Molecular Clusters
Precision Studies of Nanometer Scale Particles in the Gas Phase
Contact Information: How to get in touch with us Coworkers: Our current and previous group members Curriculum Vitae of Wolfgang Christen Publications: Publications, invited talks, and presentations of our group Open Positions: If you are studying physics or chemistry and are interested in the fascinating field of cluster science you should check here Teaching: Lectures

Research Interests: Physical Chemistry at the Nanometer Scale

We study the generation and properties of nanometer-size particles in the gas phase. We employ supersonic jet expansions to grow neutral or charged clusters from a large variety of fluids such as H2O, CO, CO2, C2H6, C3H8,... and with sizes from 2 up to 106 molecules per particle, corresponding to radii of up to 20 nm. In an ultrahigh-vacuum environment these clusters serve as well defined, wall-free test tubes that can serve, e.g., as model aerosols. Advanced analytical, mass spectrometric, and surface science techniques are combined for the exploration of fundamental processes such as homogeneous and heterogeneous condensation, collision-induced and metastable dissociation or evaporation, solubility, ion-molecule, molecule-surface, and cluster-surface reactions. Experimental studies are complemented by model calculations and supported by a continued development of suitable measurement techniques, aiming at an improved comprehension of processes at the nanometer scale.

Phase Transitions

The condensation of supersaturated gases is of fundamental relevance in a number of research fields, such as atmospheric and environmental chemistry, chemical engineering, and process technology. At present this topic experiences a remarkable renaissance owing to urging questions in climate research, fascinating applications in materials science, and also due to the significant progress achieved in modern experimental methods. These laboratory studies at accurately defined conditions are mandatory for an improved comprehension of phase transitions at the nanometer scale. In our research we study the formation and growth of clusters consisting of atmospherically important molecules such as water, carbon dioxide and hydrocarbons. The combination of unprecedented experimental precision and a highly accurate data analysis employing advanced equations of state — beyond the ideal gas approximation — opens up new possibilities and perspectives such as the determination of size-dependent cluster temperatures. As this approach is applicable to most fluid substances, it provides a fairly universal method for an improved characterization of clusters and nanoparticles.

Cluster-Surface Interactions

In the past we have studied chemical reactions of molecular cluster ions with solid surfaces at high kinetic energies, pioneering the field of intra-cluster reactions. These investigations were driven by the need to better understand physical and physicochemical behavior of matter under extreme conditions of pressure and temperature, relevant e.g. for astrophysical processes and high-barrier reactions. A suitable experimental technique was developed for the evanescent creation of ultrahigh pressure and energy density, not otherwise attainable, realized by a cluster-surface collision at hypersonic velocities.
Recently we started broadening this field of research by exploring the surface interaction of neutral clusters at thermal energies and well-defined ultrahigh-vacuum conditions. Solid surfaces can be characterized with standard surface science methods such as Auger electron spectroscopy and low-energy electron diffraction. In these studies we wish to unravel the effects of particle size, kinetic energy and charge of the clusters in promoting surface reactions.

Generation of Nanoparticles of Biochemical and Pharmaceutical Interest

In recent years supercritical fluids have gained widespread attention as solvents thanks to their unique chemical properties. This interest probably is most warranted for dissolving non-volatile or thermally sensitive molecules such as pharmaceuticals in supercritical ethylene or carbon dioxide at ambient temperatures. Employing jet expansions of supercritical solutions the biomolecules can be transferred into the gas phase without thermal strain, allowing the subsequent application of established analytical methods or the controlled vapor deposition on surfaces. We conduct studies of this type, generating and depositing nanometer-sized particles of pharmaceutical ingredients with a narrow size distribution. It is then possible to further analyze their morphology, etc.


  • Gain insight into condensation processes and thermodynamics of many particle systems at the molecular level.
  • Improve comprehension of physicochemical processes at conditions of extreme pressure and temperature, and in the vicinity of the critical point.
  • Learn about energy transfer, charge transfer, and surface reactions of molecules and molecular clusters.
  • Develop advanced experimental tools to investigate physical and chemical behavior at the nanometer scale.
  • Employ compressed gases for analytical applications.

Please feel free to contact us any time.

Go to the top of the page
Contact      Privacy      Copyright © 2001–2024 Wolfgang Christen. All rights reserved.
Send mail to with questions or comments about this Web site.     Last update: 5. April 2024