| 1. |
- Beye, Martin, et al.
(författare)
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Chemical Bond Activation Observed with an X-ray Laser
- 2016
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 7:18, s. 3647-3651
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Tidskriftsartikel (refereegranskat)abstract
- The concept of bonding and antibonding orbitals is fundamental in chemistry. The population of those orbitals and the energetic difference between the two reflect the strength of the bonding interaction. Weakening the bond is expected to reduce this energetic splitting, but the transient character of bond-activation has so far prohibited direct experimental access. Here we apply time-resolved soft X-ray spectroscopy at a free electron laser to directly observe the decreased bonding antibonding splitting following bond-activation using an ultrashort optical laser pulse.
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| 2. |
- Gladh, Jörgen, et al.
(författare)
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Detection of adsorbate overlayer structural transitions using sum-frequency generation spectroscopy
- 2015
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 633, s. 77-81
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate that temperature-programmed vibrational sum-frequency generation (SFG) spectroscopy has a unique sensitivity to certain adsorbate overlayer structural transitions. In the CO stretching vibration of co-adsorbed CO/O(2x1)/Ru(0001) we observe pronounced dips in the spectral intensity as the adsorbate overlayer undergoes structural transitions with temperature. Combining with temperature-programmed desorption (TPD) a more complete picture of temperature-dependent structural transitions is obtained. We extract kinetic parameters from the SFG data and obtain good agreement with TPD when both techniques see the same transition. Infrared-infrared visible SFG is used to determine changes in inter-adsorbate coupling that allow us to experimentally assign the structural transitions. Furthermore, density functional theory calculations of the proposed structures and energetics are performed to verify the experimental assignments.
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| 3. |
- Nilsson, Anders, et al.
(författare)
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Catalysis in real time using X-ray lasers
- 2017
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 675, s. 145-173
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Tidskriftsartikel (refereegranskat)abstract
- We describe how the unique temporal and spectral characteristics of X-ray free-electron lasers (XFEL) can be utilized to follow chemical transformations in heterogeneous catalysis in real time. We highlight the systematic study of CO oxidation on Ru(0001), which we initiate either using a femtosecond pulse from an optical laser or by activating only the oxygen atoms using a THz pulse. We find that CO is promoted into an entropy-controlled precursor state prior to desorbing when the surface is heated in the absence of oxygen, whereas in the presence of oxygen, CO desorbs directly into the gas phase. We monitor the activation of atomic oxygen explicitly by the reduced split between bonding and antibonding orbitals as the oxygen comes out of the strongly bound hollow position. Applying these novel XFEL techniques to the full oxidation reaction resulted in the surprising observation of a significant fraction of the reactants at the transition state through the electronic signature of the new bond formation.
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| 4. |
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| 5. |
- Xin, H., et al.
(författare)
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Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics on Ru(0001) Probed by Ultrafast X-Ray Spectroscopy and Ab Initio Simulations
- 2015
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 114:15
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Tidskriftsartikel (refereegranskat)abstract
- We show that coadsorbed oxygen atoms have a dramatic influence on the CO desorption dynamics from Ru(0001). In contrast to the precursor-mediated desorption mechanism on Ru(0001), the presence of surface oxygen modifies the electronic structure of Ru atoms such that CO desorption occurs predominantly via the direct pathway. This phenomenon is directly observed in an ultrafast pump-probe experiment using a soft x-ray free-electron laser to monitor the dynamic evolution of the valence electronic structure of the surface species. This is supported with the potential of mean force along the CO desorption path obtained from density-functional theory calculations. Charge density distribution and frozen-orbital analysis suggest that the oxygen-induced reduction of the Pauli repulsion, and consequent increase of the dative interaction between the CO 5 sigma and the charged Ru atom, is the electronic origin of the distinct desorption dynamics. Ab initio molecular dynamics simulations of CO desorption from Ru(0001) and oxygen-coadsorbed Ru(0001) provide further insights into the surface bond-breaking process.
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| 6. |
- Öberg, Henrik, et al.
(författare)
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Indication of non-thermal contribution to visible femtosecond laser-induced CO oxidation on Ru(0001)
- 2015
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 143:7
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Tidskriftsartikel (refereegranskat)abstract
- We studied CO oxidation on Ru(0001) induced by 400 nm and 800 nm femtosecond laser pulses where we find a branching ratio between CO oxidation and desorption of 1: 9 and 1: 31, respectively, showing higher selectivity towards CO oxidation for the shorter wavelength excitation. Activation energies computed with density functional theory show discrepancies with values extracted from the experiments, indicating both a mixture between different adsorbed phases and importance of non-adiabatic effects on the effective barrier for oxidation. We simulated the reactions using kinetic modeling based on the two-temperature model of laser-induced energy transfer in the substrate combined with a friction model for the coupling to adsorbate vibrations. This model gives an overall good agreement with experiment except for the substantial difference in yield ratio between CO oxidation and desorption at 400 nm and 800 nm. However, including also the initial, non-thermal effect of electrons transiently excited into antibonding states of the O-Ru bond yielded good agreement with all experimental results.
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| 7. |
- Öberg, Henrik, et al.
