Temperature-dependent optical Kerr effect spectroscopy of aromatic liquids

Ultrafast optical Kerr effect (OKE) spectroscopy has been used to study the temperature-dependent dynamics of five aromatic liquids: benzene, benzene-d(6), hexafluorobenzene, mesitylene, and 1,3,5-trifluorobenzene. The intermediate response time of all of the liquids was found to scale with the collective orientational correlation time, as has been observed for other simple liquids. The spectra of hexafluorobenzene, 1,3,5-trifluorobenzene, and mesitylene are qualitatively different from those of the other liquids and exhibit different behavior with temperature. These spectra allow us to assess the influence of different molecular parameters on the shape of the OKE spectrum. On the basis of these data, we propose a model that links the differences in the OKE spectra to corresponding differences in the local ordering of the liquids.     

Searching for voids in liquids with optical Kerr effect spectroscopy

It has recently been shown [J. Chem. Phys. 2005, 122, 134506; J. Am. Chem. Soc. 2006, 128, 5119] that liquid tetrahydrofuran has an unusual structure that features voids of significant dimension. Such voids should affect other observable properties of this liquid. Here we present temperature-dependent, optical Kerr effect data for tetrahydrofuran and a number of related liquids (furan, cyclopentane, tetrahydropyran, cyclohexane, diethyl ether, and n-pentane) as well as hexamethylphosphoramide to test whether this technique can be used to reveal the presence of sizable voids in liquids.     

Shape and electrostatic effects in optical Kerr effect spectroscopy of aromatic liquids

We present a detailed, temperature-dependent, optical Kerr effect (OKE) study of pyridine, pyridine-d(5), 2,4,6-trifluoropyridine, 2,4,6-trimethylpyridine, and 1,3,5-tris(trifluoromethyl)benzene. By combining these data with those for other aromatic liquids that we have studied previously (Loughnane, B. J.; Scodinu, A.; Fourkas J. T. J. Phys. Chem. B, 2006, 110, 5708), we are able to assess the relative importance of molecular shape and electrostatic forces in determining the form of the OKE reduced spectral density for such liquids.     

Structural relaxation in the hydrogen-bonding liquids N-methylacetamide and water studied by optical Kerr effect spectroscopy

Structural relaxation in the peptide model N-methylacetamide (NMA) is studied experimentally by ultrafast optical Kerr effect spectroscopy over the normal-liquid temperature range and compared to the relaxation measured in water at room temperature. It is seen that in both hydrogen-bonding liquids, beta relaxation is present, and in each case, it is found that this can be described by the Cole-Cole function. For NMA in this temperature range, the alpha and beta relaxations are each found to have an Arrhenius temperature dependence with indistinguishable activation energies. It is known that the variations on the Debye function, including the Cole-Cole function, are unphysical, and we introduce two general modifications: One allows for the initial rise of the function, determined by the librational frequencies, and the second allows the function to be terminated in the alpha relaxation.     

Depolarized light scattering versus optical Kerr effect spectroscopy of supercooled liquids: comparative analysis

Recently, heterodyne-detected optical Kerr effect (HD-OKE) spectroscopy was used to study dynamics of supercooled molecular liquids. The studies revealed an apparently new physical phenomenon that had not been reported before from the related depolarized light scattering (DLS), namely, an intermediate power law (nearly logarithmic decay) of the response functions [H. Cang et al., J. Chem. Phys. 118, 2800 (2003)]. Conceptually, HD-OKE and DLS data reflect optical anisotropy fluctuations mainly due to molecular reorientation dynamics in time and frequency domains, respectively. The above-mentioned effects are revealed in the mesoscopic range less, similar1 GHz ( greater, similar100 ps), where no direct comparison of the techniques was reported. In this Communication, we attempt such a comparison of exemplifying HD-OKE literature data of the glass-forming salol (phenyl salicylate), benzophenone, and liquid-crystal forming 4-cyano-4(')-pentylbiphenyl with DLS data of the same systems that we measured down to ca. 200 MHz by a combined tandem Fabry-Perot interferometer plus tandem-grating-monochromator technique. Generally, we find a satisfactory agreement, albeit in some cases with subtle differences at frequencies greater, similar10 GHz. We conclude that, in the mesoscopic dynamic range, HD-OKE and DLS studies provide consistent and comparable information, and therefore their conclusions must agree. We argue that the intermediate power law of HD-OKE is in essence a manifestation of the excess wing of the corresponding frequency-domain data, known long since from broadband dielectric spectroscopy and anticipated from DLS studies of supercooled liquids.     

