Small reactive organic molecules are key intermediates in interstellar chemistry, leading to the formation of biologically-relevant species as stars and planets form. These molecules are identified in space via their pure rotational spectral fingerprints in the far-IR or terahertz (THz) regime. Despite their fundamental roles in the formation of life, many of these molecules have not been spectroscopically characterized in the laboratory, and therefore cannot be studied via observational astronomy. The reason for this lack of fundamental laboratory information is the challenge of spectroscopy in the THz regime combined with the challenge of studying unstable molecules. Our laboratory research involves characterization of astrophysically-relevant unstable species, including small radicals that are the products of photolysis reactions, organic ions formed via plasma discharges, molecules released during UV and thermal processing of interstellar ice analogs, and small reactive organics that form via O(¹D) insertion reactions. Our observational astronomy research seeks to examine the chemical mechanisms at play in a range of interstellar environments and to identify chemical tracers that can be used as clocks for the star-formation process. In this talk, I will present recent results from our laboratory and observational studies that examine prebiotic chemistry in the interstellar medium. I will discuss these results in the broader context of my integrative research program that encompasses laboratory spectroscopy, observational astronomy, and astrochemical modeling.

Two dimensional (2D) semiconductors and their artificial structures hold great promises for electronic, optoelectronic, and quantum devices. We developed a facile method to disassemble van der Waals (vdW) single crystals layer-by-layer into monolayers with near-unity yield, high quality, and macroscopic dimensions, and reassemble them into artificial stacked structures. With the stacked multilayers, we have enhanced nonlinear optical response in second harmonic generation and high harmonic generation. These findings reveal versatile optical responses of the 2D semiconductor systems that can be engineered on demand for future applications.

It is well known among spectroscopists that hydrogen has two modifications: para-H₂ and ortho-H₂. Pure para-H₂ can be produced by leading "normal" H₂, a 3:1 ortho:para mixture, over a catalyst at low temperature. It is perhaps less well known that para-ortho H₂ conversion is also catalyzed by collisions with paramagnetic molecules, such as O₂. Almost ninety years ago Farkas and Sachsse measured the rate coefficient of para-ortho H₂ conversion in gas mixtures with O₂.[1] In the same year, 1933, it was proposed by Wigner [2] that it is the magnetic dipole-dipole coupling between the electron spin of O₂ and the nuclear spins of the two protons in H₂ that is responsible for the conversion. In asymmetric collisions this coupling makes the two H-nuclei inequivalent and mixes the nuclear spin functions of para- and ortho-H₂, as well as their rotational states with even and odd j values. Another mechanism, suggested to be much more effective, was proposed later: the exchange interaction with the open-shell O₂ induces spin density into the electronic wavefunction of H₂. In most collisions the spin density is different at the two H-nuclei, which makes them inequivalent by different hyperfine interactions through the Fermi contact term. An important application of para-H₂ is in NMR spectroscopy and its imaging variant, MRI. By adding para-H₂ to the sample the sensitivity of NMR can be increased by four orders of magnitude by a phenomenon called para-hydrogen induced polarization (PHIP). Para-ortho H₂ conversion by O₂ in the gas phase was remeasured in 2014 in view of this application. A detailed and quantitative understanding of the conversion process was still lacking, however. We theoretically investigated the para-ortho H₂ conversion by collisions with O₂ in a first principles approach.[3] Both mechanisms were taken into account and the corresponding coupling terms were quantitatively evaluated as functions of the geometry of the O₂-H₂ collision complex by means of ab initio electronic structure calculations. Then they were included in nearly exact quantum mechanical coupled-channels scattering calculations for the collisions between O₂ and H₂, which yielded the para-ortho H₂ conversion cross sections and the rate coefficients for temperatures up to 400 K. The conversion rate and its temperature dependence are in good agreement with the values measured in H₂-O₂ gas mixtures. The calculations provide detailed insight into the conversion process. [1] L. Farkas and H. Sachsse, Z. Phys. Chem. B 23, 1 (1933). [2] E. Wigner, Z. Phys. Chem. B 23, 28 (1933). [3] X. Zhang, T. Karman, G. C. Groenenboom, and A. van der Avoird, Nat. Sci. (2021); https://doi.org/10.1002/ntls.10002.

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