Ultrafast Dynamics of Liquid Anilines Studied by the Optical Kerr Effect

The femtosecond dynamics of liquid aniline, its N-methylated derivatives, two deuterium isotopes, and some mixtures are studied by means of the optically heterodyne detected optical Kerr effect. All data can be effectively described by three contributions: an oscillatory response associated with librational motion which is most significant at early times, a dual exponential relaxation occurring on the picosecond time scale, and a response at intermediate times which is fit to a model describing collisional dynamics, but is of uncertain origin in these complex liquids. The frequency of the librational response of aniline itself is anomalous and is interpreted in terms of a dimer like structure in the liquid. The librational response for the methylated derivatives is typical of that found in other aromatic liquids. The slowest relaxation times are not well described by simple diffusional rotational reorientation models. It is proposed that some structural relaxation may make a nonhydrodynamic contribution to the slow relaxation. For both fast and slow relaxation components the effect of deuteration is slight. The electronic hyperpolarizability is also determined from the data and increases with increasing degree of methylation. The measured pure liquid dynamics are used to calculate the solvation time correlation function, which is compared with earlier measurements of the solvation dynamics on anilines. The calculation of the solvation dynamics from the pure liquid dynamics (as measured by OHD-OKE) is not particularly successful for this class of weakly polar liquids.