A 6-18 GHZ DIRECT DIGITAL SYNTHESIS TUNABLE SEGMENTED CHIRPED PULSE FOURIER TRANSFORM MICROWAVE SPECTROMETER

Chirped pulse Fourier transform spectroscopy (CP-FTMW) has become a widely used technique for the detection of molecular rotational spectra owing to its broad frequency coverage. Traditional CP-FTMW set ups involve top-quality broadband arbitrary waveform generators (AWG), high-power amplifiers, and digitizers, which are expensive due to their specifications. One method to lower costs with only a mild sacrifice of efficiency is to divide the total bandwidth into smaller sections and step from section to section with a tunable local oscillator; these so-called “segmented” CP-FTMW spectrometers have much lower costs by decreasing the required amplifier power and digitizer bandwidth. Inspired by the work of Finneran et al.\ (Rev.\ Sci.\ Inst.\ 84, 2013, 083104), our group has designed a 6--18~GHz segmented CP-FTMW broadband spectrometer that also replaces the AWG with a direct digital synthesizer (DDS), further lowering the spectrometer cost. To our knowledge, this is the first instrument in which a DDS has been coupled with the segmented approach to achieve a tunable intermediate frequency (IF). Design, cost analysis, progress, and performance will be discussed in this talk

Explaining the Chemical Inventory of Orion KL through Machine Learning

The interplay of the chemistry and physics that exists within astrochemically relevant sources can only be fully appreciated if we can gain a holistic understanding of their chemical inventories. Previous work by Lee et al. (2021) demonstrated the capabilities of simple regression models to reproduce the abundances of the chemical inventory of the Taurus Molecular Cloud 1 (TMC-1), as well as provide abundance predictions for new candidate molecules. It remains to be seen, however, to what degree TMC-1 is a ``unicorn'' in astrochemistry, where the simplicity of its chemistry and physics readily facilitates characterization with simple machine learning models. Here we present an extension in chemical complexity to a heavily studied high-mass star forming region: the Orion Kleinmann-Low (Orion KL) nebula. Unlike TMC-1, Orion KL is composed of several structurally distinct environments that differ chemically and kinematically, wherein the column densities of molecules between these components can have non-linear correlations that cause the unexpected appearance or even lack of likely species in various environments. This proof-of-concept study used similar regression models sampled by Lee et al. (2021) to accurately reproduce the column densities from the XCLASS fitting program presented in Crockett et al. (2014).Comment: 14 pages; 6 figures, 1 table in the main text. 0 figures, 1 table in the appendix. Accepted for publication in The Astrophysical Journal. Molecular dataset for machine learning can be found in the Zenodo repository here: https://zenodo.org/record/767560

Enantioselective Synthesis of Enantioisotopomers with Quantitative Chiral Analysis by Chiral Tag Rotational Spectroscopy

Fundamental to the synthesis of enantioenriched chiral molecules is the ability to assign absolute configuration at each stereogenic center, and to determine the enantiomeric excess for each compound. While determination of enantiomeric excess and absolute configuration is often considered routine in many facets of asymmetric synthesis, the same determinations for enantioisotopomers remains a formidable challenge. Here, we report the first highly enantioselective metal-catalyzed synthesis of enantioisotopomers that are chiral by virtue of deuterium substitution along with the first general spectroscopic technique for assignment of the absolute configuration and quantitative determination of the enantiomeric excess of isotopically chiral molecules. Chiral tag rotational spectroscopy uses noncovalent chiral derivatization, which eliminates the possibility of racemization during derivatization, to perform the chiral analysis without the need of reference samples oft he enantioisotopomer

A 6-18 GHZ DIRECT DIGITAL SYNTHESIS TUNABLE SEGMENTED CHIRPED PULSE FOURIER TRANSFORM MICROWAVE SPECTROMETER

Chirped pulse Fourier transform spectroscopy (CP-FTMW) has become a widely used technique for the detection of molecular rotational spectra owing to its broad frequency coverage. Traditional CP-FTMW set ups involve top-quality broadband arbitrary waveform generators (AWG), high-power amplifiers, and digitizers, which are expensive due to their specifications. One method to lower costs with only a mild sacrifice of efficiency is to divide the total bandwidth into smaller sections and step from section to section with a tunable local oscillator; these so-called “segmented” CP-FTMW spectrometers have much lower costs by decreasing the required amplifier power and digitizer bandwidth. Inspired by the work of Finneran et al.\ (Rev.\ Sci.\ Inst.\ 84, 2013, 083104), our group has designed a 6--18~GHz segmented CP-FTMW broadband spectrometer that also replaces the AWG with a direct digital synthesizer (DDS), further lowering the spectrometer cost. To our knowledge, this is the first instrument in which a DDS has been coupled with the segmented approach to achieve a tunable intermediate frequency (IF). Design, cost analysis, progress, and performance will be discussed in this talk

Astronomical Detection of the Interstellar Anion C10H− toward TMC-1 from the GOTHAM Large Program on the Green Bank Telescope

Using data from the Green Bank Telescope (GBT) Observations of TMC-1: Hunting for Aromatic Molecules (GOTHAM) survey, we report the first astronomical detection of the C _10 H ^− anion. The astronomical observations also provided the necessary data to refine the spectroscopic parameters of C _10 H ^− . From the velocity stacked data and the matched filter response, C _10 H ^− is detected at >9 σ confidence level at a column density of ${4.04}_{-2.23}^{+10.67}\times {10}^{11}$ cm ^−2 . A dedicated search for the C _10 H radical was also conducted toward TMC-1. In this case, the stacked molecular emission of C _10 H was detected at a ∼3.2 σ confidence interval at a column density of ${2.02}_{-0.82}^{+2.68}\times {10}^{11}$ cm ^−2 . However, as the determined confidence level is currently <5 σ , we consider the identification of C _10 H as tentative. The full GOTHAM data set was also used to better characterize the physical parameters including column density, excitation temperature, line width, and source size for the C _4 H, C _6 H, and C _8 H radicals and their respective anions, and the measured column densities were compared to the predictions from a gas/grain chemical formation model and from a machine learning analysis. Given the measured values, the C _10 H ^− /C _10 H column density ratio is ∼ ${2.0}_{-1.6}^{+5.9}$ —the highest value measured between an anion and neutral species to date. Such a high ratio is at odds with current theories for interstellar anion chemistry. For the radical species, both models can reproduce the measured abundances found from the survey; however, the machine learning analysis matches the detected anion abundances much better than the gas/grain chemical model, suggesting that the current understanding of the formation chemistry of molecular anions is still highly uncertain