High Resolution THz Spectroscopy of Astrophysically Relevant Ions Open Access

McGuire, Brett Andrew (2011)

Permanent URL: https://etd.library.emory.edu/concern/etds/9019s306w?locale=en
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Abstract

Abstract
High Resolution THz Spectroscopy of Astrophysically Relevant Ions
Ions and radicals are thought to play fundamental roles in the complex chem-
istry of the interstellar medium (ISM). Identification of these species in interstellar
environments hinges on the availability of accurate laboratory spectra with which to
compare. The work presented here is focused on developing equipment and tech-
niques to study these species, with an aim towards the eventual acquisition of the
rotational spectrum of protonated methanol, an elusive organic ion that is thought to
be central to gas-phase organic chemical processes in the ISM. Protonated methanol
is a highly fluxional molecule, and this study will progress not only our knowledge
of its effects on interstellar chemistry, but also our fundamental understanding of the
intramolecular motion in organic molecules that guides reaction mechanisms. In or-
der to achieve this goal, I have designed new supersonic expansion discharge sources
that will favor protonation of organic molecules, combined these sources with multi-
pass direct-absorption spectroscopy systems, and worked towards the development of
a new, high-resolution, high-sensitivity spectrometer that operates in the terahertz
(THz) range. I have also conducted numerous broadband spectral line surveys of
a variety of interstellar sources to develop a catalog for rapid identification of new
molecules as laboratory data becomes available.

Table of Contents

1 Introduction 1

1.1 Laboratory Background . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1.1 Supersonic Expansions . . . . . . . . . . . . . . . . . . . . . . 2

1.1.2 Cavity Ringdown Spectroscopy . . . . . . . . . . . . . . . . . 3

2 Results and Discussion 6

2.1 THz Cavity Ringdown . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1.1 Initial Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1.2 Interaction of Polarized Light With System Components . . . 11

2.1.3 Current Status and Latest Results . . . . . . . . . . . . . . . 14

2.2 Spectroscopy of Transient Species . . . . . . . . . . . . . . . . . . . . 15

2.2.1 H

+5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Trans-Methyl Formate . . . . . . . . . . . . . . . . . . . . . . 22

3 Astronomical Observations 25

3.1 G+0.693-0.027 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.1.1 Observations and Data Reduction . . . . . . . . . . . . . . . . 27

3.2 L1157 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4 Future Work 32

2.2.2

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