Research/Teaching Interests

Spectroscopy of Aerosols, Clusters, and Nanoparticles

The key objective of our research is the controlled generation and detailed spectroscopic characterization of molecular nanoparticles, aerosols, and clusters from the subnanometer to the micrometer size range. This includes the study of reactive processes involving particles. The rapidly growing interest in these weakly bound huge molecular aggregates arises from their increasing relevance in many different fields. Molecular particles of widely varying composition play an important role as aerosols in atmospheric processes and as reactive sites in interstellar dust. To understand their influence on corresponding processes and to control their formation it is crucial to know their chemical composition, their size, their reactivity, and the processes by which they are formed. This knowledge opens up other fields of application such as nanoparticles of pharmaceutical agents which are attractive drug delivery systems for medical applications. The major goal here is the preparation of encapsulated or coated drug particles for the controlled drug delivery.

Our current research activities are in the following areas:

(i) Infrared Spectroscopy and Modeling of Molecularly Structured Aerosol Particles

(ii) XUV Spectroscopy, Mass Spectrometry, Photoelectron Spectroscopy of Aerosols and Cluster

(iii) Optical Trapping of Single Aerosol Particles

(iv) Metallodielectric Nanoparticles and Molecular Nanoparticles as Drug Delivery Systems

We ultimately aim at unravelling the microscopic origin of the characteristic patterns found in the spectra of these weakly bound molecular aggregates. In this context, we want to answer two central questions:(i) Which spectral properties can be described on a local or even molecular level and which are inherent ensemble properties affecting the particle as a whole? (ii) To what extend does this distinction depend on the type and strength of intermolecular forces acting in the particles? To address these questions we combine a broad range of experimental tools with specially tailored theoretical approaches.

Methods:

Particle Generation: Bath gas cooling (4-298K); Supersonic Expansion; Rapid Expansion of Supercritical Solutions (RESS); Electrospray

Particle Characterization: Spectroscopic Characterization In Situ; Laser Light Scattering; Particle Sizing; Electron Microscopy

Modelling: Standard Quantum Chemical Calculations ; Vibrational Dynamics Calculations; Molecular Dynamics Simulations; Classical Scattering Theory

Examples:

1) Shape, Size, and Surface Effects of Icy Aerosol Particles of Planetary Atmospheres: Molecular ice particles of widely varying composition influence physical-chemical processes in the atmospheres of planets, their moons and in interstellar dust. One example are methane aerosols on Titan which form the counterpart to water clouds in the Earth's atmosphere. Another example are ammonia clouds in the atmospheres of Jupiter and Saturn. In these fields spectroscopic methods play a crucial role in characterizing these weakly bound aggregates. We are interested in the spectroscopic characterization of pure and composite particles with sizes in the submicrometer range. This also includes the study of intrinsic particle properties such as shape, size, and surface effects. Left: Small ammonia ice cluster. Right: Shape effects in IR spectra of CO₂ ice particles. Experiments compared with quantum mechanical exciton calculations. The particles can be generated in supersonic expansions or by collisional cooling down to liquid helium temperatures.

2) Generation of Drug Nanoparticles: Coating and mixing of drug nanoparticles with polymers helps to prevent agglomeration of the primary particles and to control drug release. The agglomeration behaviour depends on the amout of polymer which can be controlled by FTIR-spectroscopy in situ during the particle formation. Left: SEM-Image of agglomeratedphytosterol particles with primary radii of less than 50 nm. The particles were generated by rapid expansion of supercritical CO₂ solutions. Right: In situ IR spectra of pure phytosterol particles and mixed phytosterol/L-PLA particles.

Research Projects