The Search for Interstellar Organic Ions Open Access

Abstract

Interstellar clouds are the cold, dark regions that eventually undergo gravitational collapse to form stars and ultimately new planetary systems. During star formation, complex organic

molecules that form on the icy mantles of dust grains are liberated into the gas phase in so-called “hot cores”. The molecules released encounter H3+, the most common ion in the interstellar medium (ISM). In the gas phase, the complex organic molecules can now react with H3+, which acts as a proton donor, to protonate these organic molecules. It is theorized that these protonated organic ions fuel the production of larger organic molecules in the ISM through ion-molecule reactions with polar species. The most common complex organic molecule that enters the gas phase from the icy grains is methanol and therefore the most common complex organic ion in the ISM is predicted to be protonated methanol. This ion has yet to confirmed as its rotational spectrum has not be recorded. As such, this ion has been of major interest in the field of astrochemistry. In addition, protonated methanol is highly fluxional as it has both a methyl rotor, and a H2O wagging motion and is isoelectronic to methylamine. These characteristic makes it an appealing ion to study for spectroscopists studying fundamental properties of fluxional molecules. Due to the difficulty of trying to observe and assign its rotational transitions, work has been conducted

to help improve our ability to do so. This has taken the form of instrumentation development of microwave-(sub)millimeter double-resonance and in the development of lock-in fast-sweep. Additionally, work has been done to ease the assignment of protonated methanol by removing potential spectral “weeds” that might arise during its spectral study by investigating a number of methanol clusters including argon-methanol, methanol-water, and methanol-methanol. Lastly, as a forerunner to protonated methanol experiments, attempts have been made to produce and observe protonated formaldehyde. This ion is thought to be a possible precursor to the formation of interstellar glycine, the

simplest amino acid.

Table of Contents

1 Introduction and Motivation . . . . . . . . . . . . . . . . . . . 1

1.1 The Interstellar Medium and Astrochemistry . . . . . . . . . . . . . . 1

1.2 Current Techniques and Instrumentation in the THz Regime . . . . . 5

1.3 Target Ions and Clusters . . . . . . . . . . . . . . . . . . . . . . . . . 13

2 Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.1 Discharge Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.1.1 Hollow Cathode Ion Source . . . . . . . . . . . . . . . . . . . . 21

2.1.2 Needle Electrode Source . . . . . . . . . . . . . . . . . . . . . . 22

2.1.3 Ring Electrode Source . . . . . . . . . . . . . . . . . . . . . . . 23

2.2 Vacuum Systems and Sample Delivery. . . . . . . . . . . . . . . . . . 25

2.3 Direct Absorption and Lock-In Detection . . . . . . . . . . . . . . . . 27

2.4 Fast-Sweep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

2.5 Optical Arrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3 Developments in Instrumentation. . . . . . . . . . . . . . . . 44

3.1 Pulsed Valve Triggered Lock-in Detection. . . . . . . . . . . . . . . . 44

3.2 Lock-In Fast-Sweep . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.3 Microwave-Millimeter/Submillimeter Double-Resonance. . . . . . . . 55

3.4 Cavity Enhanced Spectroscopy . . . . . . . . . . . . . . . . . . . . . . 66

3.5 Future Instrumental Developments. . . . . . . . . . . . . . . . . . . . 74

4 Protonated Formaldehyde. . . . . . . . . . . . . . . . . . . . . . 76

4.1 Protonated Formaldehyde from Paraformaldehyde Powder . . . . . . 77

4.2 Protonated Formaldehyde from Methanol . . . . . . . . . . . . . . . . 84

4.3 Protonated Formaldehyde in a Hollow Cathode. . . . . . . . . . . . . 90

4.4 Future Outlook on Protonated Formaldehyde. . . . . . . . . . . . . . 96

5 Methanol-Argon Clusters . . . . . . . . . . . . . . . . . . . . . . 98

5.1 Argon-Methanol Cluster Instrumentation . . . . . . . . . . . . . . . . 99

5.2 Argon-Methanol Cluster Results and Discussion . . . . . . . . . . . . 101

5.2.1 Argon-Methanol Cluster Fit Utilizing SPFit . . . . . . . . . . 107

6 Conclusions and Future Works. . . . . . . . . . . . . . . . . . 110

About this Dissertation

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School	Laney Graduate School
Department	Chemistry
Degree	Ph.D.
Submission	Dissertation
Language	English
Research Field	Chemistry, Physical
Keyword	Spectroscopy Ions Astrochemistry
Committee Chair / Thesis Advisor	Susanna Widicus Weaver, Emory University
Committee Members	Michael Heaven, Emory University Francesco Evangelista, Emory University

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