Unveiling the infrared and millimeter-wave spectra of elusive molecular ions with leak-out spectroscopy
Weslley G. D. P. Silva
University of Cologne, Germany
Molecular ions are key intermediates driving the chemistry of the interstellar medium through ion-molecule reactions. Yet, high-resolution laboratory data are still missing for many of these species, making them elusive to both astronomers and experimentalists. With the advent of leak-out spectroscopy (LOS) [1], high-resolution spectra of many bare molecular ions, long missing, can now be recorded with unprecedented signal-to-noise ratios.
In LOS, the internal energy of a vibrationally excited ion stored in a 22-pole cryogenic ion trap is partially converted into kinetic energy through inelastic collisions with a neutral atomic or molecular partner. This process allows the target ion to escape from the trap and be detected as a function of the excitation frequency.
In this talk, I will present both rovibrational and pure rotational spectra of astrophysically relevant cations, ranging from small hydrocarbons and cyanides to protonated forms of abundant complex organic molecules, such as protonated methanol. Because the energy of a pure rotational excitation is typically insufficient to induce the leak-out process, the ground-state rotational spectra of these ions are usually measured using millimeter-wave-infrared double-resonance spectroscopy [2]. Recently, however, we recorded a pure rotational LOS transition without the need of a double-resonance scheme: the N = 2⟵1 rotational spin manifold of the known OH⁺ ion, which was observed in our laboratory using the 2 THz source of the former upGREAT receiver (German Receiver for Astronomy at Terahertz Frequencies) from the SOFIA mission [3]. Altogether, our measurements provide the fundamental spectroscopic data needed to search for these elusive ions in space.
References:
[1] Schmid et al., J. Phys. Chem. A, 2022, 126, 8111.
[2] Asvany et al., Phys. Chem. Chem. Phys., 2023, 25, 19740.
[2] Silva et. al., Phys. Chem. Chem. Phys., 2026, 28, 12651.