Research Interests

Molecular clusters offer the unique possibility to investigate the evolution of physical and chemical properties of matter with the size of the nanoparticle, starting from the individual molecule. It is in this "cluster regime", where many properties change irregularly in a non-scalable manner with the number of molecules. Here, chemistry may be dramatically different from what is known in the macroscopic, "bulk" limit. Most favorably, molecular clusters constitute extremely well-defined test tubes, permitting investigations at the level of individual molecules. Our focus is on the formation mechanisms of these nano-droplets by condensation, the solubility of individual molecules in such clusters, and the chemical reactivity of molecular clusters with well-defined solid surfaces.

Condensation and Nucleation

The phenomenon of homogeneous nucleation is of fundamental relevance in fields such as atmospheric chemistry, astrophysics, and materials science. Surprisingly, the detailed processes involved are not well understood even today. While atmospheric aerosol formation is known to occur almost all over the world, and the importance of these particles to climate and air quality has been recognized, almost all of the processes driving aerosol formation take place below a particle diameter of approx. 3 nm. At present, observations can cover only larger particles. Here supersonic jet expansions provide excellent conditions for studying the microscopic kinetics of homogeneous nucleation.

Cluster Impact Chemistry

Chemical reactions in extreme environments with conditions beyond common temperatures and pressures play a central role both in basic and applied research. Prominent examples are the collision-induced synthesis of organic molecules by high-velocity molecular collisions between interstellar clouds as a possible explanation for the "origin of life", or high-threshold reactions that are relevant e.g. in the commercial production of fertilisers or future energy sources. Due to their high activation energies, such reactions do not take place regularly under ambient conditions. Because dissociation would precede, they cannot be initiated by conventional heating. One possible approach to overcome this problem is an energy deposition that is faster than the energy redistribution. This can be realised in an extrmely well defined manner by molecular clusters, moving at a hypersonic velocity, colliding with a solid surface.

Objectives

• Gain insight into condensation processes and thermodynamics of many particle systems at the molecular level.
• 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 extreme pressure and temperature.
• Employ compressed gases for analytical applications.

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