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Astrochemical molecules are often difficult to study in the lab because many are unstable. O(1D) insertion reactions can be used to prepare these molecules, but add difficulties to the experiment because they are exothermic and therefore produce unstable products. We plan to do terahertz spectral analysis of supersonic expansions so that we can study these reactions in the gas phase, which allows us to circumvent many of the typical problems with product stability. The goal of my project was to build a source for supersonic expansions to study the products of O(1D) insertion reactions. This source design combines two previous approaches in order to overcome the difficulties of producing O(1D) without dissociating the product molecules to be studied. This source is currently being tested to maximize the efficiency of O(1D) production and then it will be tested with organic molecules so that it can ultimately be used to produce new astrochemical molecules of interest. In companion to this laboratory research, I am participating in collection and analysis of observational spectra using the Caltech Submillimeter Observatory. These observations will enable direct comparison between the molecules studied in the laboratory and the molecules present in interstellar clouds.
Methoxymethanol, methanediol, and aminomethanol are predicted to form on grain surfaces in the initial steps of interstellar prebiotic molecular evolution [1]. Once in the gas phase, these molecules are thought to be precursors to complex molecules such as sugars and amino acids. To test these prebiotic interstellar chemical pathways a pure rotational spectrum is required for comparison to interstellar line surveys. Given the reactivity and instability of these molecules under normal laboratory conditions , an efficient gas phase chemical formation mechanism is required.
O(1D) insertion reactions are one possible production route for these molecules. O(1D) insertions into C-H bonds are highly exothermic, and therefore efficient. In low temperature matrix isolation experiments methoxymethanol and methanediol have been observed to form from O(1D) insertions [2,3] but these reactions have not been studied in the gas phase.
Figure 1. O(1D) insertion reactions to form the astrochemical molecules of interest.
I have constructed a supersonic expansion source which allows O(1D) to react with organic molecules on-the-fly, in the gas phase. The source combines a photolysis source with a fast mixing nozzle (see Figure 2).
Figure 2. a) Photolysis nozzle [4], b) fast mixing nozzle [5], and c) our new source for O(1D) insertion reactions.
We are now conducting tests to maximize the efficiency of O(1D) production. In our initial tests, O(1D) production was monitored indirectly using direct absorption spectroscopy to observe the depletion of N2O as it was photolyzed. The first trial indicates a 12% decrease in N2O. This efficiency can be increased through careful alignment of the photolysis beam. We will now install the appropriate positioning equipment to enable this alignment during the experiments.
Figure 3. a) The newly-constructed photolysis-fast-mixing supersonic expansion source for O(1D) insertion reaction studies. b) N2O spectral line signal depletion indicating production of O(1D). The blue trace is the signal with no photolysis beam, the red trace is the signal with the photolysis lamp on, and the purple trace is the difference spectrum.
Figure 4. Initial results from CSO Orion Spectral Line Survey with detailed insert.
Astronomical Observations
In addition to the laboratory investigations, we have recently completed a submillimeter spectral line survey of Orion between the frequencies of 223.838 GHz and 251.050 GHz with a new, highly-sensitive, broadband receiver at the Caltech Submillimeter Observatory (CSO). The spectra were processed using software designed specifically for the spectra acquired with this receiver. Using the spectrum shown in Figure 5, initial line assignments were made through comparison to the original line survey of Orion [6]. Additional assignments will rely upon comparison to the JPL Spectral Line Catalog [7], the Cologne Database for Molecular Spectroscopy [8], and the Splatalogue Database for Astronomical Spectroscopy [9].
We have begun collection of similar line survey information for other astronomical sources with varying physical and chemical properties. Our recent work at the CSO in July 2009 yielded similar spectral coverage for the star-forming region W51. Analysis of this spectrum has just begun. Comparison with the Orion line survey will be the first step in revealing chemical and physical triggers for complex prebiotic chemistry in interstellar clouds.
Figure 5. Spectrum of W51 obtained in July, 2009 with the CSO.
We would like to acknowledge the other Widicus Weaver Group members, including Jake Laas, Mary Radhuber, Brandon Carroll, Thomas Anderson, and Brett McGuire, as well as our collaborator Matthew Sumner from Caltech. We would also like to thank the Emory University Department of Chemistry facilities and support staff, and the Caltech Submillimeter Observatory Time Allocation Committee and local support staff. This material is based upon work supported by the Howard Hughes Medical Institute under Grant No. 52005873 and the Early Career Research Grant, Emory Department of Chemistry.
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