Three bands of tungsten sulfide, WS, in the 14,900 - 16,100 cm⁻¹ region have have been recorded in high resolution using Intracavity Laser Spectroscopy integrated with Fourier Transform detection (ILS-FTS). WS was formed in the plasma discharge resulting from a 0.05 A – 0.15 A DC current applied to a tungsten-lined copper hollow cathode within the resonator cavity of a dye laser using gas flows of Ar, CS₂, and H₂ at a pressure of approximately 1 torr. Based on isotopologue shifts, the observed WS bands are assigned as the (0,0), (1,0), and (2,0) bands of a new [15.05] Ω=0⁺ – X ³Σ(0⁺) electronic transition, with bandheads near 15,050, 15,575, and 16,094 cm⁻¹, respectively. The observed line positions were rotationally analyzed using PGOPHER, and spectroscopic constants are compared to a previous computational work [L.F. Tsang et al., J. Mol. Spec. 2019 (359), 31-36]. The results of the analysis will be presented.

The deployment of millimeter wave and terahertz heterodyne radiometers has traditionally been reserved for large flagship space missions (e.g., Hershel HIFI, UARS MLS) owing to the size, weight, and power requirements associated with this class of remote sensing instrumentation. Ongoing efforts at the Jet Propulsion Laboratory aims to reduce system complexity by utilizing high speed phase-lock loops embedded in CMOS process integrated circuitry for use as a local oscillator to pump an on chip downconversion mixer. The noise temperature of the resulting CMOS-based mm-wave heterodyne receiver system (180-190 GHz) can be reduced with custom designed InP low-noise amplifier stages to values sufficiently low (T_sys=800 - 1000 K) to allow for remote molecular detections with sub-second integration times. A deployable system, having a form factor commensurate with a 6U cubesat, can be realized by pairing this receiver with a purpose design and built 6 GS/s real-time ADC/FFT integrated circuit chip to process the intermediate frequency signal generated by the frontend receiver.Y. Kim et al., "A 183-GHz InP/CMOS-Hybrid Heterodyne-Spectrometer for Spaceborne Remote Sensing," IEEE Trans. THz Sci. Technol., vol. 9(3). pp. 313-334, 2019n engineering test flight of this system was completed as part of the Fort Sumner, NM Fall 2019 stratospheric ballooning campaign. This talk will discuss instrument performance including pre-flight laboratory testing and molecular detections observed in the radiometrically calibrated data recorded at an altitude of 38 km (125000 ft).

---

Footnotes:

Y. Kim et al., "A 183-GHz InP/CMOS-Hybrid Heterodyne-Spectrometer for Spaceborne Remote Sensing," IEEE Trans. THz Sci. Technol., vol. 9(3). pp. 313-334, 2019A

We are characterizing the vibrational relaxation dynamics of SO₂ in an Even-Lavie (EL) valve supersonic expansion. The EL valve provides rotational cooling to about 1 Kelvin with minimal clustering. In the expansion, the ~C ¹B₂ state of SO₂ is populated using laser excitation of a selected line of the ~C-~X electronic transition. The ~C state rapidly fluoresces to the ~X ¹A₁ state with known Franck-Condon (FC) factors. SO₂ is rotationally cooled in the expansion, but vibrational cooling from the initial FC distribution is slower and depends on parameters of the supersonic expansion. Using chirped-pulse millimeter-wave spectroscopy, relative intensity measurements of selected rotational transitions and their "vibrational satellites" can be performed. The observed change of the vibrational population distribution (VPD) from that of the FC distribution describes the vibrational cooling in the EL supersonic expansion. Comparing the observed VPDs with the calculated FC intensities, we characterize the relaxation dynamics as a function of various physical parameters, including the location in the expansion and the backing pressure. This experimental scheme will also be used to characterize relaxation dynamics in a cryogenic buffer gas beam. This knowledge of the nascent (unrelaxed) VPD can be used to study the transition states of photo-fragmentation reactions.

Microwave spectra have been recorded for methanesulfonic acid (CH₃SO₃H) and its –OD isotopologue. No internal rotation of the methyl group was observed, consistent with the calculated barrier of 2.7 kcal/mol. A pair of tunneling states has been observed for both species, however, and is attributed to large amplitude wagging of the hydroxyl hydrogen from one side of the molecule to the other. The predicted barrier to this motion, obtained from M06-2X/6-311++G(3df,3pd) calculations is 0.7 kcal/mol. For CH₃SO₃D, the tunneling energy was directly determined to be ∆E = 6471.9269(17) MHz from the measurement of c-type spectra. In the case of the parent species, however, transitions displaced by the tunneling energy have not been located and are likely above the maximum frequency accessible by the spectrometer (20 GHz). Thus, the value of ∆E could not be experimentally determined. Nonetheless, a satisfactory fit was obtained for transitions involving J"= 0 and J" = 1 (nine frequencies for each state). Suggestions for further work at higher frequencies will be presented.

