Formation of Environmentally Persistent Free Radicals (EPFRs) on the Phenol-Dosed -FeO(0001) Surface.

Environmentally persistent free radicals (EPFRs) are a class of toxic air pollutants that are found to form by the chemisorption of substituted aromatic molecules on the surface of metal oxides. In this study, we employ X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) to perform a temperature-dependent study of phenol adsorption on -FeO(0001) to probe the radical formation mechanism by monitoring changes in the electronic structure of both the adsorbed phenol and metal oxide substrate. Upon dosing at room temperature, new phenol-derived electronic states have been clearly observed in the UPS spectrum at saturation coverage.

Vibrational and Structural Studies of Environmentally Persistent Free Radicals Formed by Phenol-Dosed Metal Oxide Nanoparticles.

Environmentally persistent free radicals (EPFRs) are formed by the adsorption of substituted aromatic precursors on the surface of metal oxides and are known to have significant health and environmental impact due to their unique stability. In this article, the formation of EPFRs is studied by adsorption of phenol on ZnO, CuO, FeO, and TiO nanoparticles (∼10-50 nm) at high temperatures. Electron paramagnetic resonance indicates the formation of phenoxyl-type radicals.

Formation of environmentally persistent free radicals (EPFRs) on ZnO at room temperature: Implications for the fundamental model of EPFR generation.

Environmentally persistent free radicals (EPFRs) have significant environmental and public health impacts. In this study, we demonstrate that EPFRs formed on ZnO nanoparticles provide two significant surprises. First, EPR spectroscopy shows that phenoxy radicals form readily on ZnO nanoparticles at room temperature, yielding EPR signals similar to those previously measured after 250°C exposures.

High order harmonic generation from SF: Deconvolution of macroscopic effects.

We measure high order harmonics from the molecule SF over a large range of phase matching conditions and observe several features in the harmonics that are largely independent of such macroscopic conditions. The experimental data are then compared to the quantitative rescattering theory for the generation of harmonics from three orbitals. With this comparison, we are able to assign spectroscopic features in the harmonics to contributions from 1t (HOMO) and 5t (HOMO-1) orbitals.

Probing environmentally significant surface radicals: Crystallographic and temperature dependent adsorption of phenol on ZnO.

Environmentally persistent free radicals (EPFRs) are toxic organic/metal oxide composite particles that have been discovered to form from substituted benzenes chemisorbed to metal oxides. Here, we perform photoelectron spectroscopy, electron energy loss spectroscopy, and low energy electron diffraction of phenol chemisorbed to ZnO(1 0 1̱ 0) and (0 0 0 1̱)-Zn to observe electronic structure changes and charge transfer as a function adsorption temperature. We show direct evidence of charge transfer from the ZnO surfaces to the phenol.

High-resolution photoelectron spectra of the pyrimidine-type nucleobases.

High-resolution photoelectron spectra of the gas phase pyrimidine-type nucleobases, thymine, uracil, and cytosine, were collected using synchrotron radiation over the photon energy range 17 ≤ hν ≤ 150 eV. These data provide the highest resolution photoelectron spectra of thymine, uracil, and cytosine published to date. By comparing integrated regions of the energy dependent photoelectron spectra of thymine, the ionization potentials of the first four ionic states of thymine were estimated to be 8.

Electronic signatures of a model pollutant-particle system: chemisorbed phenol on TiO₂(110).

Environmentally persistent free radicals (EPFRs) are a class of composite organic/metal oxide pollutants that have recently been discovered to form from a wide variety of substituted benzenes chemisorbed to commonly encountered oxides. Although a qualitative understanding of EPFR formation on particulate metal oxides has been achieved, a detailed understanding of the charge transfer mechanism that must accompany the creation of an unpaired radical electron is lacking. In this study, we perform photoelectron spectroscopy and electron energy loss spectroscopy on a well-defined model system-phenol chemisorbed on TiO2(110) to directly observe changes in the electronic structure of the oxide and chemisorbed phenol as a function of adsorption temperature.

Vibrationally specific photoionization cross sections of acrolein leading to the X̃²A' ionic state.

