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Terahertz and Microwave Spectroscopy of Liquids and Hydrogen-Bonded Clusters


Finneran, Ian Alan (2017) Terahertz and Microwave Spectroscopy of Liquids and Hydrogen-Bonded Clusters. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9348HD4.


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.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Chemical Physics, Molecular Clusters, Liquids, Ultrafast Laser Spectroscopy, Microwave Spectroscopy, Terahertz Spectroscopy, Coherent Two-Dimensional Spectroscopy, Frequency Comb Spectroscopy
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Minor Option:Physics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Blake, Geoffrey A.
Group:Astronomy Department
Thesis Committee:
  • Beauchamp, Jesse L. (chair)
  • Okumura, Mitchio
  • Miller, Thomas F.
  • Blake, Geoffrey A.
Defense Date:1 March 2017
Non-Caltech Author Email:ifinn505 (AT)
Record Number:CaltechTHESIS:03082017-151032634
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for Ch. 2 adapted for Ch. 2 adapted for Ch. 3 adapted for Ch. 3 adapted for Ch. 4 adapted for Ch. 5 details for Ch. 5 adapted for Ch. 6 adapted for Ch. 7
Finneran, Ian Alan0000-0003-2506-4652
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:10090
Deposited By: Ian Finneran
Deposited On:21 Apr 2017 15:42
Last Modified:08 Nov 2023 00:16

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