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. (ii) In addition to the dipole-dipole interaction, dipole-induced dipole terms are included to account for the high polarizability of SF(6) and CO(2). We find SF(6) aerosol particles with a cubiclike shape directly after their formation and a change in the shape toward elongated particles with increasing time. Our microscopic model reveals that the cubic-to-monoclinic phase transition at 96 K found in the bulk cannot be observed with infrared spectroscopy because the two phases show almost identical spectra. Infrared spectra of two-component SF(6)/CO(2) particles with core-shell structure show characteristic split absorption bands for the shell. By contrast, homogeneously mixed SF(6)/CO(2) particles lead to broad infrared bands for both the core and the shell. The molecular origin of these various spectral features is uncovered by the analysis of the vibrational eigenfunctions.
Firanescu
George
G
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada.
Luckhaus
David
D
Signorell
Ruth
R