Optical Kerr Effect of Liquid Acetonitrile Probed by Femtosecond Time-Resolved X-ray Liquidography

Abstract

Optical Kerr effect (OKE) spectroscopy is a method that measures the time-dependent change of the birefringence induced by an optical laser pulse using another optical laser pulse and has been used often to study the ultrafast dynamics of molecular liquids. Here we demonstrate an alternative method, femtosecond time-resolved X-ray liquidography (fs-TRXL), where the microscopic structural motions related to the OKE response can be monitored using a different type of probe, i.e., X-ray solution scattering. By applying fs-TRXL to acetonitrile and a dye solution in acetonitrile, we demonstrate that different types of molecular motions around photoaligned molecules can be resolved selectively, even without any theoretical modeling, based on the anisotropy of two-dimensional scattering patterns and extra structural information contained in the q-space scattering data. Specifically, the dynamics of reorientational (libration and orientational diffusion) and translational (interaction-induced motion) motions are captured separately by anisotropic and isotropic scattering signals, respectively. Furthermore, the two different types of reorientational motions are distinguished from each other by their own characteristic scattering patterns and time scales. The measured time-resolved scattering signals are in excellent agreement with the simulated scattering signals based on a molecular dynamics simulation for plausible molecular configurations, providing the detailed structural description of the OKE response in liquid acetonitrile.

Ultrafast Rotational and Translational Energy Relaxation in Neat Liquids

“…Indeed, we have previously examined the dynamics of such variables for pure water (e.g., refs − ). Such “double variable” studies could relate the time scales found here and those associated with e.g reorientation in ref and solvent cage rearrangement in ref . Returning to the kinetic energy arena, a further application of the energy flow formalism could be to the examination of the participation of assorted modes of liquid motion in the energy relaxation, where resonance and coupling issues will be crucial (e.g., refs , , and ) as well as features such as transient or complete rupture of hydrogen bonds. − More generally, these several avenues would also serve to provide a detailed molecular level mechanism, both kinetic and configurational, for the energy flow.…”

“…More can be done for these solvents, and solvent classes, ,− as well as mixed solutions; clearly, polar nonhydroxylic solvents such as acetonitrile (see, e.g., ref ) merit comparable attention in the future as well. As for the present liquids, most especially water for which most experiments referred to in the Introduction have been carried out, extension of the energy flow formulation to, e.g., specific spatial and structural variables directly related to measured spectra (e.g., transition dipole orientational angles as in ref ) or configurational variables (e.g., as probed in the X-ray studies of ref ) could be carried out in the future in direct connection with the present kinetic energy studies. Indeed, we have previously examined the dynamics of such variables for pure water (e.g., refs − ).…”

Reaction dynamics studiedviafemtosecond X-ray liquidography at X-ray free-electron lasers

“…To tackle this issue, recently, Ki et al investigated OKE response in liquid acetonitrile using fs-TRXL. 150 The measured fs-TRXL signals showed anisotropic scattering patterns, as depicted in Fig. 11a.…”

“…To tackle this issue, recently, Ki et al investigated OKE response in liquid acetonitrile using fs-TRXL. 150 …”

Photoactivation of triosmium dodecacarbonyl at 400 nm probed with time-resolved X-ray liquidography

“…TRXL has proven to be an excellent tool for identifying the molecular structure of transient species in solution. [14][15][16][17][18][19][20][21][22][23][24][25][26][27] In the case of Os 3 (CO) 12 , the TRXL signal is sensitive to the metal-metal distances in the transient species, which are key structural parameters of the metal complex, in contrast to TRIR, which is sensitive to the chemical environment of the CO ligands. Accordingly, if the proposed MMBC intermediate is formed, such a change in the Os-Os distance should be evidently manifested in the measured TRXL signal.…”