(författare)
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Optical laser-induced CO desorption from Ru(0001) monitored with a free-electron X-ray laser : DFT prediction and X-ray confirmation of a precursor state
- 2015
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 640, s. 80-88
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Tidskriftsartikel (refereegranskat)abstract
- We present density functional theory modeling of time-resolved optical pump/X-ray spectroscopic probe data of CO desorption from Ru(0001). The BEEF van der Waals functional predicts a weakly bound state as a precursor to desorption. The optical pump leads to a near-instantaneous (<100 fs) increase of the electronic temperature to nearly 7000 K. The temperature evolution and energy transfer between electrons, substrate phonons and adsorbate is described by the two-temperature model and found to equilibrate on a timescale of a few picoseconds to an elevated local temperature of similar to 2000K. Estimating the free energy based on the computed potential of mean force along the desorption path, we find an entropic barrier to desorption (and by time-reversal also to adsorption). This entropic barrier separates the chemisorbed and precursor states, and becomes significant at the elevated temperature of the experiment (similar to 1.4 eV at 2000 K). Experimental pump-probe X-ray absorption/X-ray emission spectroscopy indicates population of a precursor state to desorption upon laser-excitation of the system (Dell'Angela et al., 2013). Computing spectra along the desorption path confirms the picture of a weakly bound transient state arising from ultrafast heating of the metal substrate.
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| 8. |
- Östrom, Henrik, et al.
(författare)
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Probing the transition state region in catalytic CO oxidation on Ru
- 2015
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 347:6225, s. 978-982
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Tidskriftsartikel (refereegranskat)abstract
- Femtosecond x-ray laser pulses are used to probe the carbon monoxide (CO) oxidation reaction on ruthenium (Ru) initiated by an optical laser pulse. On a time scale of a few hundred femtoseconds, the optical laser pulse excites motions of CO and oxygen (O) on the surface, allowing the reactants to collide, and, with a transient close to a picosecond (ps), new electronic states appear in the OK-edge x-ray absorption spectrum. Density functional theory calculations indicate that these result from changes in the adsorption site and bond formation between CO and O with a distribution of OC-O bond lengths close to the transition state (TS). After 1 ps, 10% of the CO populate the TS region, which is consistent with predictions based on a quantum oscillator model.
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| 9. |
- Class, Quetzal A., et al.
(författare)
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Outcome-dependent associations between short interpregnancy interval and offspring psychological and educational problems : a population-based quasi-experimental study
- 2018
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Ingår i: International Journal of Epidemiology. - Stockholm : Oxford University Press. - 0300-5771 .- 1464-3685. ; 47:4, s. 1159-1168
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Tidskriftsartikel (refereegranskat)abstract
- Background: Causal interpretation of associations between short interpregnancy interval (the duration from the preceeding birth to the conception of the next-born index child) and the offspring's psychological and educational problems may be influenced by a failure to account for unmeasured confounding.Methods: Using population-based Swedish data from 1973-2009, we estimated the association between interpregnancy interval and outcomes [autism spectrum disorder (ASD), attention-deficit/hyperactivity disorder (ADHD), severe mental illness, suicide attempt, criminality, substance-use problem and failing grades] while controlling for measured covariates. We then used cousin comparisons, post-birth intervals (the interval between the second-and third-born siblings to predict second-born outcomes) and sibling comparisons to assess the influence of unmeasured confounding. We included an exploratory analysis of long interpregnancy interval.Results: Interpregnancy intervals of 0-5 and 6-11 months were associated with higher odds of outcomes in cohort analyses. Magnitudes of association were attenuated following adjustment for measured covariates. Associations were eliminated for ADHD, severe mental illness and failing grades, but maintained magnitude for ASD, suicide attempt, criminality and substance-use problem in cousin comparisons. Post-birth interpregnancy interval and sibling comparisons suggested some familial confounding. Associations did not persist across models of long interpregnancy interval.Conclusions: Attenuation of the association in cousin comparisons and comparable post-birth interval associations suggests that familial genetic or environmental confounding accounts for a majority of the association for ADHD, severe mental illness and failing grades. Modest associations appear independently of covariates for ASD, suicide attempt, criminality and substance-use problem. Post-birth analyses and sibling comparisons, however, show some confounding in these associations.
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| 10. |
- Cotgreave, Ian, et al.
(författare)
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Pyriproxifen and microcephaly: an investigation of potential ties to the ongoing "Zika epidemic"
- 2016
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- As part of the Swetox mission to react to emerging concerns in chemical health and environmental safety, a preliminary litterature investigation was undertaken to gather all readily available scientific information on PPF with respect to safety assessment, in order to better understand potential links between chemical exposure and the devopment of microcephaly in affected areas. Therefore the contents of the report do not constitute an attempt at either questioning the use of existing regulatory data in the manner prescribed by international regulatory proceedures, or as a new risk assessment, based on the scientific information and concepts discussed. Here we report our findings, with particular emphasis on exisiting regulatory information, potential for lack of translation of results from regulatory animal testing to humans, lack of human exposure data and suggestions on plausible mode(s) of action of PPF in human neurodevelopmental adversities such as microcephaly.
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