Optically Induced Anisotropy in Time-Resolved Scattering: Imaging Molecular-Scale Structure and Dynamics in Disordered Media with Experiment and Theory

Time-resolved scattering experiments enable imaging of materials at the molecular scale with femtosecond time resolution. However, in disordered media they provide access to just one radial dimension thus limiting the study of orientational structure and dynamics. Here we introduce a rigorous and practical theoretical framework for predicting and interpreting experiments combining optically induced anisotropy and time-resolved scattering. Using impulsive nuclear Raman and ultrafast x-ray scattering experiments of chloroform and simulations, we demonstrate that this framework can accurately predict and elucidate both the spatial and temporal features of these experiments.

Figure 1

The experimental difference INXS signal (top, see Supplemental Material, Fig. 6 [15], for raw signal) and anisotropic component, $\mathrm{\Delta }{S}_2(q,t)P_2(\cos \varphi )$, of the signal obtained from experiment (middle) and theory (bottom) at times $t$ following the initial excitation of the sample. The arrow in the lower right panel defines the direction of the optical excitation electronic field.

Figure 2

The anisotropic component of the INXS signal, $\mathrm{\Delta }{S}_2(q,t)$, from experiment (top left) and simulation (bottom left). Their Fourier transforms (right) show the temporal oscillations of the anisotropic component.

Figure 3

The real-space transformed anisotropic component of the INXS signal, $\mathrm{\Delta }{g}^{(2)}(r,t)$, from (a) experiment and (c) simulation. (b) Simulated variance arising from angular fluctuations at each distance along the C-H and C-Cl axes. Comparing $\mathrm{\Delta }{g}^{(2)}(r,t)$ in (c) with the simulated chlorine-chlorine radial distribution function $g_{\text{ClCl}}(r)$ in (d) demonstrates the anisotropic nonoscillatory component is intermolecular in character since it straddles the first intermolecular (orange) peak in $g^{(0)}(r)$ but is absent from the isotropic component $\mathrm{\Delta }{g}^{(0)}(r,t)$ shown in (e).