We will describe the performance of a novel chirped-pulse microwave spectrometer for the characterization of molecular structures in the 200-300 amu mass range. The spectrometer covers the frequency range of 7-18 GHz, with a user-selectable bandwidth of up to 2 GHz. Two sampling inlets can be integrated with the system, one a modified GC inlet and the other a headspace inlet, both with fully automated sampling introduction. The chirped pulse is amplified by a pulsed solid-state amplifier with approximately 8 watts of output power. The primary goal of this instrument is to make characterization of new molecular structures by microwave spectroscopy increasingly routine and straightforward. In this talk, we will discuss the performance of this instrument and its relative sensitivity compared to both theory and our larger 3-nozzle broadband spectrometer across a series of commercially available molecules, including 4-cumylphenol, celestolide, and traseolide. Challenges related to the computational chemistry of molecules in this size range will also be discussed.

The determination of chemical mixture components is vital to a multitude of scientific fields. Oftentimes various spectroscopic methods are employed to decipher the molecular composition of these complex mixtures. The sheer density of spectral features of different molecules that are often present in such observations can make unambiguous assignment to individual species challenging. Yet, components of a mixture are commonly chemically related due to environmental processes or shared precursor molecules. Therefore, along with investigating the spectroscopic signals, analysis of the structural and chemical relevance of a molecule is an important consideration when determining which species are present in a mixture. In this talk, we present a method that combines machine-learning molecular embedding models with a graph-based ranking system to determine the likelihood of a molecule being present in a mixture based on the other known species, chemical priors, and spectroscopic information. By incorporating this metric in a rotational spectroscopy mixture analysis algorithm, we demonstrate that the mixture components can be identified with extremely high accuracy in a much more efficient manner than manual analysis.

Double resonance (DR) is a powerful tool for unraveling the level diagram of a chemical species. In this study, we tested the implementation of several double resonance schemes coupled with chirped pulse Fourier transform microwave (CP-FTMW) spectroscopy using a newly constructed 2–8 GHz spectrometer using CF₃I as a probe molecule. We employed both population labeling and coherence destruction schemes. Finally, we implemented a fully automated AMDOR (Automated Measurement of Double Resonance) measurement based on a multiplexed coherence destruction scheme for efficient streamlining of the experimental workflow. We performed the experiment under conditions in which the vibrational excited ν₆ level is populated. The double resonance information readily distinguishes transitions within two vibrational species present.

l0pt Figure The preparation of rotationally cold molecules through the use of supersonic expansions has long been a mainstay of high-resolution gas-phase microwave spectroscopy¹. An increasingly popular alternative to this method is to cool molecules of interest through continuous collisions with a large reservoir of cryogenic, inert buffer gas, typically helium². This offers several advantages over pulsed supersonic jet methods, namely higher data acquisition rates, higher sample throughput and lower electronics noise. We report on the current status of the cryo-cooled buffer gas cell at the Combustion Research Facility at Sandia National Laboratories in California. The instrument has been designed to perform broadband chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy across a frequency range of 6 to 18 GHz. A cooling capacity of 2.7 W at 4 K has been leveraged in order to accommodate a broad range of molecular sources and their associated heat loads, including flash pyrolysis reactors heated up to  ∼  1600 ^°C. Interleaved, perforated radiation shielding has been employed in an effort to reduce the frequency of degenerative thermal “crashes” that often plague such instruments. The performance of the instrument with respect to benchmark species will be reported, as well as the future of the instrument and its experimental endeavours.
[1] M. McCarthy, et. al., Astrophys. J. Supp. Ser. 2000, 129, 611 [2] J. P. Porterfield, et. al., Rev. Sci. Instrum. 2019, 90, 053104

