“…The computed activation energies for the two rotational motions are E a ( D ⊥ ) = 8.1 ± 0.6 (1 SD) kJ mol -1 and E a ( D ∥ ) = 4.5 ± 0.6 (1 SD) kJ mol- 1 . These observations [ D ∥ > D ⊥ and E a ( D ⊥ ) > E a ( D ∥ )] are qualitatively similar to results reported in other experimental studies − and MD simulations ,− of liquid benzene. 2 Rotational diffusion coefficients: circles, D ⊥ ; squares, D ∥ .…”
Equilibrium NPT and NVT molecular dynamics simulations were performed on liquid benzene over an extended range of temperature (from 260 to 360 K) using the COMPASS force field. Densities and enthalpies of vaporization (from cohesive energy densities) were within 1% of experiment at all temperatures. tumbling and spinning rotational diffusion coefficients, D(perpendicular) and D(parallel), computed as a function of temperature, agreed qualitatively with the results of earlier reported experimental and computational investigations. Generally, it was found that D(parallel)/D(perpendicular) approximately 1.4-2.5 and the activation energy for tumbling was significantly greater than for spinning about the C6 axis [Ea(D(perpendicular)) = 8.1 kJ mol(-1) and Ea(D(parallel)) = 4.5 kJ mol(-1)]. Calculated translational diffusion coefficients were found to be in quantitative agreement with experimental values at all temperatures [deviations were less than the scatter between different reported measurements]. In addition, translational diffusion coefficients were computed in the molecule-fixed frame to yield values for Dxy (diffusion in the plane of the molecule) and Dz (diffusion perpendicular to the plane). It was found that the ratio Dxy/Dz approximately 2.0, and that the two coefficients have roughly equal activation energies. This represents the first atomistic molecular dynamics study of translational diffusion in the molecular frame.
“…The computed activation energies for the two rotational motions are E a ( D ⊥ ) = 8.1 ± 0.6 (1 SD) kJ mol -1 and E a ( D ∥ ) = 4.5 ± 0.6 (1 SD) kJ mol- 1 . These observations [ D ∥ > D ⊥ and E a ( D ⊥ ) > E a ( D ∥ )] are qualitatively similar to results reported in other experimental studies − and MD simulations ,− of liquid benzene. 2 Rotational diffusion coefficients: circles, D ⊥ ; squares, D ∥ .…”
Equilibrium NPT and NVT molecular dynamics simulations were performed on liquid benzene over an extended range of temperature (from 260 to 360 K) using the COMPASS force field. Densities and enthalpies of vaporization (from cohesive energy densities) were within 1% of experiment at all temperatures. tumbling and spinning rotational diffusion coefficients, D(perpendicular) and D(parallel), computed as a function of temperature, agreed qualitatively with the results of earlier reported experimental and computational investigations. Generally, it was found that D(parallel)/D(perpendicular) approximately 1.4-2.5 and the activation energy for tumbling was significantly greater than for spinning about the C6 axis [Ea(D(perpendicular)) = 8.1 kJ mol(-1) and Ea(D(parallel)) = 4.5 kJ mol(-1)]. Calculated translational diffusion coefficients were found to be in quantitative agreement with experimental values at all temperatures [deviations were less than the scatter between different reported measurements]. In addition, translational diffusion coefficients were computed in the molecule-fixed frame to yield values for Dxy (diffusion in the plane of the molecule) and Dz (diffusion perpendicular to the plane). It was found that the ratio Dxy/Dz approximately 2.0, and that the two coefficients have roughly equal activation energies. This represents the first atomistic molecular dynamics study of translational diffusion in the molecular frame.
“…This fact indicates that external self-focusing in benzene occurs under nearly steady-state conditions for ruby laser pulses of approximately 30 ps to 40 ps duration. The orientational relaxation time of benzene of r 0^3 .1ps [40][41][42][43][44][45][46][47] (Table 1) is approximately a factor of ten shorter than the pulse duration.…”
The nonlinear refractive index of benzene is determined by measuring the reduction of the beam divergence of picosecond ruby laser pulses when passing through a benzene sample. Time-integrated spatial beam profiles give an effective refractive index while time-resolved beam profiles measured with a streak camera allow the determination of the temporal evolution of the nonlinear refractive index.
PACS:42.65, 42.65.J At high laser powers the refractive index of materials becomes intensity dependent. The spatial laser beam profile causes a spatial refractive index profile and leads to self-focusing [1][2][3][4][5][6]. The overall beam profile is responsible for whole-beam self-focusing while an intensity modulation of the spatial intensity distribution leads to a beam break-up (small-scale selffocusing) [5][6][7][8]. The combined effects of self-focusing and beam diffraction result in filament formation [1][2][3][4][5][9][10][11][12]. The self-focusing increases the pulse intensity enormously and enhances all nonlinear optical effects [13][14][15][16][17][18][19]. The abrupt rise of nonlinear optical effects is an indication of self-focusing and may be used to determine the self-focusing length. The determination of the nonlinear refractive index from the abrupt rise of nonlinear optical effects is complicated by the fact that either whole-scale self-focusing or small-scale selffocusing may act and the small-scale self-focusing parameters (ripple widths and modulation depths) are difficult to determine.The vagueness of whole-scale or small-scale selffocusing dynamics may be avoided by changing from internal self-focusing (focal point caused by nonlinear refractive index is inside sample) to external selffocusing (focal point is outside sample) [1 ]. In this case the nonlinear refractive index of the sample causes a reduction of the overall beam divergence and the effects of small ripples across the beam profile may be neglected.In our experiments we determine the reduction of beam divergence by comparing the beam diameters (FWHM) of two pulses at a certain distance behind the sample position, where one pulse passes through the sample and the other pulse is bypassed. Our timeresolved measurements of the beam diameters with a streak camera and a two dimensional readout system allow the study of the instantaneous and transient contributions to the nonlinear refractive index [12,18,20]. The effective time-averaged and the time-resolved nonlinear refractive index of benzene are measured with picosecond light pulses of a ruby laser. The reported time-averaged nonlinear refractive index coefficients n 2 of benzene vary by a factor of ten in the region between n 2 = 1.4x 10~2 1 m 2 V~2 and lxlO" 20 m 2 V-2
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