Arnaldo Naves de Brito

FAST DISSOCIATION OF RESONANTLY CORE EXCITED H2S STUDIED BY VIBRATIONAL AND TEMPORAL ANALYSIS OF THE AUGER SPECTRA

Excitation of a core electron to the lowest unoccupied orbital of H2S has been shown to give rise to dissociation that occurs faster than the Auger decay. Recent experimental data have allowed us to resolve vibrational structures connected with the resona

The resonant Auger electron spectrum of C 1s−1π* excited ethene: A combined theoretical and experimental investigation

The resonant Auger electron spectrum for ethene has been calculated with an ab initio approach using configuration-interaction energies and wave functions for the intermediate core-excited and final states. The transition rates were determined by the “one-center approximation.” The role of vibrational relaxation on the line shapes was described by a moment method which considers the case of symmetric core holes and their localization due to the vibrational relaxation of the core-excited state. The core hole localization is investigated in some detail and is found to be extremely efficient in the C 1s−1π* excited state of ethene. Another property of the core-excited state is found to be the polarization of the valence electron density toward the core hole. We demonstrate this by using three different symmetric configuration interaction representations and one nonsymmetric Hartree–Fock representation for this state. A modified improved virtual orbitals method is described and employed to obtain virtual orbi...

Surface-Altered Protonation Studied by Photoelectron Spectroscopy and Reactive Dynamics Simulations

The extent to which functional groups are protonated at aqueous interfaces as compared to bulk is deemed essential to several areas in chemistry and biology. The origin of such changes has been the source of intense debate. We use X-ray photoelectron spectroscopy and all-atom reactive molecular dynamics simulations as two independent methods to probe, at the molecular scale, both bulk and surface distributions of protonated species of cysteine in an aqueous solution. We show that the distribution of the cysteine species at the surface is quite different from that in the bulk. We argue that this finding, however, cannot be simply related to a change in the extent of proton sharing between the two conjugate acid/base pairs that may occur between these two regions. The present theoretical simulations identify species at the surface that are not present in the bulk.

Molecular alignment of ammonia studied by electron-ion-ion coincidence spectroscopy

Electron-ion-ion coincidence measurements carried out at discrete resonances near the N 1s threshold in ammonia are reported. The measured coincidence spectra show clear alignment of the molecule upon resonant core-electron excitation. The coincidence data are analyzed to extract information about the molecule in the excited state by simulating the alignment and the dissociation processes. Dynamic changes in molecular geometry are found as the photon energy is scanned through the N 1s-->4a(1) resonance, whereas for the N 1s-->2e state the geometry and kinetic energy released upon dissociation remain unchanged. The alignment of the core-excited molecules is found to be preserved even in two-step dissociation processes.

Doubly ionized states of hexafluorobenzene studied by high-resolution Auger electron and double-charge-transfer spectroscopies

Double-charge-transfer (DCT) spectroscopy has been used to measure the double-ionization energies of C6F6. OH+ and F+ projectile ions with translational energy of 6 keV, which tend to populate triplet states in the dication, were used. From the position of peaks in the resultant DCT spectra, C6F6 double ionization energies of 26.3 ± 0.5, 28.4 ± 0.5, 29.2 ± 0.5, 32.2 ± 0.5, 35.0 ± 1 and 40.0 ± 1 eV were determined. The photon-induced Auger electron spectra of C6F6 were recorded. The Auger electron spectra are dominated by transitions to final states of singlet symmetry. Ten distinct structures are observed in the C 1s Auger electron spectrum centred at double ionization energies of 26.63 ± 0.1, 30.37 ± 0.1, 34.68 ± 0.1, 36.26 ± 0.1, 37.84 ± 0.1, 39.19 ± 0.2, 42.73 ± 0.3, 46.51 ± 0.1, 49.20 ± 0.1 and 50.40 ± 0.2 eV. Five structures were observed in the F 1s outer–outer spectrum, centred at double ionization energies of 42.20 ± 0.1, 46.14 ± 0.1, 48.61 ± 0.1, 52.53 ± 0.1 and 56.9 ± 0.1. Additional structures are observed in the outer–inner and inner–inner spectrum. A comparison has been made between the results from the present work and those obtained in a recent coincidence study (K. Ibrahim, P. Lablanquie, M-J. Hubin-Franskin, J. Delwiche, M. Furan, I. Nenner, D. Hagan and J. H. D. Eland, J. Chem. Phys., 1992, 96, 1981). Greens function calculations have been performed to assist the interpretation of the spectra.

Theoretical-experimental study of formic acid photofragmentation in the valence region.

Photoionization and photofragmentation studies of formic acid (HCOOH) are performed for the valence shell electron ionization process. The total and partial ion yield of gaseous HCOOH were collected as a function of photon energy in the ultraviolet region, between 11.12 and 19.57 eV. Measurements of the total and partial ion yield of gaseous formic acid molecule are performed with a time-of-flight mass spectrometer at the Synchrotron Light Brazilian Laboratory. Density functional theory and time dependent density functional theory are employed to calculate the ground and excited electronic state energies of neutral and ionic formic acid as well as their fragments and normal vibration modes. The ionization potential energies, the stability of electronic excited states of HCOOH(+), and the energies of opening fragmentation channels are estimated from theoretical-experimental analysis. Additionally, the main formic acid photofragmentation pathways by exposition of photons within that energy range are determined experimentally. These produced ions primarily have the following mass/charge ratios: 46 (HCOOH(+)), 45 (COOH(+)), 29 (HCO(+)), and 18 (H(2)O(+)).

