Modern tools for controlling molecular collisions in the gas-phase

Microscopic understanding of molecular reactions is inherently complex because of the many possible quantum paths due to the high number of initial and final quantum states of reactants and products. My work focuses on developing tools that enable following reactions with a single quantum state resolution in the gas phase. I will present three developments bringing us closer to this “holy grail” of quantum chemistry.

Many molecules have multiple conformations (rotational isomers), which can exhibit different reactivities, opening up possibility to manipulate chemical reactions by selecting specific molecular conformations. During my PhD I developed an instrument to separate molecular conformations in a molecular beam and study their reactivity with cold ions. I will show results on the collision of conformationally selected 3-aminophenol with Coulomb crystals of laser-cooled Ca⁺ ions.

In the second part of the talk, I will present a new double velocity map imaging photoion photoelectron coincidence spectrometer for studying gas-phase reaction kinetics of transient species with high mass, energy and time resolution. This method gives us the ability to measure the kinetics of all reactants and products and has already led to the discovery of previously unknown products in photo-induced processes pertinent to atmospheric chemistry.

In the third section I will talk about ongoing work towards laser cooling of NH molecules. This will enable reactive collisions with fine control over the collision energy as well as the initial quantum states of the reactants. I will present results of hyperfine state resolved laser-induced fluorescence experiments on the cooling transition A³Π₀ ← X³Σ^- for ¹⁵NH as well as high resolution THz spectroscopy probing the X³Σ^-, N=1 ← X³Σ^-, N=0 rotational transition in the ground state. I will present our simulation results demonstrating how the narrow cooling transition can be used to efficiently laser cool the NH molecule.

References

[1] Chang, Y.-P.; Długołeçki, K.; Küpper, J.; Rösch, D.; Wild, D.; Willitsch, S. Specific Chemical Reactivities of Spatially Separated 3-Aminophenol Conformers with Cold Ca⁺ Ions. Science 2013, 342, 98-101.

[3] Rösch, D.; Almeida, R.; Sztáray, B.; Osborn, D. L. High-Resolution Double Velocity Map Imaging Photoelectron Photoion Coincidence Spectrometer for Gas-Phase Reaction Kinetics. J. Phys. Chem. A 2022, 126, 1761—1774

[5] Rösch, D.; Woo, K C.; Echternach, J. A.; Sztáray, B.; Bodi, A.; Osborn D. L. High-Resolution Time-Resolved PEPICO with Tunable Vacuum Ultraviolet Photoionization. J. Phys. Chem. A. 2026, 130, 1948-1964