Spectroscopic data for gas phase molecules in the THz domain are of significant interest for molecular physics as well as laboratory astrophysicsS. Schlemmer, Chapter 15, Frontiers and Advances in Molecular Spectroscopy. (Elsevier, 2018), pp. 471–525nd atmosphere physical chemistryA. Cuisset & al, Applied Sciences 11 (2021) However, due to the lack of powerful and tunable sources in this domain, spectroscopic measurements remain scarce. Through its large spectral coverage, high power and low noise THz radiation, synchrotron and for example the AILES beamline of SOLEIL synchrotronJ.B .Brubach & al, 5th International workshop on infrared Microscopy and spectroscopy with accelerated based sources (WIRMS 2009) are among the most suitable THz sources for spectroscopic applications. Nevertheless, commonplace Fourier transform spectrometers, used with such incoherent sources, limit the spectral resolution to tens of MHz. This resolution (30MHz on the AILES beamline), larger than the Doppler broadening at room temperature, led us to develop a new THz high-resolution spectrometerT. S. Hearne & al, Optics Express 30 (2022) This spectrometer is based on the heterodyne mixing of the THz continuum extracted by the AILES beamline with the laser lines produced by a QCL-based optically pumped molecular laserA. Pagies & al, APL Photonics 1, 031302 (2016)cting as the THz local oscillator. In the present work, we associate this spectrometer with a radio-frequency discharge set-up to produce and study radical species. Encouraging results and significant progress have been obtained: several NH₂ rotational transitions have been studied and structures previously unresolved have been revealed.

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Footnotes:

S. Schlemmer, Chapter 15, Frontiers and Advances in Molecular Spectroscopy. (Elsevier, 2018), pp. 471–525a A. Cuisset & al, Applied Sciences 11 (2021). J.B .Brubach & al, 5th International workshop on infrared Microscopy and spectroscopy with accelerated based sources (WIRMS 2009), T. S. Hearne & al, Optics Express 30 (2022). A. Pagies & al, APL Photonics 1, 031302 (2016)a

We report on the improvement of a home-built chirped-pulse spectrometer, which is operational in the millimeter-wave range between 75 and 110 GHz and which has been used to record spectra of complex molecules M. Hermanns, N. Wehres, F. Lewen, H. S.P. Müller, S. Schlemmer, Journal of Molecular Spectroscopy, 358, 25-36, 2019 The detector of our spectrometer employs a heterodyne receiver which was originally built for the use in a laboratory emission spectrometer N. Wehres, B. Heyne, F. Lewen, M. Hermanns, B. Schmidt, C. Endres, U.U. Graf, D.R. Higgins, and S. Schlemmer, Proceedings of the International Astronomical Union, 13, 332–345 (2017) Recently the performance of this instrument has been characterized thoroughly M. Hermanns, N. Wehres, B. Heyne, C. E. Honingh, U. U. Graf, and S. Schlemmer, Rev. Sci. Instrum., 94, Art. No. 034705, 2023 To date this device was not equipped for side-band separation. As a result, mirror lines of the respective other side-band appear in the spectrum. Therefore, it is difficult to assign weaker rotational transitions in the spectrum because it is not easy to distinguish them from the mirror lines. To overcome this limitation, a new baseband frequency board for signal generation, acquisition and processing has been build. This unit, called universal board (U-Board), is based on a FPGA, fast ADCs/DACs and a Jetson GPU module. In particular, this unit increases the versatility of the chirp excitation compared to a standard AWG. Here, we report the use of several excitation phases and the recording of free induction decays in a cyclic manner to perform sideband separation and signal clean-up in our setup. The new design of the spectrometer in our mm wave chirped-pulse spectrometer will be discussed, and spectra of 1-amino-2-propanol will be presented to demonstrate the performance of the improved instrument.

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Footnotes:

M. Hermanns, N. Wehres, F. Lewen, H. S.P. Müller, S. Schlemmer, Journal of Molecular Spectroscopy, 358, 25-36, 2019. N. Wehres, B. Heyne, F. Lewen, M. Hermanns, B. Schmidt, C. Endres, U.U. Graf, D.R. Higgins, and S. Schlemmer, Proceedings of the International Astronomical Union, 13, 332–345 (2017) . M. Hermanns, N. Wehres, B. Heyne, C. E. Honingh, U. U. Graf, and S. Schlemmer, Rev. Sci. Instrum., 94, Art. No. 034705, 2023.