The methanesulfonic acid water - complex (CH₃SO₃H-H₂O) has been observed using pulse-nozzle Fourier transform microwave spectroscopy. The rotational spectra for the CH₃SO₃H-D₂O and CH₃SO₃D-D₂O isotopologues have also been obtained and analyzed. DFT calculations predict the two lowest energy conformers of CH₃SO₃H-H₂O to form a strong hydrogen bond between the water molecule with the acidic proton and a second, longer hydrogen bond with one of the S=O bonds to form the 6-membered ring-like structure that is typical of oxyacid monohydrates. The observed rotational constants and isotope shifts are in best agreement with those predicted for the global minimum structure of CH₃SO₃H-H₂O, where the unbound H₂O hydrogen atom is oriented away from the methyl group. In contrast to the triflic acid monohydrate (CF₃SO₃H-H₂O) spectrum, there was no evidence of a pair of tunneling states arising from internal motion of the water. Additionally, A and E internal rotor states were not resolvable in the observed spectrum, consistent with the predicted high barrier for methyl group internal rotation (V₃=1000 cm⁻¹).

The spectroscopy of nitric oxide (NO) is of significant interest for diagnostic purposes, since NO is a harmful pollutant from combustion systems and is subject to increasingly stringent emissions requirements. The Raman spectrum of NO is of particular interest because of the spin splitting in its ground electronic level. In a recent paper by Satija et al.^a, coherent anti-Stokes Raman scattering (CARS) spectroscopy was performed and a detailed model of the pure rotational Raman spectrum of NO was developed. Evidence of an electronic Raman transition at 121 cm⁻¹ between the spin split ²Π_1/2→²Π_3/2 ground electronic levels was also observed, although the analysis of the CARS spectrum was complicated by the weakness of the electronic Raman transitions compared to the pure rotational transitions in the same spectral region. In the current work, high-resolution stimulated Raman spectroscopy measurements are performed using a high-power (18W) continuous-wave (CW) laser operating in single-frequency mode with a 0.00003 cm⁻¹ linewidth near 532 nm. These measurements will aid in the theoretical development of NO Raman spectroscopy and will lead to more definitely determining the magnitude and sign of the 〈ηv=0|^α²_q=2|ηv=0〉 tensor element. ^a A. Satija, N. Chai, M.T. Arendt, R.P. Lucht, J Raman Spectroscopy DOI: 10.1002/jrs.5836 [in-press].

Methane is the first organic molecule detected in hot-Jupiter exoplanets [1] and the observed spectra carry information about the atmospheric conditions, photochemistry, and planetary formation. To extract this information, we need accurate theoretical models of the spectra, verified by high-precision laboratory measurements. However, the energy level structure of highly excited CH₄ is poorly understood; there exist limited line-by-line assignments of laboratory spectra above the Icosad polyad ( 6,000 cm⁻¹). We perform double-resonance spectroscopy using a 3.3 μm continuous-wave pump and 1.67 μm frequency comb probe to measure sub-Doppler transitions to energy levels in the 3ν₃ band (9000 cm⁻¹range) [2]. Using a single-pass cell, we detected 36 (J’=0-2) transitions, 26 for the first time, with 1.7 MHz frequency accuracy, limited by the stability of the pump. We assign the transitions using the intensity ratios measured with parallel and perpendicular pump/probe polarizations, and by comparison of transition frequencies and intensities to predictions from the TheoReTS database [3]. Our work is the first measurement of sub-Doppler molecular response using a frequency comb and the first verification of the accuracy of the theoretical prediction that start from highly vibrationally excited methane states. Currently, experiments are being performed using an enhancement for the comb probe. [1] M. R. Swain, G. Vasisht, and G. Tinetti, Nature 452, 329 (2008). [2] A. Foltynowicz et al., Arxiv: 2001.08781 (2020). [3] M. Rey, A. V. Nikitin, Y. L. Babikov, and V. G. Tyuterev, Journal of Molecular Spectroscopy 327, 138 (2016).

