Cryogenic buffer gas cells have been widely used in experimental chemistry and physics, and have seen recent success and adoption for use for microwave spectroscopy of reactive species as well as precision measurement. Here, I present the latest in buffer gas-cooled microwave spectroscopy, including the highest resolution microwave spectroscopy ever performed, in a cryogenic, cavity enhanced buffer gas beam. The high averaging rate and low noise temperature lends extreme sensitivity to the this instrument, and measures of sensitivity will also be presented on isotopologues of carbonyl sulfide (OCS). Limitations of this technique will also be discussed.

CRESU is a French acronym for reaction kinetics in uniform supersonic flow. Since the inception of this method the overwhelming studies have been carried out using laser induced fluorescence detection. In our group we have taken up diversifying the method of detections used to probe uniform supersonic flows to study reaction kinetics at very low temperatures. One such method involves coupling Chirped-Pulse Fourier-Transform mmWave spectroscopy with the flow, “CPUF” (chirped-pulse uniform flow). However, sampling and detection using the CPUF method has its own limitations as the high-density flow and the collisional environment can interfere with the free induction decay and attenuate the signal. To prevent this and maximize the capabilities of CPUF method we have developed an extended quasi-uniform flow design. The approach involves using an extended nozzle such that the reaction of interest takes place within the nozzle itself where the flow is considered to be uniform. Then, at the nozzle exit, the flow undergoes a second expansion to lower density and temperature which is ideal for the CPUF detection. To implement this, we need to monitor the conditions within the nozzle, and commonly used impact pressure measurements are not feasible inside the nozzle. We have instead implemented a REMPI (resonance-enhanced multiphoton ionization) detection scheme which allows characterization of the conditions of the flow inside the extended nozzle. We have built and characterized an extended nozzle with a series of electrodes to capture the electron signals produced in the REMPI detection step. We have characterized this 22 K He uniform flow using (1+1) REMPI of NO. Ongoing studies with this extended flow setup involve investigation of low temperature reaction kinetics of HCO with NO and O₂.

Cavity ringdown spectroscopy (CRDS) is a valuable tool for observing the rovibrational spectra of extremely dilute molecules. By adopting laboratory techniques like CRDS and quantifying the near-IR spectra of molecules of interstellar interest, we can increase our understanding of the properties and reactions of molecules in space. I present the design and construction of a new continuous-wave CRDS system at the University of Maryland for astrochemical studies. Further, I present the rotationally resolved vibrational spectrum of the first overtone of the C-H stretch in HCN measured with this system in the near-IR (1.5 μm). In future studies, this CRDS system will be used in tandem with a cryogenic buffer-gas cell to perform low-temperature experiments.

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 spectroscopy^[1]. 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 helium^[2]. This offers several advantages over pulsed supersonic jet methods, namely higher data acquisition rates, higher sample throughput and lower electronics noise. We report the construction of such a 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 source assemblies and their associated heat loads, including flash pyrolysis reactors heated up to  ∼ 1600 ^°C. Interleaved, perforated radiation shielding has been employed in concert with a hybrid cryopump/turbopump regime 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 discussed, 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

Over 270 distinct molecules have been detected in the interstellar medium (ISM). Chemical kinetics models are used to elucidate the formation mechanisms of these species under astrophysical conditions, and these models require accurate temperature-dependent rate coefficients for gas-phase reactions. The combination of laser-induced fluorescence (LIF) with uniform supersonic molecular beams (i.e., CRESU) has proven to be a powerful method for measuring total rate constants. In recent years, there has been considerable interest in coupling chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy with the CRESU method, but the high density environment of the uniform flow causes rapid rotational dephasing, dramatically limiting its sensitivity. The Suits group at the University of Missouri-Columbia has proposed the design of an extended nozzle which contains the uniform flow, followed by a free-jet expansion where CP-FTMW spectroscopy may be employed. However the use of a nozzle extension prevents optical access to the uniform flow, complicating LIF measurements. Here, we discuss the incorporation of a fiber optic access port within the nozzle extension to provide in situ LIF measurements. Pitot tube impact pressure measurements were recorded to assess flow uniformity for solid extended nozzles, an extended nozzle with a hole, and a nozzle outfitted with a fiber optic collimator, for He and Ar nozzles spanning a range of temperatures. The effects of flow perturbation and the prospects for in situ fluorescence measurements will be discussed.

We are reporting the progress in developing a new molecular beam Fourier transform microwave (FTMW) spectrometer at Tennessee Tech University. The spectrometer uses an L-shaped vacuum chamber to house Cavity and Chirped pulse FTMW setups. This configuration allows faster switching between narrowband and broadband modes without interference from one to the other. The Fabry-Perot cavity is established using two 7.5 in. diameter and 30 cm radius of curvature Aluminum mirrors. The chamber for the chirped pulse setup is built using a polycarbonate tube, following the design of the CPUF Spectrometer from the Suits GroupOldham, James M., Chamara Abeysekera, Baptiste Joalland, Lindsay N. Zack, Kirill Prozument, Ian R. Sims, G. Barratt Park, Robert W. Field, and Arthur G. Suits. 2014. ‘A Chirped-Pulse Fourier-Transform Microwave/Pulsed Uniform Flow Spectrometer. I. the Low-Temperature Flow System’. Journal of Chemical Physics 141(15). This microwave transparent polycarbonate chamber enables mounting the horn antennas outside of the vacuum chamber for easier adjustment of their positions with respect to the supersonic expansion, which is a particularly useful feature for using FTMW spectroscopic detection with uniform supersonic flows. The initial setup will be used for recording high-resolution and broadband rotational spectra of supersonically cooled molecular systems in the 8 - 18 GHz frequency range.

