Infrared spectroscopy and vibrational exciton modeling of crystalline, polycrystalline and amorphous acetylene aerosol particles.

Mid-infrared spectra of acetylene aerosol particles generated under conditions relevant to Titan's atmosphere are analyzed in terms of the vibrational exciton model. The analysis reveals that acetylene aerosol particles do not form single crystals below 110 K as previously assumed. Instead, less ordered structures such as polycrystalline particles with orthorhombic crystalline domains or possibly partially amorphous particles derived from an orthorhombic crystal structure are found to be very stable.

Predicting the influence of shape, size, and internal structure of CO aerosol particles on their infrared spectra.

The influence of shape, size, and internal structure of CO aerosol particles on mid-infrared spectra is modeled for aggregates in the size range between 1 and 100 nm. Combining the vibrational exciton model with a molecular dynamics approach, we identify spectral features that are characteristic for the shape of the particles and for their internal structure (crystalline, amorphous, and partially amorphous) over the whole particle size range. The characteristic size-dependent patterns in the spectra of small particles (<10 nm) are lost for particle ensembles with typical size distributions.

Vibrational exciton coupling as a probe for phase transitions and shape changes of fluoroform aerosol particles.

Phase transitions and shape changes of aerosol particles play a fundamental role in atmospheric as well as technical processes involving aerosols. For the example of fluoroform particles, we demonstrate how information about both processes can be extracted from time-dependent infrared spectra by comparison with vibrational exciton calculations. We find volume crystallization rate constants for fluoroform particles in the range of J(v) = 10(8)-10(10) cm(-3) s(-1) at a temperature of T = 78 K.

Intrinsic particle properties from vibrational spectra of aerosols.

The spectroscopy of aerosols is developing into an active and important field. It allows us to characterize aerosols in a nonintrusive way, in real time, and on site. Understanding the spectroscopic features of these highly complex systems requires the development of novel experimental as well as theoretical methods.

Phase, shape, and architecture of SF6 and SF6/CO2 aerosol particles: infrared spectra and modeling of vibrational excitons.

Information on the phase, shape, and architecture of pure SF(6) and mixed SF(6)/CO(2) aerosol particles is extracted from experimental infrared spectra by comparison with predictions from quantum mechanical exciton calculations. The radius of the particles lies around 50 nm. The following extensions to our previous vibrational exciton model are included: (i) To account for the many degrees of freedom of degenerate vibrational bands of aerosol particles, we take a time-dependent approach to calculate infrared absorption spectra directly from the dipole autocorrelation function.

Size effects in the infrared spectra of NH3 ice nanoparticles studied by a combined molecular dynamics and vibrational exciton approach.

Infrared extinction spectra of ammonia ice nanoparticles with radii between 2 and 10 nm show pronounced band shape variations depending on the conditions of particle formation by collisional cooling. We present experimental and theoretical evidence showing that the variations in the region of the nu2 (umbrella) fundamental are due to changes in the particle size. The effect is analyzed in terms of an explicit atomistic model of the particles' structure and vibrational dynamics.

Large molecular aggregates: From atmospheric aerosols to drug nanoparticles.

Large molecular aggregates with sizes ranging from subnanometers to microns are ubiquitous. As atmospheric aerosols they influence our climate, in interstellar space they are discussed as reactive sites, and in medicine small particles are considered as promising candidates to achieve a targeted drug delivery. The present contribution is focused on the characterization of the physical-chemical properties of these particles and on their targeted generation.