Two Dimensional TeraHertz Spectroscopy of Molecular Liquids

California Institute Of Technology, Pasadena CA

Investigators

Abstract

In this project funded by the Chemical Structure Dynamics and Mechanism A (CSDM-A) and Chemical Measurement and Imaging (CMI) Programs of the Chemistry Division, Professors Geoffrey Blake and Tom Miller of the California Institute of Technology are using new laser techniques to study the properties of pure molecular liquids and mixtures, especially those containing hydrogen bonds (for example water and alcohols). Specifically, the new laser tools involve light at terahertz (THz) frequencies (this frequency range lies between the infrared and microwave parts of the light spectrum). These THz lasers are also designed to produce extremely short pulses of light (a few femtoseconds, where a femtosecond is one quadrillionth of one second). The femtosecond THz laser pulses are then used to probe the motions of the liquid molecules as they interact with other reactive molecules dissolved in the liquid. The THz pulses also make it possible to control the orientation of the molecules in the liquid, and to examine how energy flows between two reacting molecules or between the molecules and the liquid. Professors Blake and Miller combine the experimental data with computer simulations of liquids and clusters of molecules to build better models for understanding chemical reactions in liquids. The broader impacts of this work include potential societal benefits from an increased understanding of how liquids solvate molecules and influence chemical reactivity, as well as opportunities for the training of graduate students and undergraduates in the design and construction of advanced experimental instrumentation and complex computer simulation. The Blake and Miller research groups also engage the local K-12 community, for example via the Caltech Pre-College Science Initiative (CAPSI) program (Prof. Blake) and Prof. Miller's Team Research yielding Integrated Educational Tools (TRIDENT) program. The project focuses on the nonlinear THz spectroscopy of neat liquids and mixtures at various temperatures. Detailed molecular pictures of the structure and dynamics of liquids drive our understanding of chemistry and biology. Nonlinear two dimensional (2D)-infrared and nuclear magnetic resonance (NMR) spectroscopies have revealed many specifics of liquid behavior. Extending such tools into the THz regime would directly probe liquid dynamics supported by intermolecular forces. This research project does so by combining two intense THz pulses whose arrival at the sample can be swept over controlled delays, with the subsequent anisotropy and Raman spectrum of the liquid probed by a third pulse at 800 nm. By mapping out the liquid response over the relative delays between the three pulses, a 2D-THz-THz-Raman (2D-TTR) spectrum is obtained. This method inherently suppresses the linear response of the system, and so is sensitive to anharmonicities in the intermolecular liquid potential energy surface and the electric dipole moment and Raman polarizability surfaces. To determine the origins of the observed response, the experimental observations are interpreted with theoretical calculations both of molecular clusters and liquid models. The cluster work involves high accuracy electronic structure calculations, while the liquid simulations are based on molecular dynamics approaches.