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Footnotes:

Oldham, James M., Chamara Abeysekera, Baptiste Joalland, Lindsay N. Zack, Kirill Prozument, Ian R. Sims, G. Barratt Park, Robert W. Field, and Arthur G. Suits. 2014. ‘A Chirped-Pulse Fourier-Transform Microwave/Pulsed Uniform Flow Spectrometer. I. the Low-Temperature Flow System’. Journal of Chemical Physics 141(15)..

Rotational spectroscopy is an incredible tool for determining molecular structures, with capabilities that include experimentally establishing bond lengths and angles with incomparable precision and investigating transient, weakly bonded, chemical species. We have designed, built, and characterized a broadband microwave spectrometer based on chirped pulse excitation, built to measure rotational spectra in the 6-18 GHz range. We introduce our molecules using a supersonic expansion, thereby achieving a rotational temperature of 1-2 K for our molecules. This brings the maximum in the rotational Boltzmann distribution into the frequency range of the instrument. The spectrometer performance has been benchmarked by measuring the pure rotational spectrum of carbonyl sulfide (OCS). We are in the process of performing spectroscopic measurements of halothane. Details of how the spectrometer works and analysis of the acquired data will be presented.

Cavity measurements are well established for sensitive infrared measurements of gas-phase compounds. A recent development of a Fabry-Perot cavity by Hindle et al. [Optica, 2019, 6, 1449-1454] has allowed to adapt this technique in the sub millimeter-wave range for rotational spectral measurements using cavity enhanced absorption spectroscopy (CEAS) and cavity ring-down spectroscopy (CRDS) around 620 GHz. Recently, we have developed a similar cavity for larger wavelengths in the 150-215 GHz range in order to measure at trace levels semi-volatile organic compounds. The capability of the spectrometer to measure semi-volatiles explosive taggants such as dinitrotoluenes or 2,3-dimethyl-2,3-dinitrobutane (DMNB) and to measure at ppm level nitromethane in this spectral range will be presented. In particular, the recent measurement in cavity and the spectral analysis of DMNB based on quantum chemistry calculations and microwave spectral analysis performed at the PhLAM laboratory will be presented.

Confocal Fabry-Pérot resonators are ubiquitous in spectroscopy throughout the optical region of the electromagnetic spectrum and are often used for cavity ring down or Cavity Enhanced Absorption Spectroscopy (CEAS). Their use in the sub-THz region, however, is uncommon due to the lack of spherical dichroic mirrors at these wavelengths. To get around this, sub-THz spectroscopists often employ multi-pass or unconventional cavity geometries. Some of these, use planar free-standing wire-grid polarizers to couple radiation into and/or out of the cavity. A few years ago, we demonstrated that it is simple to fabricate a concave wire-grid polarizer by lithographically patterning and etching copper that is adhered to the concave surface of a spherical plastic blank. By placing a matching pair of these spherical mirror/polarizers in a confocal geometry, we demonstrate cavity Qs on the order of 100,000. This simple open-resonator geometry favors the TEM₀₀ mode of the field and allows us to pass a molecular beam directly through the cavity beam waste. Underscoring the increased sensitivity of this molecular beam setup, we readily observe the weak fine-structure band near 60 GHz from the magnetic dipole allowed transitions of molecular oxygen and their splitting in the earth’s magnetic field. While the setup is simple, the resulting saturated absorption/dispersion signals are surprisingly complex. At molecular beam temperatures and densities, sub-THz CEAS signals display comparable Doppler, pressure, and transit-time broadening effects. This is complicated further by the dramatic dispersion effects the molecular beam has on the cavity mode resonance. This talk will give a brief overview of the cavity and then focus on the theory for modelling and fitting the sub-THz CEAS signals.

Time-resolved, absolute HO$_{2}$ number density in O$_{2}$-He mixtures excited by a repetitive ns pulse discharge over a pool of distilled water and in atmospheric pressure plasma jets is measured in situ by Cavity Ringdown Spectroscopy (CRDS). The discharge cell with external electrodes to generate the plasma and a water reservoir are integrated into the CRDS cavity. The experimental results are obtained at near room temperature, both during the discharge pulse burst and in the afterglow. The HO$_{2}$ number density is inferred from the CRDS data using a spectral model exhibiting good agreement with previous measurements of absolute HO$_{2}$ absorption cross sections. HO$_{2}$ is generated during the discharge burst and decays in the afterglow between the bursts, on a ms time scale. Comparison with the kinetic modeling predictions demonstrates good agreement with the data and identifies the dominant HO$_{2}$ generation and decay processes. HO$_{2}$ in the plasma is formed predominantly by the recombination of H atoms, generated by the electron impact of water vapor, with O$_{2}$ molecules. Reactions with O atoms and OH radicals are among the main HO$_{2}$ decay processes in the afterglow. CRDS was also be used for HO$_{2}$ measurements in atmospheric pressure plasma jets, where the jet is integrated in the Open-air Cavity, the mirrors were protected with purge. The HO$_{2}$ number density is inferred from the CRDS data using a spectral model exhibiting good agreement with previous measurements of absolute HO$_{2}$ absorption cross sections.