Bonn 2025 – scientific programme

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MO: Fachverband Molekülphysik

MO 30: Molecular Spectroscopy and Theoretical Approaches

MO 30.2: Talk

Friday, March 14, 2025, 14:45–15:00, HS XVI

Experimental symmetry assignments of protonated methane rovibrational levels — •Samuel Marlton, Philipp Schmid, Oskar Asvany, and Stephan Schlemmer — University of Cologne, Cologne, Germany

Protonated methane (CH₅⁺) does not consist of one proton and four hydrogen atoms but all five protons are equivalent, all entertaining bonds with the central carbon atom. This makes the molecule floppy with an irregular rovibrational spectrum, which remains unassigned. [1,2] The nuclear spins of the five protons combine to a total nuclear spin of I=1/2, 3/2 or 5/2, which—due to the Pauli exclusion principle—combine with ro-vibrational states of corresponding symmetry A₂=I=5/2, G₂=I=3/2, and H₂=I=1/2 with abundance ratios of 6:4:2. Using leak-out spectroscopy (LOS),[3] we provide direct experimental symmetry assignments by measuring these abundance ratios. In our experiment, CH₅⁺ ions stored in a cryogenic ion trap are irradiated with an infrared laser. Photoexcited ions collide with neutral gas atoms (Ne) to eventually transfer vibrational energy into kinetic energy, giving the ion sufficient kinetic energy to leak out of the trap. We assign symmetry labels to transitions by measuring the fraction (2/12, 4/12, or 6/12) of CH₅⁺ of ions that leak-out of the trap when each transition is resonantly excited. This approach will help construct a ground state term diagram. [1] White et al., 1999, Science, [2] Asvany et al., 2015, Science, [3] Schmid et al., 2022, J. Phys. Chem. A.

Keywords: ions; rovibration; leak-out spectroscopy; symmetry; tunneling