Terahertz and Microwave Spectroscopy of Liquids and Hydrogen-Bonded Clusters

Abstract

The microwave (MW, 0.3-100 GHz) and terahertz (THz, 0.1-10 THz) regions of the electromagnetic spectrum are replete with a rich set of molecular motions, including soft inter- and intramolecular vibrations, torsions, and rotations. At room temperature these motions are well populated, and play an active role in condensed-phase chemistry on Earth. This work details the development of one MW and two THz spectrometers along with their application to the study of liquids and hydrogen-bonded clusters. In the first section, we cover the design and construction of a chirped pulse Fourier transform microwave (CP-FTMW) spectrometer. The instrument relies on a compact, inexpensive direct digital synthesis board to generate 2 GHz, 1 microsecond chirped pulses that, after amplification, polarize the rotational states of gas-phase molecules in a pulsed supersonic jet. In an initial demonstration, the CP-FTMW instrument is used to collect the 8-18 GHz rotational spectra of the ethanol-water and ethanol-methanol dimers. These data reveal evidence of quantum tunneling, and a complicated interplay between weak and strong hydrogen-bonds in both dimers. Next, we describe the ongoing development of a decade spanning high precision THz frequency comb, using THz time-domain spectroscopy. The instrument is capable of generating ~28000 comb teeth from 0.15-2.4 THz with a fractional precision of 1.8x10-9 and a Doppler-limited accuracy of 6.1x10-8. Further prospects for studies of intermolecular interactions in jet-cooled molecular clusters are also discussed. In the last section, we move to condensed-phase studies of THz orientational and vibrational motions of liquids. The liquids are excited with one or two intense time-delayed ultrafast THz pulses and probed with a non-resonant 40 fs Raman pulse. Initially, we use this approach to measure the picosecond molecular orientational alignment and decay timescales in several aromatic liquids. By adding a second THz pulse to the experiment and adjusting the delays between the three pulses, we control the orientational alignment of the molecules, and acquire phase-coherent 2D-THz-THz-Raman spectra in the time domain. The 2D responses of liquid CHBr3, CCl4, and CCl2Br2 show off-diagonal peaks from coupling between thermally-populated vibrational modes. In an extended bandwidth measurement, we observe photon-echo signals from liquid CHBr3 and a complicated pattern of dipole forbidden transitions. The molecular origins of the forbidden transitions are still under investigation, but are likely due to nonlinearities in the condensed-phase dipole moment surface. Coherence transfer, vibrational anharmonicity, and intermolecular coupling are also considered in this analysis.