Supersonic-jet Fourier transform microwave (FTMW) spectroscopy is a powerful tool for studying isolated gas-phase molecules where an extremely low rotational temperature near the thermodynamic zero point can be reached. The classical coaxial resonator-jet spectrometer arrangement provides unrivalled resolution, but suffers from repetitive, time-consuming frequency retuning of an high-Q resonator to acquire survey spectra. The chirp-excitation method, which employs a short but powerful frequency-ramp signal passing through molecular resonances, can reduce the survey time dramatically, but at the cost of lower resolution and sensitivity, such that many microwave laboratories are using both spectrometers. With both machines needing a vacuum system capable of handling the jet-expansion at appreciable repetition rate as well as high-frequency electronic and computerized control set-up, this tends not only to be expensive but also requires turn-over time. We developed and implemented an FTMW spectrometer capable of both techniques with an innovative and resource-efficient approach: Using a smart design, most electronic components will be used in dual-purpose while integration both jet-expansion setups in a single vacuum chamber. The tone- and chirp-excitation setups are arranged perpendicularly in the spherical chamber of a single instrument named PARIS, operated in either mode with instant turn-over. Both, the high-resolution and the broadband arrangements of PARIS, use a microwave propagation coaxial with the molecular jet-expansion. Currently, PARIS achieves a sensitivity of a few parts-per-billion (ppb) of OCS diluted in Neon (1%) on the resonator axis and a few parts-per-million (ppm) on the broadband axis. A resolution (full width at half mean, FWHM, of the amplitude spectrum) of about 3 kHz and 9 kHz can readily be achieved with the high-resolution and broadband experiments, respectively, capable of resolving the complex hyperfine structures arising from two ¹⁴N nuclei of 4-methylpyrimidine already in broadband operation.

The exothermic insertion of O(¹D) into C-H bonds can be used to produce unstable molecules of astrophysical interest. Our previous work has utilized O₃ as the precursor to O(¹D) in a supersonic expansion, but this leads to complicated side chemistry from both ozonolysis reactions and from reactions with O₂(¹ ∆) and O(³P), which are also produced from O₃ photodissociation. We are therefore exploring other sources of O(¹D) that could eliminate this side chemistry. Previous modeling work has shown that the use of N₂O as an O(¹D) precursor results in increased CH₃OH production from insertion reactions with CH₄. In addition, the commercial availability of N₂O enables fine control of experimental conditions. Here we report the results of CH₃OH formation via O(¹D) insertion into CH₄ using both O₃ and N₂O as precursors in a supersonic expansion experiment. We show differences in production of CH₃OH with both precursors. Furthermore, the decomposition pathways of excited CH₃OH in the gas phase are explored between both precursors and compared to existing literature using both theory and experiment.

Ion-ion and ion-neutral reactions dominate the chemical pathways that form complex organic molecules in the interstellar medium (ISM). Rotational spectroscopy in the tens of GHz to THz frequency range is the dominant method of detecting species in the ISM. However, the rotational spectra of many organic ions of prebiotic interest have not been measured to THz frequencies. To tackle this challenge, we present a DC-discharge hollow-cathode spectrometer designed to measure the rotational spectra of organic ions in the THz regime. The hollow-cathode instrument has been moved, rebuilt, and upgraded since its prior use. This talk focuses on the benchmarking of the upgraded instrument.

We present recent advancements in the newly built molecular beam FT-Microwave spectrometer at Tennessee Tech University. The spectrometer combines the cavity and chirped pulsed microwave setups in a single L-shaped vacuum chamber. We establish a Fabry-Perot cavity using two 7.5 in diameter (30 cm radius of curvature) mirrors, and the cavity spectrometer setup can now record rotational spectra in the 8-18 GHz frequency range. The chirped pulsed microwave setup comprises a polycarbonate tube (9.5 in internal diameter) and two horn antennas mounted on the outside of the chamber. The chirped pulse setup functions in the 26-40 GHz frequency range. The spectrometer features a Python interface for instrument operation and data acquisition. The performance of the instrument and ongoing spectral measurements will be discussed.