Scientific Highlights from PSI's research departments

NUM (Condensed Matter Research with Neutrons and Muons)

Li Diffusion in LixCoO₂ Probed by Muon-Spin Spectroscopy

Research Department Neutrons and Muons (NUM). The diffusion coefficient of Li+ ions (DLi) in the battery material LixCoO₂ has been investigated by muon-spin relaxation (µ+SR). Based on experiments in zero and weak longitudinal fields at temperatures up to 400 K, we determined the fluctuation rate (nu) of the fields on the muons due to their interaction with the nuclear moments. Combined with susceptibility data and electrostatic potential calculations, clear Li+ ion diffusion was detected above ~150 K. The DLi estimated from 'nu' was in very good agreement with predictions from first-principles calculations, and we present the µ+SR technique as an optimal probe to detect DLi for materials containing magnetic ions.

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SYN (Synchrotron Radiation and Nanotechnology)

Watching atoms move: an ultrafast phase transition

Swiss Light Source (SLS). One approach to advance our understanding of the complex interactions between different degrees of freedom in strongly correlated systems is to use time-resolved methods to study the response of a material after it has been driven out of equilibrium. Ultrafast optical techniques have demonstrated considerable potential to unravel the correlations that drive the interesting physics in such materials. Phonon dynamics in these studies are only indirectly observed via the electronic response, and are not generally able to unambiguously disentangle the dynamics of the lattice from those of the electronic subsystem. By using femtosecond x-ray diffraction to probe directly the structural response of photoexcited manganite, we have found evidence of an ultrafast laser-induced structural phase transition driven directly by electronic excitation and occuring on a sub-picosecond time scale.

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ENE (General Energy)

Evolution of Organic Aerosols in the Atmosphere

Organic aerosol (OA) particles affect climate forcing and human health, but their sources and evolution remain poorly characterized. We present a unifying model framework describing the atmospheric evolution of OA that is constrained by high time-resolution measurements of its composition, volatility, and oxidation state. OA and OA precursor gases evolve by becoming increasingly oxidized, less volatile, and more hygroscopic, leading to the formation of oxygenated organic aerosol (OOA), with concentrations comparable to those of sulfate aerosol throughout the Northern Hemisphere. Our model framework captures the dynamic aging behavior observed in both the atmosphere and laboratory: It can serve as a basis for improving parameterizations in regional and global models.

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BIO (Biology and Chemistry)

Uptake of NO₂ to deliquesced dihydroxybenzoate aerosol particles

Surface Chemistry Research Group, Head Markus Ammann. The uptake of nitrogen dioxide (NO₂), a major trace gas in the atmosphere, to deliquesced particles containing the sodium salts of hydroquinone (1,4-dihydroxybenzene) or gentisic (2,5-dihydroxybenzoic) acid was investigated at 40% relative humidity and 23 °C in an aerosol flow tube. The experiments were performed using the short-lived radioactive tracer 13N and a denuder technique. The observed uptake coefficient for NO₂ was up to ∼6 × 10⁻³ for the hydroquinone disodium salt aerosol, which exceeds previously reported data in the range 10⁻⁴ to 10⁻³. The measured time dependence of NO2 uptake was fitted using a kinetic model taking into account reactant consumption in the particle phase, and keeping the bulk accommodation coefficient, αb, and the rate constants for the reaction of dissolved NO₂ with the deprotonated forms of the mentioned phenolic compounds as variables. We obtained αb = 0.024−0.003+0.018 as a best estimate. For gentisic acid, the second-order rate constant was k2 = (2.9 ± 0.1) × 108 L mol⁻¹ s⁻¹ and is reported for the first time. The data are consistent with bulk reaction limited uptake, without indications for a surface component in the kinetics.

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GFA (Department Large Research Facilities)

Minimum emittance coupling in the SLS

Recent measurements of the vertical beam size in the SLS have shown that the value of the vertical emittance is ~ 3.2 pm-rad, close to the fundamental limit set by the quantum nature of synchrotron radiation (~ 0.5 pm-rad). The vertical beam size was determined using an optical imaging device especially built for this purpose. In this device the images are formed from vertically polarised light emitted by the electron beam in the visible to UV part of the spectrum. The new monitor allows determination of vertical beam sizes below 10 microns with sub-micron accuracy. The emittance is determined from the measured beam size and prior knowledge of the betatron and dispersion functions at the source point, determined by other means. This extremely small vertical emittance testifies to the careful reduction of transverse emittance coupling (0.05%) obtained in the SLS by virtue of carefully adjusting the settings of the ring lattice. To the best of our knowledge, this is the lowest reported coupling in an electron storage ring.