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Vibrational and rotational relaxation of hexafluorobenzene studied by Raman scattering in the supercritical domain

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Title: Vibrational and rotational relaxation of hexafluorobenzene studied by Raman scattering in the supercritical domain
Authors: Cabaço, M. Isabel1,2 isabel@cii.fc.ul.pt, Besnard, M.3, Tassaing, T.3, Danten, Y.3
Source: Journal of Molecular Liquids. Apr2006, Vol. 125 Issue 2/3, p100-106. 7p.
Subjects: Raman spectroscopy, Spectrum analysis, Supercritical fluids, Qualitative chemical analysis
Abstract: Abstract: The vibrational and rotational relaxation of hexafluorobenzene (C6F6) has been studied by Raman spectroscopy in the supercritical domain. The polarized I VV(ν) and depolarized I HV(ν) profiles associated with the vibrational ν 1(A 1g) ‘breathing’ mode of the molecule have been analysed for the fluid under isothermal (T ⁎ = T/T C ∼1.23 and 1.02) and isobaric conditions (P ⁎ = P/P C ∼3.34 and P ⁎ ∼4.89) in a wide density domain (0.02≤ ρ ⁎ = ρ/ρ C ≤3). The band shapes were found to be close to Lorentzian profiles in the density and temperature range investigated. The band centre position shifts towards higher frequency values with density. These shifts have been discussed in terms of the competition between repulsive and attractive forces using the Buckingham-Ben-Amotz approach. It was found that repulsive interactions are dominant and that the attractive interactions are mostly effective at low densities. As for the vibrational relaxation, the analysis has been done in the framework of the dephasing mechanism. In the domain of liquid-like densities the experimental vibrational times follow the Isolated Binary Collision (IBC) model (Fischer-Laubereau), derived for a repulsive interaction. In contrast, along the isotherms in the gas-like density values, such a model does not apply. As a consequence, it appears that attractive interactions play a significant role, in the gas-like density domain. As the rotational dynamics of the main molecular axis (tumbling motion) obeys the fast modulation condition, the molecular reorientation process is governed by a diffusional process. Along the near critical isotherm and in the high compressible domain, local density inhomogeneities are found to have strong influence on both measured shifts and rotational correlation times. A local density excess δρ ⁎ = ρ loc ⁎− ρ ⁎ has been featured from these two observables. Furthermore, a very large activation volume was found (800 cm3 mol−1) which is due to the clustering phenomenon taking place in the hyper-compressible domain. [Copyright &y& Elsevier]
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Abstract:Abstract: The vibrational and rotational relaxation of hexafluorobenzene (C6F6) has been studied by Raman spectroscopy in the supercritical domain. The polarized I VV(ν) and depolarized I HV(ν) profiles associated with the vibrational ν 1(A 1g) ‘breathing’ mode of the molecule have been analysed for the fluid under isothermal (T ⁎ = T/T C ∼1.23 and 1.02) and isobaric conditions (P ⁎ = P/P C ∼3.34 and P ⁎ ∼4.89) in a wide density domain (0.02≤ ρ ⁎ = ρ/ρ C ≤3). The band shapes were found to be close to Lorentzian profiles in the density and temperature range investigated. The band centre position shifts towards higher frequency values with density. These shifts have been discussed in terms of the competition between repulsive and attractive forces using the Buckingham-Ben-Amotz approach. It was found that repulsive interactions are dominant and that the attractive interactions are mostly effective at low densities. As for the vibrational relaxation, the analysis has been done in the framework of the dephasing mechanism. In the domain of liquid-like densities the experimental vibrational times follow the Isolated Binary Collision (IBC) model (Fischer-Laubereau), derived for a repulsive interaction. In contrast, along the isotherms in the gas-like density values, such a model does not apply. As a consequence, it appears that attractive interactions play a significant role, in the gas-like density domain. As the rotational dynamics of the main molecular axis (tumbling motion) obeys the fast modulation condition, the molecular reorientation process is governed by a diffusional process. Along the near critical isotherm and in the high compressible domain, local density inhomogeneities are found to have strong influence on both measured shifts and rotational correlation times. A local density excess δρ ⁎ = ρ loc ⁎− ρ ⁎ has been featured from these two observables. Furthermore, a very large activation volume was found (800 cm3 mol−1) which is due to the clustering phenomenon taking place in the hyper-compressible domain. [Copyright &y& Elsevier]
ISSN:01677322
DOI:10.1016/j.molliq.2005.11.002