Small molecules containing refractory elements are important keys to understanding the connection between the gas-phase and solid-state constituents of the interstellar medium. They also have relevance for the origin of life and the delivery of the biogenic elements to planet surfaces. Studies of these types of molecules in interstellar space have clearly been driven by laboratory spectroscopy. For almost three decades, the Ziurys lab has been conducting measurements of rotational spectra of highly reactive metal and phosphorus-bearing species, and subsequently searching for these molecules in interstellar space with radio telescopes. Critical to this endeavor has been the development of unusual synthetic methods to create these unstable molecules, and the challenge of unraveling spectra of states with high spin and orbital angular momenta. Molecules of particular interest include metal dicarbides and metal-phosphorus species. An overview of this spectroscopy will be presented, as well as new results for SiP and VO, and the current astrophysical implications.

It is well known among spectroscopists that two modifications of hydrogen exist: para-H₂ and ortho-H₂. Pure para-H₂ can be produced by leading `normal' H₂, a 1:3 para:ortho mixture, over an iron-containing catalyst at low temperature, and can be kept for a long time also at higher temperature in specially prepared gas cylinders. It is perhaps less well known that para-ortho H₂ conversion is also accelerated by interactions with paramagnetic molecules, such as O₂. 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 through a phenomenon called para-hydrogen induced polarization (PHIP). The para-ortho H₂ conversion by O₂ was recently measured in view of this application.[1] Two mechanisms have been suggested for the para-ortho H₂ conversion by collisions with O₂. The first one, proposed in 1933 by Eugene Wigner,[2] is the magnetic dipole-dipole coupling between the electron spin of O₂ and the nuclear spins of the two protons in H₂. 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, proposed by Minaev and Ågren[3] in 1995, is that the overlap of the O₂ and H₂ wavefunctions in a collision complex transfers some of the spin density of O₂ to the wavefunction of H₂. The spin densities induced at the two H-nuclei may be different, which causes a different hyperfine interaction through the Fermi contact term. Wigner made a crude estimate of the para-ortho H₂ conversion rate with the use of some kinetic gas data. Minaev and Ågren suggested, however, that the second mechanism is much more effective. We investigated the para-ortho H₂ conversion by collisions with O₂ by a first principles approach. Both mechanisms are included: the corresponding coupling terms are quantitatively evaluated as a function of the geometry of the O₂-H₂ collision complex by means of ab initio electronic structure calculations. Then they are included in nearly exact quantum mechanical coupled-channels scattering calculations for the collisions between O₂ and H₂, which yield the para-ortho H₂ conversion cross sections and the rate coefficients for a range of temperatures. The conversion rate at room temperature is compared with the value measured in H₂-O₂ gas mixtures.[1] [1] S. Wagner, Magn. Reson. Mater. Phys., Biol. Med. 27, 195 (2014). [2] E. Wigner, Z. Phys. Chem. B 23, 28 (1933). [3] B. F. Minaev and H. Ågren, J. Phys. Chem 99, 8936 (1995).

A molecule is chiral if its mirror image cannot be superimposed onto itself. Chirality serves an essential function in life. Our research program centres on understanding mechanisms of chirality recognition/transfer/amplification at the molecular level. To achieve this goal, we apply and develop new spectroscopic tools to characterize structural and dynamical properties of chiral molecules and non-covalent interactions among them in the gas phase, solution, cold rare gas matrices and at liquid-liquid interfaces. In particular, we emphasize the connection between the gas phase results obtained by using high resolution spectroscopy with those obtained in the condense phases. In the first example, we investigate tetrahydro-2-furoic acid, a chiral carboxylic acid, using broadband chirped pulse-Fourier transform microwave spectroscopy. Aided by a semiempirical quantum chemistry code for conformational search and DFT calculations, we examine its conformational landscape and chirality recognition in its binary adducts. The unusual conformational distributions and chirality controlled conformational preferences will be discussed. We further investigate the acid in cold rare gas matrices and as a liquid using vibrational circular dichroism to extract dominant species under these conditions. In the second example, I will discuss extremely strong chirality transfer from a chiral nickel complex to solvent molecules detected as Raman optical activity. We propose a chirality transfer/amplification mechanism based on quantum plasmons, i.e. plasmons in the few-atom limit. The interplay between experiment and theory is essential for all the work described.

Photo-induced charge separation in transition metal dichalcogenide heterobilayers is being explored for moiré excitons, spin-valley polarization, and quantum phases of excitons/electrons. While different momentum spaces may be critically involved in charge separation dynamics, little is known directly from experiments. Here we determine momentum-resolved electron dynamics in the WS₂/MoS₂ heterobilayer using time and angle resolved photoemission spectroscopy (TR-ARPES). Upon photoexcitation in the K valleys, we detect electrons in M/2, M, and Q valleys/points on time scales as short as  ∼ 70 fs, followed by dynamic equilibration in K and Q valleys in  ∼ 400 fs. The interlayer charge transfer is accompanied by momentum-specific band renormalization. These findings reveal the essential role of phonon scattering, the coexistence of direct and indirect interlayer excitons, and constraints on spin-valley polarization.

Biomolecules couple to their aqueous environment through a variety of noncovalent interactions. Local hydration structures at the surface of DNA and RNA are frequently determined by directed hydrogen bonds with water molecules, complemented by non-specific electrostatic and many-body interactions. I will present recent results from 2D-IR spectroscopy of sugar-phosphate backbone vibrations of native and artificial DNA and RNA, together with theoretical calculations of molecular couplings and molecular dynamics simulations. The results reveal the femtosecond fluctuation dynamics of the water shell, a short-range character of Coulomb interactions, and the strength and fluctuation amplitudes of interfacial electric fields [1,2]. Recent applications of phosphate vibrations [3,4] as probes for local hydration patterns and contact ion pair configurations hold strong potential for quantifying folding-induced changes of the ion distribution around DNA and RNA on a multitude of time scales.
References: 1. T. Siebert, B. Guchhait, Y. Liu, B. P. Fingerhut, T. Elsaesser, J. Phys. Chem. Lett. 7, 3131 (2016). 2. E. M. Bruening, J. Schauss, T. Siebert, B. P. Fingerhut, T. Elsaesser, J. Phys. Chem. Lett. 9, 583 (2018). 3. J. Schauss, F. Dahms, B. P. Fingerhut, T. Elsaesser, J. Phys. Chem. Lett. 10, 238 (2019). 4. A. Kundu, J. Schauss, B. P. Fingerhut, T. Elsaesser, J. Phys. Chem. B, DOI:10.1021/acs.jpcb.0c01474 (2020).