r0pt Figure Methyl formate plays an important role in multiple combustion mechanisms, such as the oxidation of dimethoxymethane, warranting further study of its absorbance spectra at elevated temperatures. However, pyrolysis reactions make broadband measurements at elevated temperatures difficult, and currently available spectra are mostly limited to lower temperatures around 296 K. In this study, we have shock heated methyl formate, dilute in argon, at temperatures up to 1000 K and measured the cross sections from 1655 to 1875 cm⁻¹. Measurements within the short ms-scale test times were achieved through a rapid-tuning, broad-scan external-cavity quantum-cascade laser. We have also supplemented these elevated temperature measurements with elevated pressure cross sections at room temperature in a static cell, at pressures up to 35 atm. Our measurements were validated through excellent 296 K agreement with that in the literature. The elevated temperature cross sections reveal an additional absorbance structure near 1800 cm⁻¹, possibly the emergence of a combination band. Interestingly, the cross sections at elevated pressure conditions display a dependence on pressure, contrary to its common use and implementation in previous literature. These cross sections expand our ShockGas-IR database (https://searchworks.stanford.edu/view/wt021dc3029), containing elevated temperature cross sections of many other molecules important for combustion mechanisms.

Millimeter and sub-millimeter differential absorption radar (DAR) systems, which measure the attenuation of a transmitted beam as function of both frequency and range, are currently in development for a variety of Earth and planetary science applications. This talk will summarize efforts to realize a portable DAR system optimized to profile the 557 GHz 1₁₀-1₀₁ pure rotational transition of water that is suited for humidity measurements between scatter targets in low-pressure Martian-like environments. This emerging class of active remote sensing instrumentation, if deployed on future Mars lander/rover missions, could provide local near-surface humidity profiles that are unresolvable to the current generation of passive orbiting sensors. This presentation will include an overview of DAR operational principles, system architecture, and deployment scenarios. Room temperature laboratory measurements recorded with the DAR prototype system of the 557 GHz pure rotational water transition broadened by carbon dioxide in a sample mixture that is reasonably analogous to that found on Mars ( ∼ 200 ppm H₂O in 5 Torr CO₂) will be presented. Observed results will be discussed in the context of previously measured lineshape parameters with extrapolation made to the lower surface temperatures (200 - 250 K) found on Mars.

In this talk, we report the first room-temperature optical detection of radiocarbon dioxide (¹⁴CO₂) samples at concentrations below the natural abundance level (1.2 parts per trillion, ¹⁴C/C), using the recently-developed two-color, mid-IR, pump-probe, cavity ringdown (CRD) technique. With 3 minutes of averaging, our two-color CRD method successfully differentiates, with an accuracy of 8% of the ¹⁴C natural abundance, five combusted ¹⁴C standards with ¹⁴CO₂ concentrations ranging from petrogenic (zero ¹⁴C/C) to approximately double the contemporary abundance. Room-temperature quantification of ¹⁴CO₂ is not possible with any existing one-photon cavity-enhanced techniques at our demonstrated ¹⁴C concentration levels, due to severe spectral overlap between the very weak target ¹⁴CO₂ ν₃-band transitions ( ∼ 5/s ringdown rate at natural abundance) and the strong hot-band transitions of CO₂ isotopologues ( > 10000/s). All previous CRD-based, one-photon ¹⁴CO₂ measurements at the sub-natural-abundance level required cooling of the test gas (-20 to -100^°C) to mitigate the strong background absorption. Our unprecedented high-sensitivity, high-selectivity detection of ¹⁴CO₂ at room temperature is made possible by the dual-background compensation capabilities of the two-color CRD technique. The two-color measurement utilizes two cavity-enhanced pump and probe lasers to excite, respectively, the ν₃=1←0, P(14) and ν₃=2←1, R(13) rovibrational transitions of ¹⁴CO₂. With the pump radiation switched off during every other probe ringdown events ( > 2 kHz rate), the CRD rate fluctuations and strong one-photon absorption interference are effectively cancelled out during the two-color measurements. Highly-selective, room-temperature detection of weak ¹⁴CO₂ absorption signals reduces the technical and operational burdens for cavity-enhanced measurements of radiocarbon. This is a crucial achievement that will enable laser-based radiocarbon quantification outside a laser laboratory setting, and benefit a wide range of scientific applications, such as ¹⁴C-labeling analysis of biomedical samples and field monitoring of fossil fuel emission.

