Cosmic ice analogue experiments are an important aspect of astrochemistry because they help researchers construct the chemical pathways leading to molecules found in young stellar objects, comets, and meteorites. Decades of cosmic ice experiments have demonstrated the formation of various organics and how ice composition is affected by UV photons and temperature. The ice chemistry can be challenging to elucidate, and structure-specific techniques are required to uniquely identify products. We present the Sublimation Laboratory Ice Millimeter/submillimeter Experiment (SubLIME), which uses rotational spectroscopy to complement previous laboratory ice studies. Using this technique, we can detect a wide range of products, including structural and conformational isomers, of UV-photolyzed ice samples from a single spectrum. Furthermore, this technique can be used to model the observational spectra of pre- and protostellar cores and cometary comae. We will present the SubLIME setup and new spectroscopic results of sublimated UV-photolyzed ice samples containing water (H₂O) and carbon monoxide (CO).

The ortho-to-para ratios (OPRs) of interstellar molecules with symmetric hydrogen nuclei are thought to provide information about their thermal and chemical history. The OPR of formaldehyde (H₂CO) is sometimes used to predict whether this compound formed on a cold interstellar grain or in the gas phase at warmer temperatures, but the full meaning of the OPR detected in interstellar space is still a topic of debate. This work aims to unravel more information about the OPR of H₂CO formed on icy interstellar grains through laboratory experiments. In these experiments, the OPR of H₂CO is measured using submillimeter spectroscopy after low-temperature formation by ultraviolet photolysis of interstellar ice analog samples containing water (H₂O), carbon monoxide (CO), and/or methanol (CH₃OH). The experimental approach and preliminary results will be discussed.

Dark and cold regions in space, including regions like prestellar cores and protoplanetary disks, have been shown to harbor high densities of complex organic molecules. Many organic molecules can form in interstellar ices and be deposited into the gas phase via heating, shocks, or other desorption mechanisms. Nonetheless, the expectation is that the density of large organics in cold, dark regions should be low because the molecules readily freeze out onto ices during collisions. Therefore, there is a debate about how molecules like methanol can be detected in the gas phase in regions where they should be depleted on the surface of dust particles. We have developed a new experimental technique, Sublimation Laboratory Ice Millimeter/submillimeter Experiment (SubLIME), to study these processes. We will present the latest experimental findings using SubLIME2, the newest ultra-high vacuum setup focused on detecting complex organics from interstellar ice analogs studied at cryogenic temperatures. With these experiments, we perform FTIR spectroscopy in the mid-IR to monitor solid-phase molecules, mass spectrometry to detect the molecules in the gas phase, and millimeter/submillimeter rotational spectroscopy from 100 to 1000 GHz to look for complex molecules desorbing from the solid to the gas phase. Here we will report on our recent experiments to study photolysis and photodesorption of simple ices containing water, methanol, and carbon monoxide.

Density function theory calculations in 17H₂O and 24H₂O clusters were used to study the deposition and subsequent chemistry of MgNC, the first metal-containing molecule identified in interstellar space. MgNC is a reactive radical with a mixture of covalent and ionic bonding between the Mg and NC. We found that H can react facilely with adsorbed MgNC to form HMgNC, a known astromolecule; there is sufficient energy to eject HMgNC into the gas phase. Acetylene (HCCH) and hydrogen cyanide (HCN) reactions with adsorbed MgNC were also characterized. While there are barriers to forming complexes in both cases, they appear to be submerged below the reactant asymptote. Among the outcomes of these reactions are the formation of the vinyl radical (C₂H₃) from HCCH and the methaniminyl radical (H₂CN) from HCN. Deposition of compounds containing Na and Al will also be summarized.

The weak, often vanishing, dipole moments of polycyclic aromatic hydrocarbons pose a challenge to exploring their interstellar chemistry through radio astronomy. Funtionalization of a pure hydrocarbon with a highly polar nitrile (−CN) group yields a useful proxy, so long as the spectroscopy and chemistry of such CN-tagged molecules are well understood. In this talk, we present recent laboratory measurements of the CN-substituted cyclic hydrocarbons cyanocyclopentadiene, C₅H₅CN, and cyanoindene, C₉H₇CN, produced in a discharge expansion source and probed by cavity-enhanced Fourier transform microwave spectroscopy. We discuss the role that resonantly stabilized radical intermediates play in the likely formation chemistry of these species and the astrochemical implications of their abundances in the cold, dense molecular cloud TMC-1.

