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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 3749-3756 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report on an experimental investigation of the temperature dependence of the intermolecular dynamics in liquid benzene and toluene. With the use of THz time domain spectroscopy we measured the complex dielectric function (0.2–3.3 THz) of the liquids, at temperatures between −6 °C and 75 °C. By analyzing the dielectric loss (as opposed to the absorption coefficient) we found three contributions to the dielectric function for toluene and two for benzene. In the dipolar liquid toluene we observed a contribution from rotational diffusion at lower frequencies in addition to the two high-frequency librations also observed in benzene. The temperature and density dependence were different for the two librational bands, probably due to the different effect of three-body interactions for the two processes. Furthermore, we present measurements of the low-frequency depolarized Raman spectra as a function of temperature for benzene and toluene. These have been compared with the dielectric loss at similar temperatures. Moreover, for benzene a similar comparison is made between the dielectric loss and the reduced Raman susceptibility obtained by optical Kerr effect spectroscopy. Libration of the symmetric top axis is found in all spectra. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 5246-5255 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dielectric function of liquid benzene at −4 °C, 21 °C, and 65 °C has been calculated from molecular dynamics simulations. The simulated dielectric loss curves reproduce the experimental temperature and density trends. In order to investigate the detailed influence of temperature and density changes as well as the underlying molecular mechanism we have taken advantage of different partitioning and projection schemes for the total dipole moment time correlation function (TCF). The study of the n-body partitioning showed that the temperature dependence of the two- and three-body contributions at t=0 can be explained solely by density change arguments. The molecular projection scheme showed that the dielectric loss is governed by out-of-plane libration at all temperatures. In-plane libration was found to contribute significantly only below 2 THz. Below 1 THz, diffusion, manifested as the negative cross correlation between the out-of-plane and the in-plane TCF's, plays a role very different from that of the directly observed diffusion in dipolar liquids. It has further been established that it is highly problematic to carry out an analysis of the dielectric loss function in terms of the molecular axis rotational TCF's which is a common procedure for the absorption spectrum. This problem was, however, solved by employing a molecular projection scheme. © 2001 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 5319-5331 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report measurements of the real and imaginary part of the dielectric constant of liquid water in the far-infrared region from 0.1 to 2.0 THz in a temperature range from 271.1 to 366.7 K. The data have been obtained with the use of THz time domain reflection spectroscopy, utilizing ultrashort electromagnetic pulses generated from a photoconductive antenna driven by femtosecond laser pulses. A Debye model with an additional relaxation time is used to fit the frequency dependence of the complex dielectric constants. We obtain a fast (fs) and a Debye (ps) relaxation time for the macroscopic polarization. The corresponding time correlation functions have been calculated with molecular dynamics simulations and are compared with experimental relaxation times. The temperature dependence of the Debye relaxation time is analyzed using three models: Transition state theory, a Debye–Stoke–Einstein relation between the viscosity and the Debye time, and a model stating that its temperature dependence can be extrapolated from a singularity of liquid water at 228 K. We find an excellent agreement between experiment and the two latter models. The simulations, however, present results with too large statistical error for establishing a relation for the temperature dependence. © 1997 American Institute of Physics.
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