Ultrafast Dynamics of Polar Monosubstituted Benzene Liquids Studied by the Femtosecond Optical Kerr Effect

Abstract

The femtosecond and picosecond dynamics of liquid aniline, nitrobenzene, and benzonitrile have been recorded through measurements of the optically heterodyne detected optical Kerr effect. A major part of the subpicosecond dynamics is assigned to librational motion. This assignment is supported by studies of para-substituted benzonitrile derivatives with differing moments of inertia. The librational frequencies of the three liquids are only weakly dependent on temperature but shift to lower frequency on dilution in inert solvents. The picosecond relaxation dynamics are well described by a biexponential function. The slowest relaxation time behaves, at least qualitatively, as predicted by hydrodynamic models of orientational diffusion. However, quantitative analysis suggests that some of the assumptions concerning the molecular shape or hydrodynamic boundary condition are inadequate to completely describe diffusive orientational motion in these liquids. Measurements of the slowest relaxation time as a function of dilution in nonpolar solvent showed that static orientational pair correlation is negligible in these liquids. The faster of the two picosecond exponential relaxation times could not be ascribed to orientational diffusion, and is instead proposed to arise from structural relaxation occurring on the picosecond time scale in these liquids.

Optically-heterodyne-detected optical Kerr effect (OHD-OKE): Applications in condensed phase dynamics

“…The linear concentrationindependent relationship found in all cases is strong evidence for a di usive reorientation and suggests that g 2 is close to 1 in this molecule. Similar results were obtained for benzonitrile [47]. A value of g 2 close to 1 for these polar molecules suggests that orientational correlation is less in¯uenced by dipole moment than other factors (for example molecular shape).…”

“…h/T (cP/K) Figure 6. Plot of the slowest relaxation time for nitrobenzene, ½c, as a function of temperature and dilution [47]. That the points lie on a single line strongly suggests that g2ˆ1.…”

“…Under other assumptions about molecular shape the modi®ed Debye model indeed predicts a multiexponential relaxation [57]. However, there are no reasonable models of molecular shape which yield a relaxation time which is a factor of 10 shorter than the long relaxation time, as observed experimentally [47]. It has therefore to be concluded that the shorter of the two exponential relaxation times observed cannot be assigned to di usive reorientation.…”

“…This component is assigned to di usive molecular reorientation. Some quite detailed attempts have been made to analyse the relaxation times in terms of the Debye model of orientational relaxation [42,47,49], which predicts, for a molecule represented by an ellipsoidal shape with two equal axes [57±59],…”

Polarizability anisotropy relaxation in pure and aqueous dimethylsulfoxide

“…36 Kerr effect spectroscopy has proven a remarkable tool to explore the inter-and intramolecular dynamics of liquids and has been successfully applied to liquid mixtures in recent years. 21,[37][38][39][40][41] The present work is largely motivated by the recently reported OKE spectroscopy analysis of DMSOwater mixtures by Castner and co-workers. 21 The results reported here provide a complementary view on the mixtures dynamics observed in the OKE measurements, specially with respect to the intermolecular motions and the separate contributions from each co-solvent to the system's total response.…”