An Investigation of Methanol Photolysis Branching Ratios and Their Implications for Chemistry in the Interstellar Medium Open Access

Abstract

Methanol is ubiquitous in star-forming regions, and recent astrochemical

models have shown that methanol photolysis contributes

largely to more complex organic chemistry which occurs

in interstellar ices. While some of the branching ratios for

methanol photolysis have been measured via mass spectrometric

techniques, the photolysis products methoxy (CH3O) and

hydroxymethyl (CH2OH) cannot be distinguished using these

methods. Additionally, recent experiments focused on the photoprocessing

of interstellar ice analogues have led to conflicting

theories about the branching ratios because the radical products

can undergo further reactions in the ice matrix. To address these

challenges, we have implemented pure rotational spectroscopy

in the millimeter and submillimeter regimes to quantitatively

probe the products of methanol photolysis. We conduct experiments

in the gas phase, where the photoproducts can be

stabilized in a supersonic expansion, and secondary reactions of

the radical photolysis products can be minimized. We use a UV

excimer laser to dissociate methanol near the throat of a supersonic

expansion, and probe the products after they have undergone

sufficient rotational cooling. Rotational diagram analysis

is then performed with collected spectra to determine the relative

density of each photolysis product relative to methanol. We

have thus far detected formaldehyde, methoxy, hydroxymethyl,

and carbon monoxide as products of methanol photolysis. We

present here the experimental setup and the initial results for

methanol photolysis branching ratios. Additionally, production

of isotopic formaldehyde as a photolysis product, and detection

of water are discussed, as are future directions for the project.

Table of Contents

1 Introduction 1.1 General Introduction to the ISM 1.2 Methanol Photolysis and Branching Ratios in the ISM 2 Experimental Setup 3 Results and Summary 3.1 Methanol Photolysis 3.1.1 Rotational Diagram Analysis 3.1.2 Spectral Roadmap 3.1.3 Initial Detection of Methanol, and Calculation of a Rotational Energy Diagram 3.1.4 Improvements to Spectrometer, Detection of Formaldehyde, and Explanations for Asymmetry/Differences in Production 3.1.5 Production of Methoxy 3.1.6 Tentative Hydroxymethyl Detection 3.1.7 Implementation of 3D Translation Stage, and Methoxy Raster Tests 3.1.8 Introduction of Spherical Focusing Lens 3.1.9 Detection of Isotopic Formaldehyde 3.1.10 Detection of CO and H2O 3.1.11 Future Directions 4 Conclusions Bibliography

 

About this Master's Thesis

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School	Laney Graduate School
Department	Chemistry
Degree	M.S.
Submission	Master's Thesis
Language	English
Research Field	Chemistry, Physical
Keyword	Rotational Spectroscopy Astrochemistry Radicals Physical Chemistry Gas Phase
Committee Chair / Thesis Advisor	Widicus Weaver, Susanna, Emory University
Committee Members	Kindt, James, Emory University Heaven, Michael, Emory University

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