Core localization and sigma(*) delocalization in the O 1s core-excited sulfur dioxide molecule

Electron-ion-ion coincidence measurements of sulfur dioxide at discrete resonances near the O 1s ionization edge are reported. The spectra are analyzed using a model based upon molecular symmetry and on the geometry of the molecule. We find clear evidence for molecular alignment that can be ascribed to symmetry properties of the ground and core-excited states. Configuration interaction (CI) calculations indicate geometry changes in accord with the measured spectra. For the SO(2) molecule, however, we find that the localized core hole does not produce measurable evidence for valence localization, since the transition dipole moment is not parallel to a breaking sigma* O-S bond, in contrast to the case of ozone. The dissociation behavior based upon the CI calculations using symmetry-broken orbitals while fixing a localized core-hole site is found to be nearly equivalent to that using symmetry-adapted orbitals. This implies that the core-localization effect is not strong enough to localize the sigma* valence orbital.

Ethanol Solvation in Water Studied on a Molecular Scale by Photoelectron Spectroscopy

Because of the amphiphilic properties of alcohols, hydrophobic hydration is important in the alcohol-water system. In the present paper we employ X-ray photoelectron spectroscopy (XPS) to investigate the bulk and surface molecular structure of ethanol-water mixtures from 0.2 to 95 mol %. The observed XPS binding energy splitting between the methyl C 1s and hydroxymethyl C 1s groups (BES_[CH3-CH2OH]) as a function of the ethanol molar percentage can be divided into different regions: one below 35 mol % with higher values (about 1.53 eV) and one starting at 60 mol % up to 95 mol % with 1.49 eV as an average value. The chemical shifts agree with previous quantum mechanics/molecular mechanics (QM/MM) calculations [ Löytynoja , T. ; J. Phys. Chem. B 2014 , 118 , 13217 ]. According to these calculations, the BES_[CH3-CH2OH] is related to the number of hydrogen bonds between the ethanol and the surrounding molecules. As the ethanol concentration increases, the average number of hydrogen bonds decreases from 2.5 for water-rich mixtures to 2 for pure ethanol. We give an interpretation for this behavior based on how the hydrogen bonds are distributed according to the mixing ratio. Since our experimental data are surface sensitive, we propose that this effect may also be manifested at the interface. From the ratio between the XPS C 1s core lines intensities we infer that below 20 mol % the ethanol molecules have their hydroxyl groups more hydrated and possibly facing the solutions bulk. Between 0.1 and 14 mol %, we show the formation of an ethanol monolayer at approximately 2 mol %. Several parameters are derived for the surface region at monolayer coverage.

Surface Propensity of Atmospherically Relevant Amino Acids Studied by XPS

Amino acids constitute an important fraction of the water-soluble organic nitrogen (WSON) compounds in aerosols and are involved in many processes in the atmosphere. In this work, we applied X-ray photoelectron spectroscopy (XPS) to study aqueous solutions of four amino acids, glycine, alanine, valine, and methionine, in their zwitterionic forms. We found that amino acids with hydrophilic side chains and smaller size, GLY and ALA, tend to stay in the bulk of the liquid, while the hydrophobic and bigger amino acids, VAL and MET, are found to concentrate more on the surface. We found experimental evidence that the amino acids have preferential orientation relative to the surface, with the hydrophobic side chain being closer to the surface than the hydrophilic carboxylate group. The observed amino acid surface propensity has implications in atmospheric science as the surface interactions play a central role in cloud droplet formation, and they should be considered in climate models.

P2-424 Mild food insecurity is associated with obesity among Brazilian women

Introduction To determine if food insecurity was associated with a higher prevalence of obesity in a large random sample of Brazilian women of reproductive age. Methods The data were derived from the third edition of the Childrens and Womens National Demographic and Health Survey, conducted in 2006–2007. This was a nationally representative cross-sectional study. Obesity (BMI≥30 kg/m2) was the outcome variable. Associations were measured using crude and adjusted prevalence ratios (PR) with 95% CI through Poisson regression models taking into account the complex sampling design. The sample included 10 226 women from 18 to 45 years. Results The prevalence of any level of food insecurity measured by the Brazilian Food Insecurity Scale was 40.9%, with 25.5% light, 10.1% mild and 5.3% severe food insecurities. The prevalence of obesity was 17.4%. We found borderline effect of light food insecurity and increased prevalence of obesity in Brazil (PR=1.16; 95% CI 0.98 to 1.38; p>0.05). Women with mild food insecurity had a higher risk of being obese than their counterparts without food insecurity (PR = 1.49; 95% CI 1.17 to 1.90; p=0.010), after adjustment for skin colour/ethnicity, years of schooling, macro-region, income, age and marital status. Conclusion Mild food insecurity was associated with obesity as measured by BMI, even after adjusting for various confounding factors in this large cross-sectional survey performed in a middle-income country undergoing the nutrition transition.