AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Infrared Spectra of Individual Wavelength-Scale Particles: Spectral Challenges and Novel Techniques
ARUNA RAVI, Antriksh Luthra, James Coe, The Ohio State University
Abstract Number: 656 Working Group: Aerosol Chemistry
Abstract The characterization of infrared (IR) extinction and absorption spectra of single particles of mixed composition has wide use in astronomy, geology, atmospheric sciences, and nanotechnology. To quantify the amount of a material in a bulk model, one might employ the Beer–Lambert law, whereas with single particles one might have a specific orientation and require a nonlinear, Mie-like particle theory. A “scatter-free” infrared library of 63 previously airborne dust particles of 3-5μm size was obtained with an imaging infrared microscope by trapping the particles in the holes of plasmonic metal mesh, providing a qualitative analysis of the occurrence of common materials in our lab air [K.E. Cilwa et al, J. Phys. Chem. C. 115, 16910 (2011)]. A recent study by our group [D.B. Lioi et al, J. Phys. Chem. A. (2013), under review] presents a quantitative analysis of inhalable dust using IR spectra of 94 individual dust particles of known composition. Single particle spectra present a unique set of challenges, for instance, spectral lineshapes of wavelength scale particles can be distorted by shape, phase, and orientation. Another recent study by our group [A. Ravi et al, Phys. Chem. Chem. Phys., 2013, 15, 10307-10315] uses three dimensional finite difference time domain (3D-FDTD) calculations on single particles to illustrate some of the challenges – in particular the distortion of lineshapes by strong phonon vibrations that lie within a range of strong scattering. It is observed that at small sizes relative to the wavelength, particles exhibit Beer-Lambert-like behavior. However, as the particle size increases to about the wavelength, this behavior begins to diverge from Beer-Lambert. A Mie–Bruggeman model for single particle spectra is presented to isolate the effects of strong phonon vibrations on lineshapes which has utility for analyzing the spectra of single, mixed-composition particles. This model enables the determination of volume fractions of components in single particles that are mixtures of many materials with strong phonons, as are the dust particles breathed into people’s lungs.