Control over the quantum states of atoms and molecules can lead to a fundamentally new understanding of how these particles interact and react. This knowledge has the potential to impact our ability to probe processes in planetary atmospheres and in the interstellar medium. Experimental techniques developed for control and measurement of atoms are now being used to study more and more complex molecules. We study these rich systems at low temperatures, where we can trap and examine their properties for many minutes, as compared to small fractions of a second in standard experiments. Using these cold, trapped molecular ions, we investigate mechanisms of ion-molecule reactions to gain insights into the mechanisms driving these processes.

The method of slow electron velocity-map imaging of cryogenically cooled anions (cryo-SEVI) is a variant of anion photoelectron spectroscopy with a resolution as high as 1-2 cm⁻¹. It has proved invaluable in obtaining well-resolved photoelectron spectra of molecular anions, bare and complexed metal oxide clusters, and anionic precursors to transition states for bimolecular and unimolecular reactions. However, cryo-SEVI is subject to the same selection rules that govern photoelectron spectroscopy, and thus yields frequencies for only a small set of vibrational modes of the neutral species created by photodetachment. We have recently incorporated a tunable infrared laser into our experiment with the goal of vibrationally pre-exciting the anions prior to photodetachment. Results on diatomic and polyatomic anions will be presented that illustrate the capabilities and complexities of this new type of spectrscopy, IR cryo-SEVI.

Vibrational spectroscopists profit enormously from the advances in quantum chemistry codes for spectral assignment and interpretation. This is certainly true for the study of weakly bound complexes, often generated in supersonic jet expansions with limited equilibration. It is time to pay back for this valuable support by generating dedicated new experimental data which can help theory to assess the quality of their approximate methods. This is complementary and in the end perhaps superior to important theory-internal benchmarking activities, which rely on a hierarchy of methods and may be limited in molecular complexity [1]. Experimental benchmarking only works if every effort has been made to secure the results. It comes in two variants, which can alternate with each other, to systematically approach the right answers for the right reasons. Blind challenges hide the experimental result and invite theory groups to predict it [2], followed by full disclosure and a discussion of the theory performance. Databases provide carefully curated compilations of spectroscopic observables [3], to save the interested theoreticians from tedious searches and misinterpretation. The talk will address a few examples from the Göttingen research training group BENCh [4], to convey the general idea. Close competition between conformational energies or for spectral properties are particularly valuable, because they invite binary decisions [5]. If you got a nice spectroscopy experiment running in your lab, choose and publish your systems and observable quantities in a way which is useful and challenging for the benchmarking of computational methods! If your experiment is reasonably unique but still well tested against others, consider to organize a blind challenge! If you are a theoretician, try to put your models to the (blind) test by experiment, although it seems so much easier to compare them to a reference theory! [1] R. A. Mata, M. A. Suhm: Angew. Chem. Int. Ed. 56 (2017) 11011–11018 [2] T. L. Fischer, M. Bödecker, A. Zehnacker-Rentien, R. A. Mata, M. A. Suhm: Phys. Chem. Chem. Phys. 24 (2022) 11442–11454 [3] Challenges for Numerical Quantum Chemistry: https://qmbench.net/ [4] Benchmark Experiments for Numerical Quantum Chemistry: https://uni-goettingen.de/en/587836.html [5] R. Medel, J. R. Springborn, D. L. Crittenden, M. A. Suhm: Molecules 27 (2022) 101

Non-covalent interactions have been proposed to play a large role in organic and organometallic chemistry, with the magnitude of the interactions increasing rapidly with the number of atoms. Detailed experimental tests of the proposition in the gas phase focus on small molecules, for a variety of technical reasons. For small molecules, though, the magnitude of the interactions is small, which introduces its own suite of technical constraints. We report four different experimental systems, examined with a range of gas-phase, physical techniques ranging from energy-resolved collision-induced dissociation cross-sections to ion spectroscopy by cryogenic ion vibrational predissociation spectroscopy or IRMPD, of electrosprayed molecular ions, in which the non-covalent interactions become large enough that the experiments can provide hard benchmarks against which theory can be tested. We find that the present generation of dispersion-corrected DFT methods appear to overestimate the attractive component of the interaction, at least for certain interaction geometries.