The effects of molecular geometry on the depolarized stimulated gain spectra of simple liquids

Impact of deuteration on the ultrafast nonlinear optical response of toluene and nitrobenzene

Nonlinear pulse propagation inside highly nonlinear media requires accurate knowledge on the temporal response function of the materials used particular in the case of liquids. Here we study the impact of deuteration on the ultrafast dynamics of toluene and nitrobenzene via all optical Kerr gating, showing substantially different electronic and molecular contributions, which was quantified by fitting a multichannel decay model to the data points. Specifically we found that deuteration imposes the time-integrated nonlinearities to reduce particular for toluene which could be caused by both reduced electronic hyperpolarizabilities as well as weaker intermolecular interactions. The results achieved reveal that deuterated organic solvents represent promising materials for infrared photonics since they offer extended infrared transmission compared to their non-deuterated counterparts while maintained strong nonlinear responses.

Polarizable empirical force field for aromatic compounds based on the classical drude oscillator

The polarizable empirical CHARMM force field based on the classical Drude oscillator has been extended to the aromatic compounds benzene and toluene. Parameters were optimized for benzene and then transferred directly to toluene, with parameters for the methyl moiety of toluene taken from the previously published work on the alkanes. Optimization of all parameters was performed against an extensive set of quantum mechanical and experimental data. Ab initio data was used for determination of the electrostatic parameters, for the vibrational analysis, and in the optimization of the relative magnitudes of the Lennard-Jones parameters. The absolute values of the Lennard-Jones parameters were determined by comparing computed and experimental heats of vaporization, molecular volumes, free energies of hydration, and dielectric constants. The newly developed parameter set was extensively tested against additional experimental data such as diffusion constants, heat capacities at constant pressure, and isothermal compressibilities including data as a function of temperature. Moreover, the structures of liquid benzene, liquid toluene, and solutions of each in water were studied. In the case of benzene, the computed and experimental total distribution function were compared, with the developed model shown to be in excellent agreement with experiment.

Molecular dynamics investigation with the time resolved optical Kerr effect on the CS2–C6H6 mixtures

An investigation of the molecular dynamics in pure liquids and in mixtures through the technique of time resolved optical Kerr effect is performed. The samples studied were the mixtures of carbon disulfide (CS(2)) with benzene (C(6)H(6)). The molecular dynamics of the pure liquids is briefly discussed while the main results are obtained for the mixtures. A slow dynamics component is observed for the optical heterodyne detected optical Kerr effect transient decaying exponentially with time constants on picosecond time scale. The fast subpicosend time relaxations are analyzed in terms of the nondiffusive component of the spectral response that is associated with the molecular dynamics. The modifications of the spectrum are quantified, and the explanation of the observed changes is given in terms of the structural interaction configurations that produced changes in the intermolecular potential within which the molecules execute librational motions.

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.

Why Does the Intermolecular Dynamics of Liquid Biphenyl so Closely Resemble that of Liquid Benzene? Molecular Dynamics Simulation of the Optical-Kerr-Effect Spectra

The combination of optical-Kerr-effect (OKE) spectroscopy and molecular dynamics simulations has provided us with a newfound ability to delve into the librational dynamics of liquids, revealing, in the process, some surprising commonalities among aromatic liquids. Benzene and biphenyl, for example, have remarkably similar OKE spectra despite marked differences in their shapes, sizes, and moments of inertia--and even more chemically distinct aromatics tend to have noticeable similarities in their spectra. We explore this universality by using a molecular dynamics simulation to investigate the librational dynamics of molten biphenyl and to predict its OKE spectrum, comparing the results with our previous calculations for liquid benzene. We suggest that the impressive level of quantitative agreement between these two liquids is largely a reflection of the fact that librations in these and other aromatic liquids act as torsional oscillations with oscillator frequencies selected from the liquid's librational bands. Since these bands are centered about the librational Einstein frequencies, the quantitative similarities between the liquids are essentially reflections of the near identities of their Einstein frequencies. Why then are the Einstein frequencies themselves so insensitive to molecular details? We show that, for nearly planar molecules, mean-square torques and moments of inertia tend to scale with molecular dimensions in much the same way. We demonstrate that this near cancellation provides both a quantitative explanation of the close relationship between benzene and biphenyl and a more general perspective on the similarities seen in the ultrafast dynamics of aromatic liquids.

Polarizability response in polar solvents: Molecular-dynamics simulations of acetonitrile and chloroform

The relaxation of the many-body polarizability in liquid acetonitrile and chloroform at room temperature was studied by molecular-dynamics simulations. The collective polarizability induced by intermolecular interactions was included using first- and all-orders dipole-induced-dipole models and calculated considering both molecule-centered and distributed site polarizabilities. The anisotropic response was analyzed using a separation scheme that allows a decomposition of the total response in terms of orientational and collision-induced effects. We found the method effective in approximately separating the contributions of these relaxation mechanisms, although the orientational-collision-induced interference makes a non-negligible contribution to the total response. In both liquids the main contribution to the anisotropic response is due to orientational dynamics, but intermolecular collision-induced (or translational) effects are important, especially at short times. We found that higher-order interaction-induced effects were essentially negligible for both liquids. Larger differences were found between the center-center and site-site models, with the latter showing faster polarizability relaxation and better agreement with experiment. Isotropic and anisotropic spectra were computed from the corresponding time correlation functions. The lowest-frequency contributions are largely suppressed in the isotropic spectra and their overall shape is similar to the purely collision-induced contribution to the anisotropic spectra, but with an amplitude which is smaller by a factor of approximately 5 in acetonitrile and approximately 3 in chloroform.