The recent astronomical observations of the simplest aromatic nitrile benzonitrile, c-C₆H₅CN, followed by a five-membered [1], [2] and a bicyclic [3] CN functionalized ring in TMC-1 have opened up a new field of complex organic molecules (COMs) in space. These new findings provided an impetus for the laboratory rotational spectroscopy studies of larger -CN functionalized rings. One such example is 2,4,6-cycloheptatriene-1-carbonitrile (2,4,6-CHT-1-CN), a seven-membered ring with a -CN group attached to the sp³-hybridized carbon atom. With a permanent electric dipole moment of 4.3 D and a low boiling point, the molecule is an excellent candidate for laboratory rotational spectroscopy. Experiments were performed in the 18-26 GHz and 75-110 GHz frequency ranges in a supersonic expansion setup and a room temperature flow cell setup, respectively. The measurements across the 18-110 GHz region enabled the identification and assignment of the vibronic ground state, singly substituted rare-atom isotopologues, and vibrationally excited states. In this work, we report the precise determination of the rotational constants, quartic centrifugal distortion constants, as well as nitrogen nuclear quadrupole coupling constants for the vibronic ground state. The rotational spectroscopy study of 2,4,6-CHT-1-CN presented here forms the basis for future astronomical detection of this molecule. [1] M. C. McCarthy et al., “Interstellar detection of the highly polar five-membered ring cyanocyclopentadiene,” Nat Astron, vol. 5, no. 2, pp. 176–180, Feb. 2021, doi: 10.1038/s41550-020-01213-y. [2] K. L. K. Lee et al., “Interstellar Detection of 2-cyanocyclopentadiene, C5H5CN, a Second Five-membered Ring toward TMC-1,” ApJL, vol. 910, no. 1, p. L2, Mar. 2021, doi: 10.3847/2041-8213/abe764. [3] B. A. McGuire et al., “Detection of two interstellar polycyclic aromatic hydrocarbons via spectral matched filtering,” Science, vol. 371, no. 6535, pp. 1265–1269, Mar. 2021, doi: 10.1126/science.abb7535.

Vibrational spectra of the three singly substituted ¹³C isotopic species of ethyl cyanide, aka propionitrile (CH₃CH₂CN), have been studied at high spectral resolution at the synchrotron facility SOLEIL using Fourier-transform far-infrared spectroscopy. The measurements, recorded up to 700 cm⁻¹, cover the fundamental modes of the CCN in-plane bending ν₁₃, the methyl torsion ν₂₁, the CCN out-of-plane bending ν₂₀ as well as the CCC in-plane bending ν₁₂. A first spectroscopic analysis has been performed using the Automated Spectral Assignment Procedure (ASAP)M. A. Martin-Drumel, C. P. Endres, O. Zingsheim, T. Salomon, J. van Wijngaarden, O. Pirali, S. Gruet, F. Lewen, S. Schlemmer, M. C. McCarthy, and S. Thorwirth 2015, J. Mol. Spectrosc. 315, 72o derive accurate excited-state rotational level energies with a focus on the ν₂₀ and the ν₁₂ vibrational modes. M. A. Martin-Drumel, C. P. Endres, O. Zingsheim, T. Salomon, J. van Wijngaarden, O. Pirali, S. Gruet, F. Lewen, S. Schlemmer, M. C. McCarthy, and S. Thorwirth 2015, J. Mol. Spectrosc. 315, 72t

Protonated ethyl cyanide, CH₃CH₂CNH⁺, a likely intermediate in interstellar clouds and in the planetary atmosphere of Titan, has been detected at high spectral resolution by means of Fourier transform microwave spectroscopy at centimeter wavelengths. From 13 a-type rotational transitions between 8 and 44 GHz, the three rotational constants have been determined to better than 0.05%, and two of the leading centrifugal distortion terms to a few percent. Since nitrogen hyperfine structure in the lower rotational transitions is highly compact, only the quadrupole coupling tensor element along the a-inertial axis χ_aa(N) could be determined. The agreement between the experimental rotational constants and those calculated theoretically is very good, of order 0.2%, a clear indication that the CCSD(T) level of theory provides an accurate treatment of the electronic structure. By scaling to isoelectronic butyne, even better agreement between the two is achieved ( << 0.1%). The similarity of the eQq(N) values derived along the C-N bond axis for both protonated vinyl cyanide and protonated ethyl cyanide along with the very small magnitudes of these constants implies a quadruply-bound nitrogen atom and an \ceH−N⁺#C−R type structure that is affected little by protonation. Closely spaced torsional doublets in one K_a=0 line and three K_a=+1 lines allow an estimate of the threefold barrier to internal rotation of V₃=2.50±0.09 kcal mol⁻¹, which is within 4% of that calculated theoretically. Ethyl cyanide has a high proton affinity and is abundant in rich astronomical molecular sources, implying its protonated variant is a good candidate for astronomical detection, particularly since this species is calculated to possess a sizable dipole moment along the a-inertial axis (2.91 D).

Diffuse interstellar bands (DIBs) are optical absorption lines by electronic transitions of interstellar molecules in diffuse clouds. Almost all bands are not identified yet, except for C₆₀⁺. As a hint of DIB carriers, the presence of C₆₀⁺ infers that molecules in diffuse clouds are ionized. Additionally, the molecules would frequently contain a cyano group and more or less include a halogen atom. Hence, halogen cyanide cations are good carrier candidates. To identify origin molecules of DIBs, laboratory data of band profiles of electronic transitions are essential as well as those of their wavelengths. Generally, a band profile is determined by a structural change of an electronic transition. In this work, the high-resolution spectrum of the A²Σ⁺-X²Π_3/2 electronic transition for ICN⁺, which is one of the halogen cyanide cations, was observed for the first time by cavity ringdown spectroscopy. The rotational constants were determined to be 0.10700(12) and 0.11002(12) cm⁻¹for the A²Σ⁺ and X²Π_3/2 states, respectively. Therefore, the rotational constant ratio β = (B′−B")/B" was derived to be -2.7 %. This small β suggests that the profiles of the absorption bands of the halogen cyanide cations have symmetric structures irrespective of diffuse-cloud temperature. This information allows us to search the halogen cyanide cations in space.

