Infrared spectroscopy of particulate matter: between molecular clusters and bulk

The present contribution focuses on the vibrational dynamics of large molecular aggregates with sizes ranging from the subnanometre to the micrometre region. These aggregates or particles bridge the gap between molecular clusters and bulk matter. Depending on the kind of intermolecular interactions, the vibrational dynamics of the particles can be strongly influenced by intrinsic particle properties such as size, shape, or surface area. Such phenomena are discussed for several examples. In contrast, for some substances the vibrational spectra are mainly governed by local effects, i.e. by the interaction of neighboring molecules. The comparison of the experimental results with different model calculations leads to a deeper insight into the microscopic origin of the characteristic patterns observed. The vibrational dynamics are studied by Fourier transform infrared spectroscopy. In addition, the particle properties are characterized by microscopy and measurements of the size distribution. Four different particle generation methods are discussed. It is shown that the methods of generation can even influence the chemical composition of the particles.