Phase, shape, and architecture of SF6 and SF6∕CO2 aerosol particles: Infrared spectra and modeling of vibrational excitons

Abstract

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.

Study of cluster formation in low-temperature systems. Spectral manifestation of resonance dipole–dipole interactions between nondipole polyatomic molecules

“…Few conclusions can be drawn from this figure. First, the calculated correlation of n 3 and n 2 parameters by using the experimental data for CF 4 solution is in accordance with the suggested model. Second, for the NF 3 molecule, the discrepancy between the predicted and experimental behavior of n 3 vs n 2 is observed in the concentration range C = (0.2−0.5) mf.…”

“…At the same time, the absorption bands in the spectra of pure liquids exhibit complex asymmetric profiles extending over ≈ 60 cm −1 . It seems likely that the main mechanism of the band shape formation is the RDD interaction because the dipole moment derivative 3 P′ has a high value of 0.55 D for SF 6 and 4 . In addition, it can be seen from Figs.…”

“…Note that there exists some ambiguity in the parameter a for the studied liquid. For example, a = 5.5 cm −1 for liquid CF 4 at temperature close to the melting point [18] and a = 7.0 cm −1 found from the density of liquid at the same temperature. Figure 10,a displays the calculated correlation of n 3 and n 2 parameters by using the experimental data on the M(2) and M(3) spectral moments for solution of CF 4 in LAr with a varying from 5.5 to 7.1 cm…”

“…IR spectra of SF 6 , SiF 4 and CF 4 in low-temperature Ar, N 2 and Ne matrices were recorded at 11 K (Ar, N 2 matrices) and 4.7 K (Ne matrix) with a resolution of 0.25 and 0.12 cm −1 by means of a Bruker 113v FTIR and Bruker IFS/66S…”

“…The intense fundamental band and overtone bands for a mode with a large value of P′ in the IR absorption are asymmetrically broadened by RDD interaction in the Raman [12−14] and IR absorption spectra of lowtemperature molecular liquids consisting of polar or nonpolar molecules [15−17]. By using the RDD interaction formalism, band shape simulations were performed for the absorption spectra of several liquids (CF 4 , SF 6 , SiF 4 , CF 3 Cl and CF 3 Br) at temperatures close to the melting point [18], and the results obtained are in quantitative agreement with the experimental data.…”