Pure rotational spectra of Sc¹³C₂ (~X²A₁) and Sc¹²C¹³C (~X²A^′) have been obtained using Fourier Transform microwave/millimeter-wave methods. These molecules were synthesized from the combination of scandium vapor, produce via laser ablation, with mixtures of ¹³CH₄ or ¹³CH₄/¹²CH₄, diluted in argon. The four lowest a-type rotational transitions were observed for both species in the frequency range of 14 – 61 GHz. Each exhibit hyperfine splittings due to the nuclear spins of ¹³C (I = 1/2) and/or Sc (I = 7/2). Rotational, spin-rotation, and hyperfine parameters have been determined for these isotopologues, and a refined structure for ScC₂ established. In addition, a quartic force field was calculated for ScC₂ and its isotopologues using a highly accurate coupled cluster-based composite method, incorporating complete basis set extrapolation, scalar relativistic corrections, outer core and inner core electron correlation, and higher-order valence correlation effects. The ratio of experimental to theoretical (B+C) values is 1.005 for all calculated isotopologues, yielding a promising route towards predictive gas phase rotational spectroscopy for new metal-carbon bearing radicals.

Organic acids and amides make up much of the necessary material a planet needs to support life. The simplest forms of these molecular classifications have previously been detected in the interstellar medium (ISM); both formic acid (CH₂O₂) and formamide (CH₃NO) are well-known interstellar molecules. In this study we chose to look for the next step in molecular complexity, carbonic acid (H₂CO₃) and glycolic acid (C₂H₄O₃) in addition to the amide counterparts carbamic acid (CH₃NO₂) and glycolamide (C₂H₅NO₂). All of these molecules are predicted to form via grain-surface reactions in the ISM. While some spectroscopic information for carbonic acid and glycolamide is available, it is of limited frequency coverage. No spectroscopic information is available for glycolid acid and carbamic acid. We have therefore undertaken experiments to collect spectra in the millimeter and submillimeter regime so as to compare with observational results. We will report on the current status of assigning spectra of glycolic acid and glycolamide. We will also report on the current progress towards formation of carbonic and carbamic acid via O(¹D) insertion reactions with formic acid and formamide, respectively. The laboratory spectra of these molecules will provide many insights into if and how the building blocks of life find their start in the ISM.

Third-row transition metal complexes, particularly those containing platinum and iridium, are ubiquitous photosensitizers and catalysts, and transient x-ray absorption near edge structure (XANES) spectroscopy at the metals’ L-edges has been increasingly used to study their photodynamics. While synchrotron XANES provides high photon flux and ultrafast time resolution, beamtime is unfortunately limited. We have examined 5d metal complexes using a tabletop XANES spectrometer which generates femtosecond XUV pulses (30-100 eV, 30 fs IRF) via high-harmonic generation (HHG), where the N_6,7-edge (4f-5d) and O_2,3-edge (5p-5d) of these metals appear. We show the ground state N-edge and O-edge XUV spectra of several platinum and iridium complexes. We will use transient XUV spectroscopy to demonstrate that these edges can be used to track oxidation state changes at the metal center after excitation of a MLCT transition in Ir(ppy)₃. Overall, this will provide information on the metal electronic structure and allow for parsing out the impact of metal vs ligands in the excited states when combined with other transient spectroscopies.

The dissociation mechanism of N₂O²⁺ has been investigated experimentally after double ionization by a single EUV photon with an energy of 38.5 eV using reaction microscope. From the ion-ion time-of-flight coincidence spectrum, the de-nitrogenation (N₂O²⁺ → N⁺ + NO⁺) and de-oxygenation (N₂O²⁺ → O⁺ + N₂⁺) channel of N₂O²⁺ are identified. It is found that, after removal of two electrons, the N₂O²⁺ dications are mainly promoted to the 1¹∆ and 1¹Σ⁺ states for de-nitrogenation channel. In its KER spectra, a main peak centered at 6.4 eV with a shoulder towards higher energies is observed. The major peak is derived from the direct dissociation of N₂O²⁺ from the 1¹∆ and 1¹Σ⁺ potential curve to the N⁺(¹D) + NO⁺(¹Σ⁺) (second dissociation limit L2). The shoulder is originated from the dissociation of N₂O²⁺ from the 1¹∆ and 1¹Σ⁺ states to the N⁺(³P) + NO⁺(¹Σ⁺) (first dissociation limit L1) via the crossing of 1³Π state in the potential energy curves. In addition, the relative contributions from each dissociating processes are obtained for de-nitrogenation channel. Our work shows that the table-top monochromatic EUV beam line provides great advantages in detailed studies of the electronic states and dissociation dynamics.