Hydrogen cyanide (HCN) is a molecule of importance in astrochemistry. To prepare for experiments to study its reactivity, we selectively produced a molecular beam of monomeric HCN using a cryogenic buffer-gas source. The HCN beam was first interrogated by condensing it on a 10K substrate using argon as a bath gas to create an inert matrix. Based on a comparison of the resulting infrared spectrum with experiments that use conventional effusive sources, HCN polymers can be nearly eliminated from the matrix using a cryogenic buffer-gas beam source. Our experiments suggest that HCN undergoes polymerization in the gas phase and may exist, to some extent, as a dimer under ambient conditions. We will discuss further investigations using continuous-wave cavity ringdown spectroscopy to examine the first vibrational overtone of the alkynyl C-H stretch of HCN monomer and dimer in the near infrared.

Para-hydrogen (p-H₂) is used as a host matrix in matrix-isolation experiments because of its unique properties to act as a quantum solid. However, p-H₂ is not commercially available and needs to be produced in house with a custom-built p-H₂ converter. Throughout this presentation, we will describe the design and building phases of the custom-built p-H₂ converter at the University of Maryland. Instrument drawings, schematics, and preliminary results will be presented. This talk will also explore the spectroscopic techniques that are used to both prove the enrichment of p-H₂ and determine the purity of p-H₂. The p-H₂ will be used in future experiments to study novel astrochemistry interactions and reactions in the interstellar medium (ISM). The production of p-H₂ is critical for future astrochemistry relevant experiments as its properties as a quantum solid allow us to further understand molecular properties and interactions that would be otherwise unattainable with rare-gas host matrices.

We built a chirped-pulse Fourier transform millimeter-wave spectrometer (CPFTS) [1], which is operational between 75 and 110 GHz. The design and operation of the instrument (excitation, optical path and detection scheme) will be discussed. The detector is based on a heterodyne receiver of an emission spectrometer [2] which we built and used before to sensitively record rotational spectra of complex molecules. The performance of the CPFTS instrument is analysed by recording spectra of methyl cyanide as well as products from a DC discharge of this molecule. Based on the quantitative calibration of the detector we compare the operation of the instrument as CPFTS with that of the emission spectrometer. We find molecular signals much higher in intensity and much lower in noise for the CPFTS operation. We demonstrate how the detection of the coherent molecular signal (FID) reduces the noise more efficiently compared to the detection of the emitted power when operating the system as an emission spectrometer.
References
[1] M. Hermanns, N. Wehres, B. Heyne, C. E. Honingh, U. U. Graf and S. Schlemmer, in preparation
[2] N. Wehres, B. Heyne, F. Lewen, M. Hermanns, B. Schmidt, C. Endres, U. U. Graf, D. R. Higgins and S. Schlemmer, IAU Symposium, 2018, pp. 332-345

Acetone-¹³C₁ is a complex organic molecule with two internal methyl (-CH₃) rotors having relatively low barriers to internal rotation of 251 cm⁻¹ [1]. This leads to two low-lying torsional modes and five internal rotation components resulting in a dense and complex spectrum. Similar conditions can be found in many complex molecules, with isotopologues, hyperfine structure, and interactions being additional factors for the presence of even more crowded spectra than that of acetone. Measurements of acetone-¹³C₁ were performed with an isotopically enriched sample in the frequency range from 37-1102 GHz. Loomis-Wood plots (LWPs) are one approach to improve and fasten the analysis of such crowded spectra. Here, an updated version of the LLWP software was used which relies on LWPs for fast and confident assignments. Additionally, LLWP focuses on being user-friendly, intuitive, and applicable to a broad range of assignment tasks. The software will be presented here and is available together with its full documentation at llwp.astro.uni-koeln.de. Predictions of acetone-¹³C₁ created with ERHAM [2] allow for future radio astronomical searches. [1] P. Groner, J. Mol. Struct. 550-551 (2000) 473-479. [2] P. Groner, J. Chem. Phys. 107 (1997) 4483-4498. [3] P. Groner, J. Mol. Spectrosc. 278 (2012) 52-67.

