Structure and intermolecular dynamics of liquids: Femtosecond optical Kerr effect measurements in nonpolar fluorinated benzenes

A comparative study of the collective polarizability anisotropy dynamics of benzene, 1,3,5-trifluorobenzene, and hexafluorobenzene was carried out by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) with 45 fs laser pulses. The OPID-RIKES data were analyzed by using a model-dependent approach and a Fourier transform approach, which yields a spectral density for the liquid. From an analysis of the long-time tails, collective reorientation times of 2.45 +/- 0.06, 9.05 +/- 0.10, and 13.5 +/- 0.1 ps were obtained respectively for C6H6, 1,3,5-C6F3H3, and C6F6 The spectral densities are narrower for 1,3,5-C6F3H3 and C6F6 than for C6H6 by roughly a factor of 2. Information about the intermolecular vibrational modes of the liquid is contained in the reduced spectral density which is obtained by subtracting the tail-matched diffusive reorientational response from the OHD-RIKES data. In the case of C6H6 and C6F6, the intermolecular spectral density can be decomposed into at least two broad overlapping bands. In contrast, the intermolecular spectral density for 1,3,5-C6F3H3 is characterized by a single band. These spectra are rationalized in terms of the librational dynamics of perpendicular dimers in liquid C6H6 and C6F6 and parallel dimers in liquid 1,3,5-C6F3H3.