The recent detection of cyano naphthalenes within TMC-1 using radioastronomyB.A. Mc Guire, R.A. Loomis, A. M. Burkhardt, K.L. K. Lee, C. N. Shingledecker, S. B. Charnley, I. R. Cooke, M. A. Cordiner, E. Herbst, S. Kalenskii, M. A. Siebert, E. R. Willis, C. Xue, A. J. Remijan, and M. C. Mc Carthy, Science, 371, 1265 (2021)rovided the first unambiguous confirmation of the interstellar PAH’s hypothesis. In the mid-infrared (IR) domain, the launch of the James Webb Space Telescope opens exciting perspectives to collect information about polycyclic aromatic compounds. In this context, high resolution (HR) IR studies' enabling to resolve the rotational structure of vibrational bands of large aromatic species mainly used synchrotron-based Fourier Transform (FT) spectroscopy coupled to room temperature long path cells but the spectral analysis of such recordings remains very challenging. Nowadays, very few set-ups combining HR IR spectroscopy to the supersonic jet technique were developed to target low volatile PAH compoundsB. E. Brumfield, J. T. Stewart and B. J. McCall, J. Phys. Chem. Lett 3, 1985 (2012) O. Pirali, M. Goubet, T. Huet, R. Georges, P. Soulard, P. Asselin, J. Courbe, P. Roy and M. Vervloet, PCCP. 15, 10141 (2013) A tunable mid-IR quantum cascade laser spectrometer coupled to a pulsed supersonic jet (SPIRALES set-up) recently implemented allows to record the rotationally resolved spectra of large molecules at low temperatures. We report the jet-cooled rovibrational IR study of three centrosymmetric two-ring PAH molecules: naphthalene, 1,5-naphthyridine and biphenyl in both regions of in plane ring C-H bending and C-C ring stretching vibrations, enabling to extract reliable spectroscopic parameters both in ground and excited vibrational states. Comparison between experiment and quantum chemistry calculations give confidence in the predictive power of corrected calculated rotational parameters. Last, experimental inertial defects of naphthalene and 1,5-naphthyridine complemented by similar two-ring and larger species agree well with an extended Oka’s empirical formula developed for estimating the inertial defects of aromatic ring compounds.

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Footnotes:

B.A. Mc Guire, R.A. Loomis, A. M. Burkhardt, K.L. K. Lee, C. N. Shingledecker, S. B. Charnley, I. R. Cooke, M. A. Cordiner, E. Herbst, S. Kalenskii, M. A. Siebert, E. R. Willis, C. Xue, A. J. Remijan, and M. C. Mc Carthy, Science, 371, 1265 (2021)p B. E. Brumfield, J. T. Stewart and B. J. McCall, J. Phys. Chem. Lett 3, 1985 (2012) O. Pirali, M. Goubet, T. Huet, R. Georges, P. Soulard, P. Asselin, J. Courbe, P. Roy and M. Vervloet, PCCP. 15, 10141 (2013).

Small molecules made of refractory materials are thought to play an important role in the dust formation processes around late-type stars. Likewise, they take part in the opacity process of variable late-type stars, as has been shown for the molecule TiO.M.J. Reid & J.E. Goldston, The Astrophysical Journal 568, 931 (2002)ecause of similar formation conditions, the diatomic molecule vanadium oxide (VO) is thought to occur in similar locations around stars as TiO.L.K. McKemmish et al., Monthly Notices of the Royal Astronomical Society 463, 771 (2016)O has already been detected in the near-infrared region in the envelope of the red hypergiant VY CMaJ. Bernal et al., 74th International Symposium on Molecular Spectroscopy (2019) but due to the lack of high-resolution laboratory spectra, no astrophysical search of VO in the mid-IR region has been performed. In this work, we report the ro-vibrational absorption spectrum of X⁴Σ⁻ VO, including its hyperfine structure. In our experiment we used a frequency modulated quantum cascade laser in combination with Herriott-type multipass optics. The molecules were produced by laser ablation of a vanadium rod and an N₂O/He buffer gas, which was subsequently adiabatically expanded into a vacuum chamber. The rotationally cooled spectrum was analyzed using the pgopher software and the molecular constants were determined. The experimental data as well as line predictions will enable a dedicated search for this molecule in space at mid-IR wavelengths.

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Footnotes:

M.J. Reid & J.E. Goldston, The Astrophysical Journal 568, 931 (2002)B L.K. McKemmish et al., Monthly Notices of the Royal Astronomical Society 463, 771 (2016)V J. Bernal et al., 74th International Symposium on Molecular Spectroscopy (2019),