Watching molecules jump: Ultrafast nonlinear teraHertz spectroscopy of liquids and binary mixtures

Mol. dynamics studies in liqs. perennially provides a fertile ground for expanding our knowledge of a liq.'s structure. While great progress has been made over the last few decades, results obtained from new exptl. methods, such as time-domain TeraHertz (THz) or 2D Raman-THz spectroscopy, have provided fundamental insight into how the dynamics of liqs. relates to their structure on an ultrafast timescale. Nonlinear THz spectroscopy would provide a unique opportunity to study the low-frequency, soft modes of liqs., which play key roles in important chem. processes such as enzyme catalysis. Yet nonlinear, 2D THz-THz spectroscopy, to-date, has only succeeded in measuring 2D spectra of systems with significantly larger dipole moments than mols. in soln. Ultrafast TeraHertz Kerr effect (TKE) has recently been demonstrated as a time-resolved, nonlinear THz technique capable of measuring the dielec. relaxation of liqs. The true power of this technique lies in its ability to provide complementary information to measurements taken using heterodyne-detected optical Kerr effect (OKE) spectroscopy. The optical pulses in OKE measurements interact with the sample via the mol. polarizability, a rank-two tensor, in contrast with THz pulses that can interact with the mols. via the dipole moment, a rank-one tensor. Given the different light-matter interactions in the two techniques, TKE measurements help complete the phys. picture of intermol. interactions at short timescales. We report here our implementation of heterodyne-detected TKE spectroscopy, and our subsequent measurements on polar and non-polar liqs., and binary mixts. Our spectrometer uses either the org. crystal emitter DSTMS, producing THz pulses that provide a 500 fs time resoln., or a two-color plasma filament as a THz pulse source, thus allowing us to achieve sub-100 fs time resoln. Our studies of rigid-rotor mols. such as benzene and carbon disulfide provide a robust set of systems where we can compare TKE reorientational time consts. with those from OKE spectroscopy to gain insight into the mol. dynamics. We also report data from THF and pyridine, which demonstrate the high-sensitivity achieved by our instrument. This spectrometer represents another important step towards a 2D THz spectrometer capable of studying mol. systems