Experimental verification of agglomeration effects in infrared spectra on micron-sized particles

¹ Heidelberg University, Kirchhoff-Institute for Physics, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany
² Max Planck Institute for the Science of Light, Guenther-Scharowsky-Str. 1, 91058 Erlangen, Germany
³ University of Oldenburg, Division Microrobotics and Control Engineering (AMiR), Uhlhornsweg 84, 26129 Oldenburg, Germany
⁴ Tohoku University, Astronomical Institute, 6-3 Aramaki, Aoba-ku, Sendai, Miyagi, 980-8578 Japan
⁵ Friedrich-Schiller-University Jena, Astrophysical Institute and University Observatory, Schillergaesschen 3, 07745 Jena, Germany
e-mail: mutschke@astro.uni-jena.de

Received: 28 March 2018
Accepted: 26 August 2018

Abstract

Context. Detailed analysis of observed infrared (IR) dust emission spectra is often performed in order to derive information about mineralogy, particle size, and temperature of the dust. However, the IR bands are also influenced by agglomeration of the dust particles. Light scattering theory simulating agglomeration and growth effects is especially challenged by the consideration of highly absorbing particles.

Aims. To clarify the influence of agglomeration on the diagnostic phonon bands of amorphous SiO₂ particles, we experimentally measure the extinction spectra of systematically arranged particle configurations and compare the measured spectra with the spectra obtained from different theoretical approaches.

Methods. We construct artificial particle agglomerates by means of the dedicated robotic manipulation (DRM) technique. IR microspectroscopic extinction measurements of these arranged particles are performed at the French National Synchrotron Facility, SOLEIL, in the mid-IR region considering polarization effects. The theoretical approaches applied are the discrete dipole approximation (DDA) as well as T-matrix and finite-difference time-domain methods.

Results. In both the experimental spectra and the theoretical calculations, we find that the Si–O stretching vibration band at about 9 μm is clearly broadened on the long-wavelength side by the agglomeration of particles. This is mainly caused by the radiation components, which are polarized in directions in which the agglomerate is extended, while the extinction band profile of the component polarized perpendicular to the long axis of an elongated agglomerate is close to the spectrum of the single sphere. All of the theoretical simulations predict these effects in qualitatively good agreement.

Conclusions. Our comparative study of the experimentally measured and theoretically calculated IR extinction spectra of well-defined agglomerate structures makes obvious how the various particle arrangements in small clusters might contribute to average spectra of dust. Therefore the study might help to improve the precision of light scattering calculations as well as their specific applicability.