Methanol Photodissociation as a Case Study of Complex Interstellar Organic Chemistry Öffentlichkeit

Abstract

Methanol is ubiquitous in interstellar molecular clouds and other molecule-rich extraterrestrial environments. Its reaction dynamics are therefore expected to be highly-influential on related chemistries. This was verified through astrochemical modeling, when it was found that the fractional abundances of methyl formate (HCOOCH3) and its structural isomers glycolaldehyde (HCOCH2OH) and acetic acid (CH3COOH) can change by up to an order of magnitude if changes to the branching ratios of the cosmic-ray induced photodissociation of methanol are made. This is due to the fact that methanol photodissociation is the primary formation pathway for the molecular radicals methoxy (CH3O), hydroxymethyl (CH2OH), and methyl (CH3) on grain surfaces under hot core/corino conditions, and these radicals may then go on to form larger complex organic molecules through radical-radical addition reactions. Motivated by these results, a millimeter/submillimeter absorption spectrometer has been implemented to quantitatively measure these branching ratios in the gas-phase, as well as provide more complete rotational line catalogs for methoxy and hydroxymethyl. Spectral line surveys of a statistically-significant population of star-forming regions have also been acquired through observational radio astronomy so as to better constrain astrochemical models by cataloging their molecular inventories. I present here the results of the modeling, laboratory, and observational studies, and provide a discussion of each in the context of methanol playing a critical role in the complex organic chemistry of the interstellar medium.

Table of Contents

1 Introduction..............................................................................................1

1.1 Interstellar Chemistry and Our Molecular Origins..................................1

1.2 Methanol Photodissociation...............................................................3

1.3 Thesis Overview.............................................................................4

2 Astrochemical Modeling..............................................................................5

2.1 Background....................................................................................5

2.2 Methyl Formate Formation................................................................6

2.2.1 Radical-Radical Addition Mechanism.........................................7

2.2.2 Gas-Phase Ion-Neutral Mechanisms........................................14

2.2.3 Conclusions........................................................................24

2.2.4 Impact and Future Work.......................................................27

3 Laboratory Spectroscopy............................................................................31

3.1 Background...................................................................................31

3.2 Design and Benchmarks of a mm/submm Spectrometer........................32

3.2.1 Background........................................................................32

3.2.2 Experimental Methods..........................................................33

3.2.3 Methanol Dissociation Dynamics as a Case Study......................38

3.2.4 Results and Analysis............................................................40

3.2.5 Conclusions........................................................................45

3.3 Application to Molecular Systems......................................................48

3.3.1 Methanol Photodissociation Studies.........................................48

3.3.2 Methyl Hypochlorite Trials.....................................................53

3.3.3 A Spectral Search for Hydroxymethyl.....................................54

3.3.4 Methoxy: Extending its Spectral Assignments...........................55

4 Observational Astronomy with the CSO........................................................60

4.1 Background..................................................................................60

4.2 Data Reduction..............................................................................61

4.3 Interstellar Methoxy.......................................................................63

4.4 Spectral Fits.................................................................................65

5 Conclusions and Future Outlook..................................................................67

Appendices

A MM/SUBMM Optics...................................................................................71

A.1 VDI AMC Band 2 (55-60 GHz)..........................................................71

A.2 VDI AMC Band 4 (135-145 GHz).......................................................73

A.3 VDI AMC Band 5 (145 GHz, 190 GHz, 220 GHz)...................................75

A.4 VDI AMC Band 6 (247 GHz, 303 GHz)................................................76

A.5 Multipass Optical Cell.....................................................................77

B Misc. Electronics......................................................................................79

B.1 in vacuo Optical Chopper.................................................................79

B.1.1 DC Motor and Chopper Wheel................................................79

B.1.2 External Communication.......................................................80

B.2 Pre-amps for ZBD Use....................................................................81

B.2.1 Gain-of-10 Inverter..............................................................81

B.2.2 Gain-of-200 Inverter, with DC Offset.......................................83

C CSO DSB Spectral Deconvolutions................................................................86

D Methoxy vs CSO Line Surveys.....................................................................89

About this Dissertation

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School	Laney Graduate School
Department	Chemistry
Degree	PhD
Submission	Dissertation
Language	English
Research Field	Physics, Molecular Chemistry, Physical Physics, Astronomy and Astrophysics
Stichwort	Molecular Spectroscopy Astrochemistry
Committee Chair / Thesis Advisor	Widicus Weaver, Susanna, Emory University
Committee Members	Heaven, Michael, Emory University Bowman, Joel M, Emory University

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