Welcome to the research group on clusters and nanostructures!
Our main field of research is the investigation of clusters, i.e. of particles as small as only a few nanometers, or even less. Such small particles are benchmark systems for nanostructured materials and offer the unique chance to explore the development of many-body phenomena in finite quantum systems. We study the properties of free atomic clusters and complexes embedded in ultracold helium droplets, with a special focus on the interaction between light and matter, ranging from electron emission up to ultrafast nonlinear dynamics in nanoplasmas. Read more...
Highly charged Rydberg ions from the Coulomb explosion of clusters
Ion emission from a nanoplasma produced in the interaction of intense optical laser pulses with argon clusters is studied resolving simultaneously charge states and recoil energies. By applying appropriate static electric fields we observe that a significant fraction of the ions Arq+ ( q = 1 − 7 ) have electrons with binding energies lower than 150 meV, i.e. nRyd ≥ 15 levels are populated. Charge state changes observed on a μs time scale can be attributed to electron emission due to autoionizing Rydberg states, indicating that high- ℓ Rydberg levels are populated as well. The experiments support theoretical predictions that a significant fraction of delocalized electrons, which are bound with hundreds of eV to the nanoplasma after the laser exposure, fill up meV bound ion states in the adiabatic expansion. We expect the process to be relevant for the long-term evolution of expanding laser-induced dense plasmas in general.
Nanoplasmonic electron acceleration by attosecond-controlled forward rescattering in silver clusters
In the strong-field photoemission from atoms, molecules, and surfaces, the fastest electrons emerge from tunneling and subsequent field-driven recollision, followed by elastic backscattering. This rescattering picture is central to attosecond science and enables control of the electron’s trajectory via the sub-cycle evolution of the laser electric field. Here we reveal a so far unexplored route for waveform-controlled electron acceleration emerging from forward rescattering in resonant plasmonic systems. We studied plasmon-enhanced photoemission from silver clusters and found that the directional acceleration can be controlled up to high kinetic energy with the relative phase of a two-color laser field. Our analysis reveals that the cluster’s plasmonic near-field establishes a sub-cycle directional gate that enables the selective acceleration. The identified generic mechanism offers robust attosecond control of the electron acceleration at plasmonic nanostructures, opening perspectives for laser-based sources of attosecond electron pulses.
High performance charge-state resolving ion energy analyzer optimized for intense laser studies on low-density cluster targets
We report on a versatile ion analyzer which is capable to resolve ion charge states and energies with a resolution of E/ΔE = 100 at 75 keV/nucleon. Charge states are identified by their characteristic deflection in a magnetic field, whereas the ion energies are independently determined by a time-of-flight measurement. To monitor the signals a delay-line detector is used which records ion impact positions and times in each laser shot. Compared to conventional Thomson parabola spectrometers our instrument provides a low background measurement, hence a superior dynamic range. Further features are an improved energy resolution and a significantly increased transmission. We demonstrate the performance by showing charge-state resolved ion energy spectra from the Coulomb explosion of a low-density target, i.e., silver clusters exposed to intense femtosecond laser pulses.
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.
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.
The 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.