Scientific Interests

The term “cluster” describes, in physical and chemical science, a group of atoms or molecules formed by different kind of bonding interactions. These interactions can vary from weak van-der-Waals contacts to ionic bonds. All these systems, no matter if formed by one or multiple elements, share the same characteristic. Very often clusters show different bonding arrangements and therefore different geometric structures compared to known bulk phases or molecular compounds. Therefore, the physical and chemical properties of small clusters vary with size and composition in a way which can not easily described by simple empirical scaling laws. In the research group of Prof. Dr. Schäfer we focus on experimental investigations of bare and supported clusters with the goal to quantify and understand the size and composition depended properties of metallic and semiconducting aggregates.

Cluster Catalysis and Cluster Surface Interactions

Schematic experimental setup.
Schematic experimental setup.

An isolated cluster in the gas phase often shows different geometrical structures and bonding arrangements compared to clusters in solution or on surfaces. A first step towards a practicable application is to study the interaction of clusters with well defined environments, e.g. surfaces. For this purpose a laser vaporization cluster source is coupled with a collinear time-of-flight mass spectrometer and an electric mirror, enabling investigations of mass selected charged clusters [1]. The mass selected clusters can be guided to the surface of interest with a previously defined kinetic energy. Consequently, the interactions of clusters with a surface can be studied as a function of there kinetic impact energy. This allows soft landing, surface induced dissociation, pinning and implanting of clusters. Hence, soft landing, pinning or implanting of clusters enables us to perform experiments with the supported metal particles, for example catalysis [2], while surface induced dissociation gives insight of the gas phase cluster energetics [3,4].

[1] M. Turra, B. Waldschmidt, B. Kaiser, R. Schäfer, An improved time-of-flight method for Cluster Deposition and Ion-Scattering Experiments, Rev. Sci. Instrum. 79, 013905 (2008).

[2] R. Joshi, B. Waldschmidt, J. Engstler, R. Schäfer, J. J. Schneider, Generation and agglomeration behaviour of size-selected sub-nm iron clusters as catalysts for the growth of carbon nanotubes, Beilstein J. Nanotechnol. 2, 734 (2011).

[3] B. Waldschmidt, M. Turra, R. Schäfer, Surface-Induced Dissociation as a Probe for the Energetics and Structure of Lead Clusters, Z. Phys. Chem. 221, 1569 (2007).

[4] B. Waldschmidt, S. Barman, C. Rajesh, C. Majumder, G. P. Das, R. Schäfer, Energetics and Fragmentation of single-doped Tin and Lead Clusters, Phys. Rev. B 79, 045422 (2009).

Collision induced dissociation

Laser Spectroscopy

Schematic experimental setup.
Schematic experimental setup.

When a gas phase cluster interacts with light various processes can occur. A possible one photon process is the fragmentation of a neutral cluster, if the sum of the absorbed photon energy and the internal energy of the cluster is above the dissociation threshold. Due to the small cluster concentration in the gas phase it is not possible to perform a “normal” absorption experiment. Therefore, the spectral information is gained by coupling the experiment to mass spectrometric measurements. A typical experiment is performed in a supersonic molecular beam, where all objects travel on the same axis. If a photon induced cluster fragmentation occurs, the produced fragments will most likely leave the predefined molecular beam axis. Consequently, a mass spectrum recorded on the beam axis and behind the laser excitation region will show a depletion of the fragmented cluster. Therefore, measuring the depletion of the mass signal as a function of photon energy enables to record the UV-Vis spectrum of gas clusters. The experiment is realized in a high-vacuum apparatus, equipped with a laser vaporization source, an orthogonal time-of flight spectrometer and flash lamp ionization source. The tuneable laser source (NIR-UV) is an optical parametric oscillator pumped by a third harmonic of a Nd:YAG and the corresponding second harmonic generator unit.

Clusters in electric and magnetic fields

Schematic experimental setup.
Schematic experimental setup.

A cluster in a static electric or magnetic field interacts with the field via its electric or magnetic dipole moment. The first is sensitive to chemical bonding and in some cases to the geometric structure of the cluster while the second gives information on the electronic state of the isolated gas phase species. Experimentally, the described properties are accessible by beam deflection experiments. The clusters are produced in a laser vaporization source and a supersonic molecular beam is formed. This beam passes an inhomogeneous electric or magnetic field in which the cluster experiences a force due to the presence of an electric or magnetic dipole moment. A movable slit and an Excimer laser are used to detect the molecular beam deflection and ionize the clusters, whereas the mass distribution is analyzed by a time-of-flight mass spectrometer. A quantitative analysis of the field induced beam deflection allows extracting electric [1] or magnetic [2] dipole moments from experiment. Combined with theoretical computations, beam deflection measurements are a powerful tool to investigate the electronic and geometric structure of clusters.

[1] S. Heiles, S. Schäfer, R. Schäfer, On the rotational temperature and structure dependence of electric field deflection experiments: A case study of germanium clusters, J. Chem. Phys. 135, 034303 (2011).

[2] U. Rohrmann, S. Schäfer, R. Schäfer, Size- and Temperature-Dependent Magnetic Response of Molecular Cage Clusters: Manganese-Doped Tin Clusters, J. Phys. Chem. A 113, 12115 (2009).