The vibrational branching ratios in the photoionization of acrolein for ionization leading to the X̃²A' ion state were studied. Computed logarithmic derivatives of the cross section and the corresponding experimental data derived from measured vibrational branching ratios for several normal modes (ν9, ν10, ν11, and ν12) were found to be in relatively good agreement, particularly for the lower half of the 11-100 eV photon energy range considered. Two shape resonances have been found near photon energies of 15.

EPFR Formation from Phenol adsorption on Al2O and TiO: EPR and EELS studies.

We have examined the formation of environmentally persistent free radicals (EPFRs) from phenol over alumina and titania using both powder and single-crystal samples. Electron paramagnetic resonance (EPR) studies of phenol adsorbed on metal oxide powders indicates radical formation on both titania and alumina, with both oxides forming one faster-decaying species (lifetime on the order of 50-100 hours) and one slower-decayng species (lifetimes on the order of 1000 hours or more). Electron energy loss spectroscopy (EELS) measurements comparing physisorbed phenol on single-crystal TiO(110) to phenoxyl radicals on the same substrate indicate distinct changes in the π-π* transitions from phenol after radical formation.

High-throughput Toroidal Grating Beamline for Photoelectron Spectroscopy at CAMD.

A 5 meter toroidal grating (5m-TGM) beamline has been commissioned to deliver 28 mrad of bending magnet radiation to an ultrahigh vacuum endstation chamber to facilitate angle resolved photoelectron spectroscopy. The 5m-TGM beamline is equipped with Au-coated gratings with 300, 600 and 1200 lines/mm providing monochromatized synchrotron radiation in the energy ranges 25-70 eV, 50-120 eV and 100-240 eV, respectively. The beamline delivers excellent flux (~10-10 photons/sec/100mA) and a combined energy resolution of 189 meV for the beamline (at 1.

Vibrational branching ratios in the (b(2u))(-1) photoionization of C6F6.

The vibrational branching ratios in the photoionization of C(6)F(6) leading to the C (2)B(2u) state of C(6)F(6)(+) are considered. Computational and experimental data are compared for the excitation of two totally symmetric modes. Resonant features at photon energies near 19 and 21 eV are found.

Mode-specific photoionization dynamics of a simple asymmetric target: OCS.

Vibrationally resolved photoelectron spectra of OCS(+)(C (2)Sigma(+)) are used to probe coupling between photoelectron motion and molecular vibration for a simple asymmetric system. Spectra are reported over the photon energy range of 21 View Article and Find Full Text PDF

Vibrationally resolved photoionization dynamics of CF4 in the D 2A1 state.

Vibrationally resolved photoelectron spectroscopy of the CF4+ (D 2A1) state is studied for the first time over an extended energy range, 26.5 View Article and Find Full Text PDF

Quasibound continuum states in SiF4 (D2A1) photoionization: photoelectron-vibrational coupling.

The authors report a fully vibrationally resolved photoelectron spectroscopy investigation of a nonplanar molecule studied over a range of excitation energies. Experimental results for all four fundamental vibrational modes are presented. In each case significant non-Franck-Condon effects are seen.

Launching a particle on a ring: b2u-->ke2g ionization of C6F6.

Evidence is presented demonstrating that an electron launched into the continuum is trapped in an unprecedented quasibound state, namely, one that extends through the backbone of the six-member carbon ring of C6F6. The mode specificity of the vibrational sensitivity to the electron trapping provides an experimental signature for this phenomenon, while adiabatic static model-exchange scattering calculations are used to map the wave function, which corroborate the interpretation.

An infrared and X-ray spectroscopic study of the reactions of 2-chlorophenol, 1,2-dichlorobenzene, and chlorobenzene with model cuO/silica fly ash surfaces.

The surface-mediated reactions of 2-chlorophenol, 1,2-dichlorobenzene, and chlorobenzene were studied using CuO/ SiO2 as a fly ash surrogate. These compounds served as model precursors that have been implicated in the formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). With FTIR, we determined that reactions of the model precursors with a substrate composed of CuO dispersed on silica result in the formation of a mixture of surface-bound phenolate and carboxylate partial oxidation products from 200 to 500 degrees C.