The exploration of icy body composition in the solar system has primarily involved spectroscopic measurements of volatiles through remote sensing, in which materials naturally expelled from the surface enter the exosphere and potentially escape into space. Landed missions on comets have brought focus onto the development of small, sensitive instrumentation capable of similar composition measurements of the nascent surface and near-surface materials. We present an evolution of our compact millimeter-wave cavity spectrometer that is tuned for sensitivity at 80.6 and 183 GHz where HDO and H₂O exhibit resonance features. In this presentation we will discuss both a low SWaP (size-weight and power) architecture that uses custom micro-chip transceiver elements that is suitable for maturation to deployable systems and a modular configuration using traditional GaAs based millimeter wave hardware suitable for laboratory studies. New design features for these systems including the quartz based coupler, thermal management, and separate clocking board will be discussed in addition to sensitivity studies and preliminary work detecting sublimated ice samples.

Cavity-Enhanced Absorption Spectroscopy (CEAS) and Cavity Ring-Down Spectroscopy (CRDS) are well established for sensitive infrared measurements of gas phase compounds at trace level using their rovibrational signatures. The recent successful development of a THz Fabry-Perot spectrometer shows that the adaptation of such techniques to the THz and submillimeter is possible¹ by probing rotational transitions of light polar compounds. Here we report on the development of a new millimeter resonator based on a low-loss corrugated waveguide with highly reflective photonic mirrors obtaining a finesse above 3500 around 150 GHz. With an effective path length of one kilometer, a significant sensitivity has been evaluated by the measurement of line intensities as low as 10⁻²⁶cm⁻¹/(molecule/cm²). This spectrometer will be used to detect semi-volatile organic vapors at trace level which could not be envisaged with a conventional detection technique.^{2,3 1}Francis Hindle et al. Optica, vol.6, 1449-1454, (2019). ² Gaël Mouret et al. IEEE Sensors, vol.11(1), 133-138, (2013). ³Roucou et al., CHEMPHYSCHEM, 19, 1056-1067, (2018). Acknowledgment: This work received financial support from the French Agence Nationale de la Recherche via funding of the project Millimeter-wave Explosive Taggant vapors Investigations using Spectral taxonomy (METIS) under contract number ANR-20-ASTR-0016-03.

r0pt Figure Coherences at 386.2 THz and 384.1 THz, corresponding to the 5d_5/2 – 5p_3/2 and 5p_3/2 – 5s_1/2 difference frequencies, respectively, have been established in the Rb atom during pump-probe experiments involving pairs of identical  150 fs pulses produced by a Ti:Al₂O₃ laser and a Michelson interferometer. The interference between the two coherences within the atom is observed through a parametric four-wave mixing process in Rb and detection of the signal wave intensity at 420 nm. Scanning the time delay between the pump and probe pulses over a 600 ps interval produces a sampling rate over 300 THz and allows for a spectral domain resolution of 0.05 cm⁻¹ to be achieved. The figure at right shows several spectra recorded near 2.1 THz, the (5d_5/2 – 5p_3/2) - (5p_3/2 – 5s_1/2) difference frequency, with varying angle between the pump and probe pulses (i.e., the phase matching angle) and varying Rb background density. A Fano interference window is clearly observed, and analysis of these and similar spectra demonstrates that the amplitude and phase of the coherently-coupled, three Rb state system can be controlled precisely.