discrete and continuum regions of sulphur dioxide from 3.5 to 51 eV have been measured using high resolution (∼0.05 eV FWHM) dipole ( e , e ) spectroscopy. A wide-range spectrum, covering both the valence shell and the S 2p and 2s inner shells, has also been obtained from 5 to 260 eV at low resolution (∼1 eV FWHM), and this has been used to determine the absolute oscillator strength scale using valence shell TRK (i.e., S(0)) sum-rule normalization. The present measurements have been undertaken in order to investigate the recently discovered significant quantitative errors in our previously published low resolution dipole ( e , e ) work on sulphur dioxide (Cooper et al., Chem. Phys. 150 (1991) 237; 150 (1991) 251). These earlier measurements were also in poor agreement with other previously published direct photoabsorption measurements. We now report new absolute photoabsorption oscillator strengths using both high and low resolution dipole ( e , e ) spectroscopies. These new measurements cover a wider energy range and are much more consistent with the previously published direct photoabsorption measurements. The accuracy of our new measurements is confirmed by an S(−2) dipole sum-rule analysis which gives a static dipole polarizability for sulphur dioxide in excellent agreement (within 3.5%) with previously reported polarizability values. Other dipole sums S( u ) ( u =−1,−3 to −6,−8,−10) and logarithmic dipole sums L( u ) ( u =−1 to −6) are also determined from the presently reported absolute oscillator strength distributions.

Dipole (e,e) spectroscopic studies of benzene: quantitative photoabsorption in the UV, VUV and soft X-ray regions

Abstract Absolute UV and VUV photoabsorption oscillator strengths (cross sections) for the valence shell discrete and continuum regions of benzene (C 6 H 6 ) have been measured from 4 to 56 eV using high resolution (∼0.05 eV fwhm) dipole (e,e) spectroscopy. A wide-range spectrum, spanning the UV, VUV and soft X-ray regions, from 5 to 200 eV has also been obtained at low resolution (∼1 eV fwhm), and this has been used to determine the absolute oscillator strength scale by employing valence shell TRK [i.e., S(0)] sum-rule normalization. The presently reported high and low resolution absolute photoabsorption oscillator strengths are compared with previously published data from direct photoabsorption measurements in those limited energy regions where such data are available. Evaluation of the S(−2) sum using the presently reported absolute differential photoabsorption oscillator strength data gives a static dipole polarizability for benzene in good agreement (within ∼3.4%) with published polarizability values obtained from optical measurements. Other dipole sums S( u ), ( u =−1, −3, −4, −5, −6, −8, −10), and logarithmic dipole sums L( u ), ( u =−1 to −6), are also determined from the presently reported absolute differential photoabsorption oscillator strength data using dipole sum-rules.

Absolute photoabsorption cross-sections (oscillator strengths) for ethanol (5–200 eV)

Abstract Absolute photoabsorption oscillator strengths (cross-sections) in the valence-shell discrete and continuum regions of ethanol (C 2 H 5 OH) have been measured from 5 to 32 eV using high-resolution (∼0.05 eV fwhm) dipole (e,e) spectroscopy. A wide-range spectrum, spanning the UV, VUV and soft X-ray regions, from 5 to 200 eV has also been obtained at low-resolution (∼1 eV fwhm), and this has been used to determine the absolute oscillator strength scale by employing valence-shell TRK (i.e., S (0)) sum-rule normalization. The presently reported high- and low-resolution absolute photoabsorption oscillator strengths are compared with previously published data from direct photoabsorption measurements in those limited energy regions where such data are available. Evaluation of the S (−2) sum using the presently reported absolute differential photoabsorption oscillator strength data gives a static dipole polarizability for ethanol in excellent agreement (within 0.5%) with previously reported polarizability values. Other dipole sums S ( u ), ( u =−1,−3,−4,−5,−6,−8,−10), and logarithmic dipole sums L ( u ), ( u =−1 to -6), are also determined from the presently reported absolute differential photoabsorption oscillator strength data using dipole sum-rules.

Photofragmentation study of core-excited NO

The production of anionic and cationic fragments of NO has been measured after photoexcitation in the vicinity of the nitrogen 1s threshold. The partial ion yield for the anionic fragment O−, in conjunction with the partial cation yields, can clearly distinguish the shape resonances from doubly excited states. On the N 1s → π* resonances, which are composed of the three intermediate states 2Δ, 2Σ− and 2Σ+ in the order of increasing photon energy, the intensity ratios for the NO++, O+ and O++ fragments are very different from the total ion yield. An explanation for this behaviour in terms of symmetry is proposed.

Absolute oscillator strengths for hydrogen sulphide: II. Ionic photofragmentation and photoionization in the valence shell continuum regions (10–60 eV)

Abstract Absolute oscillator strengths (cross sections) of ionic photofragmentation and photoionization for hydrogen sulphide in the valence shell continuum regions have been determined using dipole (e,e+ion) coincidence spectroscopy (∼1 eV fwhm) at equivalent photon energies from the first ionization threshold to 60.2 eV. These have been determined from the recently published absolute photoabsorption oscillator strengths [Feng et al., Chem. Phys. 244 (1999) 127] together with the photoionization efficiency and ionic photofragmentation branching ratios obtained from time-of-flight mass spectra reported in the present work. Consideration of the presently reported data together with the photoelectron branching ratios for H 2 S and ionization potentials obtained from previously reported photoelectron and dipole (e,2e) spectroscopies yields quantitative information on the breakdown pathways of H 2 S following absorption of radiation in the VUV and soft X-ray regions. A new photofragmentation product (H 2 + ) and the doubly charged molecular ion (H 2 S 2+ ) from hydrogen sulphide have been found in the present work. Partial photoionization oscillator strengths for production of the four valence shell electronic states of H 2 S + have also been derived from the molecular and dissociative partial photoionization oscillator strengths over the entire energy range up to 60.2 eV.

