Dynamics and structure of liquids studied by femtosecond optical Kerr effect spectroscopy

Ultrafast optical Kerr effect (OKE) spectroscopy is a simple tool for monitoring the dynamics of liquids with high temporal resolution and an excellent signal-to-noise ratio. The principles of OKE spectroscopy and its realization in recording the dynamics of bulk and confined liquids are described. The data are discussed in terms of the underlying molecular orientational librational and diffusive motions and the microscopic structure of liquids.; The collective orientational dynamics of 2-butyne in nanoconfinement at 293 K have been investigated. In all pore sizes, three exponential decays are observed. The fastest decay arises from the molecules in the pore centers. The intermediate decay is attributed to surface molecules that rotate off of the pore walls, and its time constant is exactly that expected based on hydrodynamic volume effects. The slowest decay is attributed to inhibition of reorientation along the pore surfaces due to reduced-dimensionality effects.; The collective orientational dynamics of water confined in hydrophilic and hydrophobic nanopores have been studied. Orientational relaxation is significantly faster at hydrophobic surfaces than hydrophilic surfaces, whereas the relaxation of the liquid in the pore centers is independent of the chemical nature of the pore surfaces.; OKE spectroscopy has been used to examine mixtures of CS₂ with 2-methylhexane and with n-hexane. Obtaining data in isoviscous mixtures allowed us make a direct comparison of orientational and intermolecular dynamics as a function of the CS₂ volume fraction, and to gain insight into the structure of the mixtures. In both cases the inhomogeneity in the intermolecular spectrum is found to stay roughly constant and the intermolecular potential to soften as the CS₂ volume fraction is decreased.; The underlying molecular structures of benzene, benzene-d₆, hexafluorobenzene and 1,3,5-trifluorobenzene have been investigated by observing the evolution of their intermolecular spectra and the static orientational correlation parameter as a function of temperature. This study shows evidence of a significant degree of perpendicular local ordering in benzene and benzene-d ₆.; Finally, OKE experiments have been performed on mixtures of benzene and hexafluorobenzene at 293 K. Comparison of collective orientational correlation times for hexafluorobenzene with single-molecule orientational times in the same mixtures suggests that even at low concentrations, hexafluorobenzene exists preferentially in 2:1 sandwich structures with benzene.