Nanostructural organization and anion effects on the temperature dependence of the optical Kerr effect spectra of ionic liquids

The intermolecular spectra of three imidazolium ionic liquids were studied as a function of temperature by the use of optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The ionic liquids comprise the 1,3-pentylmethylimidazolium cation ([C(5)mim]+), and the anions, bromide (Br-), hexafluorophosphate (PF(6)-), and bis(trifluoromethanesulfonyl)imide (NTf(2)-). Whereas the optical Kerr effect (OKE) spectrum of [C(5)mim][NTf(2)] is temperature-dependent, the OKE spectra of [C(5)mim]Br and [C(5)mim][PF6] are temperature-independent. These results are surprising in light of the fact that the bulk densities of these room temperature ionic liquids (RTILs) are temperature-dependent. The temperature independence of the OKE spectra and the temperature dependence of the bulk density in [C(5)mim]Br and [C(5)mim][PF(6)] suggest that there are inhomogeneities in the densities of these liquids. The existence of density inhomogeneities is consistent with recent molecular dynamics simulations that show RTILs to be nanostructurally organized with nonpolar regions arising from clustering of the alkyl chains and ionic networks arising from charge ordering of the anions and imidazolium rings of the cations. Differences in the temperature dependences of the OKE spectra are rationalized on the basis of the degree of charge ordering in the polar regions of the RTILs.     

Depolarized light scattering versus optical Kerr effect. II. Insight into the dynamic susceptibility of molecular liquids

We have previously discussed [J. Chem. Phys. 125, 114502 (2006)] that optical Kerr effect (OKE) and depolarized light scattering (DLS) data of molecular liquids reveal, each in their native domain, the same characteristic signatures of the glass transition dynamics; in particular, the intermediate power law of OKE is equivalent with the excess wing of the frequency-domain data, long since known in dielectric spectroscopy. We now extend the discussion to show that the excess wing is an equally common feature in DLS. We further discuss the time-temperature superposition property of OKE data in relation to our DLS and literature dielectric-spectroscopic results, and the merits of their mode coupling theory analyses. Spectroscopic signatures of a liquid-crystal-forming system (nematogen) are discussed in the same frame.     

Analysis of anisotropic polarizability of molecules in the gas phase and in liquids and solutions

Conclusions The anisotropic polarizabilities of molecules in the gas phase and in dilute solutions in isotropic solvents, as determined from data on Rayleigh light scattering and the Kerr effect, are completely consistent with each other.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya No. 2, pp. 335–339, February, 1981.     

Ultrafast optical Kerr effect spectroscopy of water confined in nanopores of the gelatin gel

We report on the investigation of a short-time collective dynamics of water confined in the pores of the gelatin gel, using the femtosecond optical Kerr effect spectroscopy. The ultrafast responses of water molecules obtained in bulk liquid and in three concentrations of gelatin gels are explained theoretically, both in a long time and in a short time regime, taking into account all molecular motions. We prove that the contribution of molecules involved in tetrahedral, strongly H-bonded structures stabilizing the gel network increases with the gel concentration. On the other hand the long-time relaxation of water molecules is significantly slowed down in the gel pores.     

A new ultrafast technique for measuring the terahertz dynamics of chiral molecules: the theory of optical heterodyne-detected Raman-induced Kerr optical activity

Optical heterodyne-detected Raman-induced Kerr optical activity (OHD-RIKOA) is a nonresonant ultrafast chiroptical technique for measuring the terahertz-frequency Raman spectrum of chirally active modes in liquids, solutions, and glasses of chiral molecules. OHD-RIKOA has the potential to provide much more information on the structure of molecules and the symmetries of librational and vibrational modes than the well-known nonchirally sensitive technique optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES). The theory of OHD-RIKOA is presented and possible practical ways of performing the experiments are analyzed.     

Low-frequency modes of the benzoic acid dimer in chloroform observed by the optical Kerr effect

The low frequency Raman spectral density associated with the intermolecular hydrogen-bonding interaction of benzoic acid in chloroform was investigated through the ultrafast optically-heterodyne-detected optical Kerr effect. The low-frequency solute Raman spectrum was obtained by Fourier transform analysis and subtraction of the solvent spectrum from the solution spectrum. The resulting difference spectrum has a broad band below 150 cm(-1) with a peak at around 80 cm(-1). Previous studies of aromatic liquids suggest that the origin of such a low-frequency band is librational motion, although intermolecular hydrogen-bonding modes in benzoic acid may also contribute. To clarify these contributions to the low-frequency band, methyl benzoate was used to estimate the librational component; its structure is similar to benzoic acid, but it forms no intermolecular hydrogen bonds. Both librational and intermolecular modes were found to contribute to the low-frequency Raman spectrum of the dimer and thus can be separated. These experimental results were compared with the results of density functional theory calculations. In addition, the effect of deuteration on the Raman spectrum was also investigated.     

Relaxation of laser induced transient plasmas in liquids

Transient plasma is induced in H₂O, D₂O, CS₂, optical glass, and Plexiglas by single pulses of aQ-switched neodymium laser in the power density range of 0.4 to 0.9 GW cm^–2. The relaxation times for the simple exponential decay of intensity of a white continuum emitted by all samples have been determined. Possible mechanisms of the formation and decay of the plasmas are discussed.With 2 FiguresThe experimental part of this work was performed at the University of Utah and was supported in part by the Directorate of Chemical Sciences, Air Force Office of Scientific Research under Grant AFOSR-69-1717-D.