Eline Plaar

Cyanooxomethylium, NCCO$^+$, a fundamental linear acylium ion, has been observed spectroscopically for the first time using action spectroscopy in ion trap apparatuses. A first low-resolution infrared spectrum was obtained between 500 to 1400 cm$^{-1}$ and 2000 to 2500 cm$^{-1}$ using the Free Electron Laser for Infrared eXperiments (FELIX) and the FELion apparatus, employing infrared predissociation of the weakly bound NCCO$^+$-Ne complex. Subsequently, high-resolution studies of the bare ion were performed with the COLtrap II setup, one targeted at the CN-stretching mode $ν_2$ around 2150 cm$^{-1}$ using leak-out spectroscopy and one at the pure rotational spectrum employing a leak-out infrared/millimeter-wave double resonance approach covering transition frequencies as high as 246 GHz. Spectroscopic detection and analysis were guided by high-level quantum-chemical calculations performed at the CCSD(T) level of theory. The collected data permit accurate frequency predictions to support future astronomical searches with sensitive radio telescopes.

The rovibrational spectrum of the molecular ion H$_2$CCCH$^+$ was investigated in a 4~K cryogenic ion trap instrument employing the leak-out spectroscopy method. Transitions within the fundamental $ν_1$ (C-H stretch) and the combination band $ν_3$+$ν_5$ (C-C stretches) were detected, the search aided by high level quantum chemical calculations. The analysis of the rovibrational measurements enabled us to predict the rotational structure of the ground state. Using a rotational-vibrational double-resonance scheme, 14 pure rotational transitions were measured. This, in turn, led to the radio astronomical detection of H$_2$CCCH$^+$ in the interstellar medium, as recently reported (Silva et al., Astron. Astrophys. 676, L1, 2023).

The antisymmetric C-H stretching vibration $ν_3$ ($^2Π$ $\leftarrow$ $^2Π$) of ionized acetylene, C$_2$H$_2^+$, has been revisited using a cryogenic 22-pole ion trap machine. Two action spectroscopic techniques, the novel leak-out spectroscopy (LOS) method and the more established laser-induced reactions (LIR) method, are applied and compared. Mass selectivity and cryogenic temperatures down to 4~K enabled the observation of uncontaminated spectra in which the $Λ$-doubling components of this open-shell molecule are mostly well resolved, leading to a slight refinement of the spectroscopic parameters.