Dipole (e,e+ion) spectroscopic studies of benzene: absolute oscillator strengths for molecular and dissociative photoionization in the VUV and soft X-ray regions

Abstract The branching ratios for molecular and dissociative photoionization of benzene (C 6 H 6 ) have been measured in the VUV and soft X-ray regions using dipole (e,e+ion) coincidence spectroscopy (∼1 eV FWHM) at equivalent photon energies from the first ionization threshold up to 80 eV. The absolute partial oscillator strengths (cross-sections) for molecular and dissociative photoionization have been determined by combining previously reported absolute photoabsorption oscillator strength data [J. Electron Spectrosc. Relat. Phenom. 123 (2–3) (2002) 211–223] together with the photoionization branching ratios and the (multi-dissociative-corrected) photoionization efficiency obtained from time-of-flight mass spectra reported in the present work. The fact that the photoionization efficiency has been measured as greater than unity above ∼25 eV indicates the existence of multi-dissociative products from Coulomb explosion of multiply charged ions. Appearance potentials of all ion products from benzene are also reported. The presently reported results are compared with the previously published data where possible.

UV, VUV and soft X-ray photoabsorption of dimethyl ether by dipole (e,e) spectroscopies

Absolute UV and VUV photoabsorption oscillator strengths (cross-sections) for the valence shell discrete and continuum regions of dimethyl ether (CH3OCH3, DME) have been measured from 5 to 32 eV using high resolution (HR) (0.05 eV f.w.h.m.) dipole (e,e) spectroscopy. A wide-range spectrum, spanning the UV, VUV and soft X-ray regions, from 5 to 200 eV has also been obtained at low resolution (LR) (1 eV f.w.h.m.), and this has been used to determine the absolute oscillator strength scale by employing valence shell Thomas‐Reiche‐Kuhn (i.e., S(0)) sum-rule normalization. The presently reported HR and LR absolute photoabsorption oscillator strengths are compared with previously published data from direct photoabsorption measurements in those limited energy regions where such data are available. Evaluation of the SOˇ2U sum using the presently reported absolute diAerential photoabsorption oscillator strength data gives a static dipole polarizability for dimethyl ether in excellent agreement (within 0.5%) with previously reported polarizability values. Other dipole sums SOuU,Ouaˇ1;ˇ3;ˇ4;ˇ5;ˇ6;ˇ8;ˇ10U, and logarithmic dipole sums LOuU ,( uaˇ 1t oˇ6), are also determined from the presently reported absolute diAerential photoabsorption oscillator strength data using dipole sum rules. ” 2000 Elsevier Science B.V. All rights reserved.

Commissioning of the VESPERS Beamline at the Canadian Light Source

VESPERS beamline is a hard X‐ray microprobe beamline at the Canadian Light Source (CLS). It is designed to measure material structure and composition using X‐ray diffraction (XRD) and X‐ray fluorescence (XRF) spectroscopy. Four widely differing bandwidths, ∼0.01%, ∼1.6%, ∼10%, and fully polychromatic beam, are selectable to simplify the Laue diffraction analysis and to optimize XRF excitation. Also, X‐ray absorption spectroscopy (XAS) can be employed using ∼0.01% bandwidth to provide chemical analytical information. Currently, the beamline is in its late commissioning phase. A number of test experiments have been performed to show the Laue XRD, XRF and XAS capabilities of the beamline. Beamline control and data acquisition software have been developed to include the essential functions for user operation. Full user operation starts in 2010.

Promoting Effect of Ni(OH)2 on Palladium Nanocrystals Leads to Greatly Improved Operation Durability for Electrocatalytic Ethanol Oxidation in Alkaline Solution

Most electrocatalysts for the ethanol oxidation reaction suffer from extremely limited operational durability and poor selectivity toward the CC bond cleavage. In spite of tremendous efforts over the past several decades, little progress has been made in this regard. This study reports the remarkable promoting effect of Ni(OH)2 on Pd nanocrystals for electrocatalytic ethanol oxidation reaction in alkaline solution. A hybrid electrocatalyst consisting of intimately mixed nanosized Pd particles, defective Ni(OH)2 nanoflakes, and a graphene support is prepared via a two-step solution method. The optimal product exhibits a high mass-specific peak current of >1500 mA mg-1Pd , and excellent operational durability forms both cycling and chronoamperometric measurements in alkaline solution. Most impressively, this hybrid catalyst retains a mass-specific current of 440 mA mg-1 even after 20 000 s of chronoamperometric testing, and its original activity can be regenerated via simple cyclic voltammetry cycles in clean KOH. This great catalyst durability is understood based on both CO stripping and in situ attenuated total reflection infrared experiments suggesting that the presence of Ni(OH)2 alleviates the poisoning of Pd nanocrystals by carbonaceous intermediates. The incorporation of Ni(OH)2 also markedly shifts the reaction selectivity from the originally predominant C2 pathway toward the more desirable C1 pathway, even at room temperature.