Our research focuses on the
spectroscopic investigation of the properties of molecular nanoparticles
and aerosols. They play a crucial role in the atmosphere of the earth and in
interstellar space. The subject of another very active field
are nanoparticles of pharmaceutical agents
which are attractive drug delivery systems for medical applications. Our main
interest is to learn more about the relation between the properties of the
particles and the nature of the intermolecular interactions that hold them
together.
Nanoparticles which are built up from molecules
exhibit a variety of interesting effects in the size range between one nanometer and one micrometer. The lower nanometer
range provides the link to molecular clusters. In this region, the properties
of the particles are mainly determined by the large surface to volume ratio.
The interaction of light in the mid-infrared with particles around one hundred nanometers leads to characteristic size and shape dependent
resonance phenomena. In bigger particles, such phenomena appear together with
the characteristic scattering effects in the spectra. Scattering with
mid-infrared light becomes most prominent in the micrometer region, where the
particle properties resemble those of the macroscopic material. But not only
size and shape can influence particle spectra. The particle phase
(amorphous/crystalline) also determines the spectral features. The most
prominent influence, however, is to be expected from chemical reactions. An
important property for heterogeneous reactions is for example the large
specific surface in particulate matter.
Experimentally, the main challenge
is to generate particles with a well defined size distribution. Up to now, we
apply three different methods to generate the particles. Particles of
non-volatile substances can be produced by spraying solutions and subsequent
drying of the primary droplets. Spraying in a high-voltage field, the so called
electrospray generation, leads to small particles with
narrow size distributions. The generation of nanoparticles
by Rapid Expansion of Supercritical Solutions (RESS) opens a broad spectrum of applications. This
includes the micronization of pharmaceutical
substances. For volatile substances, the particles can be produced by collisional cooling directly from the gas phase. In a collisional
cooling cell,
introducing the warm gas phase into a cold bath gas leads to supersaturation and thus to particle formation. Cooling
with liquid helium, the temperature of the buffer gas can be reduced down to
almost 4K. This influences the particle formation and enables us to perform
temperature dependent studies. A detailed size characterization is the basis
for a quantitative analysis of the results. The size distribution of particles
can be determined from scattering experiments such as 3-Wavelenghts-Extinction
measurements. The sizing and subsequent counting of particles in an electric
mobility analyser
and a condensation
particle counter
leads to well defined size distributions. This method, however, is only
applicable to non-volatile particles. Shape and size can also be obtained from
electron microscopy.
Fourier transform infrared
spectroscopy is ideally suited for the investigation of intermolecular
interactions in molecularly structured nanoparticles
and aerosols. It allows to study the characteristic
absorption bands and scattering phenomena of the often broad bands of the
particulate phase. At the same time the gas phase of aerosols can be
investigated, an important prerequisite for studying heterogeneous reactions.
The investigations aim at a
better understanding of the properties of molecular nanoparticles
in relation to the intermolecular forces acting between the constituent
molecules. A true understanding, however, requires quantitative models of the
phenomena observed experimentally. To cover the enormous range of several
orders of magnitude from less than one nanometer up
to micrometers poses a major challenge to theory. Today, very small particles
or clusters can be modelled as supermolecules with
quantum chemical methods. To analyze the spectroscopic properties as a function
of size, model potentials and quantum chemical calculations for small particles
can serve as a starting point. For particles in the intermediate size range,
the molecular picture can be retained, but here one depends on classical
molecular dynamics methods. Classical continuum models - frequency-dependent
optical constants- together with classical electrodynamics are appropriate to
describe large particles and their interaction with light. To bring these three
different approaches together in a consistent picture is still an active field
of research.