Photoelectron trapping in N2O 7sigma-->ksigma resonant ionization.

Vibrationally resolved photoelectron spectroscopy of the N2O+(A 2Sigma+) state is used to compare the dependence of the photoelectron dynamics on molecular geometry for two shape resonances in the same ionization channel. Spectra are acquired over the photon energy range of 18< or =hv< or =55 eV. There are three single-channel resonances in this range, two in the 7sigma-->ksigma channel and one in the 7sigma-->kpi channel.

Electronically forbidden (5sigmau-->ksigmau) photoionization of CS2: mode-specific electronic-vibrational coupling.

Vibrationally resolved photoelectron spectroscopy of the CS(2) (+)(B (2)Sigma(u) (+)) state is used to show how nontotally symmetric vibrations "activate" a forbidden electronic transition in the photoionization continuum, specifically, a 5sigma(u)-->ksigma(u) shape resonance, that would be inaccessible in the absence of a symmetry breaking vibration. This electronic channel is forbidden owing to inversion symmetry selection rules, but it can be accessed when a nonsymmetric vibration is excited, such as bending or antisymmetric stretching. Photoelectron spectra are acquired for photon energies 17 View Article and Find Full Text PDF

Vibrational branching ratios in photoionization of CO and N2.

We report results of experimental and theoretical studies of the vibrational branching ratios for CO 4sigma(-1) photoionization from 20 to 185 eV. Comparison with results for the 2sigma(u)(-1) channel of the isoelectronic N2 molecule shows the branching ratios for these two systems to be qualitatively different due to the underlying scattering dynamics: CO has a shape resonance at low energy but lacks a Cooper minimum at higher energies whereas the situation is reversed for N2.

Mode-specific photoelectron scattering effects on CO2(+)(C2Sigmag+) vibrations.

Using high-resolution photoelectron spectroscopy, we have determined the energy dependent vibrational branching ratios for the symmetric stretch [v+ = (100)], bend [v+ = (010)], and antisymmetric stretch [v+ = (001)], as well as several overtones and combination bands in the 4sigmag(-1) photoionization of CO2. Data were acquired over the range from 20-110 eV, and this wide spectral coverage highlighted that alternative vibrational modes exhibit contrasting behavior, even over a range usually considered to be dominated by atomic effects. Alternative vibrational modes exhibit qualitatively distinct energy dependences, and this contrasting mode-specific behavior underscores the point that vibrationally resolved measurements reflect the sensitivity of the electron scattering dynamics to well-defined changes in molecular geometry.

Observation of the symmetry-forbidden 5sigmau-->ksigmau CS2 transition: a vibrationally driven photoionization resonance.

Vibrationally resolved photoelectron spectroscopy and Schwinger calculations are used to characterize a new resonance phenomenon in the 5sigma(u)-->ksigma(u) photoionization of CS2. This resonant channel is symmetry forbidden, yet is observable because it is activated by the antisymmetric stretching vibration. In addition, we show that a Franck-Condon breakdown occurs even though the energy dependence of the cross section is insensitive to geometry changes, which is unprecedented in photoionization.

X-ray spectroscopic studies of the high temperature reduction of Cu(II)O by 2-chlorophenol on a simulated fly ash surface.

The reaction of 2-chlorophenol on Cu(II)O at 375 degrees C is studied using X-ray absorption near edge structure (XANES) spectroscopy. A mixture of copper(II) oxide and silica is prepared to serve as a surrogate for fly ash in combustion systems. 2-Chlorophenol is utilized as a model precursor for formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F).

Cu(0) nanoclusters derived from poly(propylene imine) dendrimer complexes of Cu(II).

A family of diaminobutane core, poly(propylene imine) dendrimers coordinated to Cu(II), DAB-Am(n)-Cu(II)x (n = 4, 8, 16, 32, 64, x = n/2), was studied by means of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopies. The geometry of the dipropylene triamine (dpt)-Cu(II) end-group complexes for all dendrimer generations is reported for the first time and is found to be that of a square-based pyramid with each Cu ion bound to three nitrogen atoms (Cu-N distance approximately 2.03 A) of the dpt end group of the dendrimer.