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Recent Topics
Two-Color Strong-Field Photoelectron Spectroscopy and the Phase of the Phase
The presence of a weak second-harmonic field in an intense-laser ionization experiment affects the momentum-resolved electron yield, depending on the relative phase between the ω and the 2ω component. The proposed two-color “phase-of-the-phase spectroscopy” quantifies for each final electron momentum a relative-phase contrast (RPC) and a phase of the phase (PP) describing how much and with which phase lag, respectively, the yield changes as a function of the relative phase. Experimental results for RPC and PP spectra for rare gas atoms and CO2 are presented. The spectra demonstrate a rather universal structure that is analyzed with the help of a simple model based on electron trajectories, wave-packet spreading, and (multiple) rescattering. Details in the PP and RPC spectra are target sensitive and, thus, may be used to extract structural (or even dynamical) information with high accuracy.
S. Skruszewicz et al., Phys. Rev. Lett. 115, 043001 (2016)
Morphological impact on the reaction kinetics of size-selected cobalt oxide nanoparticles
Apart from large surface areas, low activation energies are essential for efficient reactions, particularly in heterogeneous catalysis. Here, we show that not only the size of nanoparticles but also their detailed morphology can crucially affect reaction kinetics, as demonstrated for mass-selected, soft-landed, and oxidized cobalt clusters in a 6 nm to 18 nm size range. The method of reflection high-energy electron diffraction is extended to the quantitative determination of particle activation energies which is applied for repeated oxidation and reduction cycles at the same particles. We find unexpectedly small activation barriers for the reduction reaction of the largest particles studied, despite generally increasing barriers for growing sizes. We attribute these observations to the interplay of reactionspecific material transport with a size-dependent inner particle morphology.
S. Bartling et al., J. Phys. Chem. 143, 114301 (2015)
Scattering image of an individual Ag nanoparticleThe 3D-architecture of individual free silver nanoparticles captured by X-ray scattering
Determining the three-dimensional shape of individual nanoparticles in flight is a challenging task. While for nanostructures at surfaces various tomographic methods exist free particles elude 3D experimental access because they cannot be immobilized without introducing additional interactions with the environment. Here we show that wide-angle soft x-ray scattering can be utilized for obtaining the full 3D morphology of individual silver nanoparticles in a single shot. Surprising geometries such as icosahedra and extremely flat particles are revealed in a so far unexplored size regime.
I. Barke et al.,Nature Communications 6, 6187 (2015)
A new design for imaging of fast energetic electrons
We report on an essentially improved version of the classical Eppink–Parker velocity map imaging spectrometer design (Rev. Sci. Instrum. 68, 3477 (1997)). By adding electrostatic lenses with an opposite polarity to the extraction system we succeeded in extending the range of detection of energetic particles up to the keV regime at moderate (<20 kV) extraction voltage conditions. With respect to Eppink–Parker the energy range has been extended by a factor of 2.5. Moreover, particle trajectory simulations demonstrate that the energy resolution can be improved by about 20%.
S. Skruszewicz et al., Int. Journal of Mass Spectrometry 365-366, 338 (2014)
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