The research in the aggregation and kinetics group aims at exploring aggregation phenomena under reactive conditions
at the molecular level. The atmospheric oxidation of certain organic substances and the oxidation of hydrocarbon fuels
in diesel engines are well known processes leading to particle formation. In the atmosphere such aggregates containing
a large fraction of organic matter are called secondary organic aerosol. Soot particles are responsible for the black
colour of exhaust gas from diesel fuel combustion. Aerosols influence visibility, climate, and human health and their
chemistry is an active field of research with many open issues. An example is the climate effect: It is very large but
the mechanisms behind are only poorly characterized.
In this context an important question is the exact structure of the smallest aggregates at the beginning of aggregation
processes. Such small aggregates are called clusters and we examine their structure with the help of reactive sodium atoms.
When attached to an aggregate and irradiated by ultraviolet or visible light the weakly bound sodium 3s electron is ejected
and the cluster softly ionized without fragmentation. Additional irradiation with an infrared laser provides the infrared
spectrum of the aggregate. The infrared spectrum is our basis for examining the cluster structure, but we need support from theory
for arriving at structural assignments. When aggregation occurs under reactive conditions a zoo of stable and unstable species is formed and it is very difficult
to identify those, which initiate the formation of aggregates. Here our trick is first to study the kinetics of the gas phase
chemistry, which we can do well and second to analyze the time evolution of the particle size distribution, which we also can do well.
The links we find between gas phase kinetics and the particle dynamics are our key for unravelling the aggregation mechanism. With a newly developed code we can simulate now
for sulphuric acid vapour the complete aggregation process from reactive monomer formation in the gas phase to the time evolution of the emerging particle size distribution. Moreover we continuously
develop complex reaction mechanisms for model fuels being used in industrial combustion research for optimizing engine performance and minimizing pollutant formation.