Gas-Phase Terahertz Spectroscopy and the Study of Complex Interstellar Chemistry

Citation

Braakman, Rogier (2010) Gas-Phase Terahertz Spectroscopy and the Study of Complex Interstellar Chemistry. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/R2KK-E302. https://resolver.caltech.edu/CaltechTHESIS:09252009-142627047

Abstract

Terahertz spectroscopy holds great promise in the advancement of the field of astrochemistry. The sensitive observation of interstellar THz radiation is expected to lower detection limits and allow the study of larger and more complex species than is currently possible at millimeter wavelengths, which will place further constraints on chemical models and permit a direct comparison to the organic compounds seen in carbonaceous chondrites. With the successful recent launch of the Herschel Space Telescope, which will give high-fidelity access to interstellar THz radiation for the first time, and the completion of the Atacama Large Millimeter Array (ALMA) by 2013, the THz astronomy era is upon us. Unfortunately, laboratory THz spectroscopy presents significant challenges and will be soon be lagging behind the newly available observational platforms. Technologies to extend the capabilities of high-resolution spectroscopic systems into the THz domain are actively being pursued on many fronts, but affordable systems that are broadly tunable, sensitive and achieve the necessary resolution are not yet available. The work in this thesis should therefore be seen as part of the effort in the transition from centimeter-/millimeter-wave to THz spectroscopy that is currently taking place in the astrochemistry community.

As part of this thesis, observational searches for the complex organics hydroxyacetone (CH₃COCH₂OH), 2-cyanoethanol (OHCH₂CH₂CN) and methoxyacetonitrile (CH₃OCH₂CN) were attempted at millimeter wavelengths. The unsuccessful nature of these searches highlight the current limits of studying interstellar chemistry using pure rotational spectroscopy. The characterization of the laboratory spectra of these molecules is nonetheless important as it will aid in the assignment and description of the rotational substructure and band shapes of their THz torsional spectra, features that may allow their interstellar detection; and this thesis presents methods by which such complex spectra may be rapidly and efficiently collected and fit using automated spectrometers and modern software tools.

The description of the spectrum of hydroxyacetone is furthermore of interest due to the presence of the very low barrier to internal rotation in this molecule. Many interstellar compounds, both known and potential future targets, have functional groups capable of internal rotation in their structure; and so the effort in understanding the complex effects of the low barrier rotor in this case will benefit the general effort to further understand internal rotation.

In searching for new interstellar molecules, both at millimeter wavelengths and at higher THz frequencies, characterization of the complete spectra of known interstellar molecules is of great importance to allow substraction of their contribution to observational spectra. In this thesis, the ground-state rotational spectrum of methanol, the most important "interstellar weed", is catalogued and described in detail through most of the THz region that will be accessible with Herschel and ALMA.

Lastly, as part of the effort to increase the sensitivity of THz spectrometers, the use of Fabry-Perot cavities at these frequencies is explored. Such resonant cavities hold the potential to significantly increase the possible path lengths in spectroscopic system and to allow novel and sensitive detection techniques. Optimal configurations and the limits on achievable path lengths and Q-factors of such cavities are discussed, as are the possible extensions of Fourier Transform MicroWave (FT-MW) techniques to THz frequencies.

Thesis Files