Probing Intermolecular Dynamics with Nonlinear Ultrafast THz Spectroscopy

California Institute Of Technology, Pasadena CA

Investigators

Abstract

With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) program in the Division of Chemistry, Geoffrey Blake and Scott Cushing of the California Institute of Technology (Caltech) are developing sophisticated spectroscopies to study the structure and dynamics of liquids and liquid mixtures. The forces between molecules that determine the properties of liquid water, for example, can be directly probed only using light that is much lower frequency than that of visible light, on the order of a few terahertz (THz). Still, the motions in liquids are very fast, and a significant technical challenge is to develop very short, yet tailored, pulses of THz light. Drs. Blake and Cushing and their students will advance ultrafast nonlinear THz spectroscopies to study the intermolecular forces in hydrogen bonding liquids (e.g., water) in the presence of dissolved ions. Their discoveries could lead to new THz methodologies that advance applications in sustainable energy, telecommunications, and national security. The project will provide a highly interdisciplinary environment that includes graduate students and undergraduates, not only from Caltech but from nearby minority/Hispanic serving institutions, lowering the barriers for entry into quantum science. Direct probes of intermolecular forces in liquids and other molecular materials, especially those involving hydrogen bonds, require THz photons, yet the necessary time resolution is sub-picosecond. Thus, multi-octave pulses with controllable frequency and carrier envelope phase content must be generated. This collaborative Caltech team will combine the near-IR/Visible pulse compression expertise in the Cushing group with novel THz emitters fabricated at Caltech to drive pump-probe and THz-THz-Raman (TTR) approaches pioneered in the Blake group into the 10-30 femtosecond regime. The principal scientific focus will be studies of the hydrogen bond dynamics in liquid water and mixtures, especially at high ionic strength and with components such as alcohols that demonstrate unusual thermodynamics of mixing. The broader scientific applications of the tailored THz pulse capabilities include the selective nonlinear studies of a broad range of processes in sustainable energy generation and storage, and in quantum information science. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.