High-Resolution Ro-Vibrational and Rotational Spectroscopy of the Open-Shell, Linear CCH⁺ Ion (³Π)

In this work, we report on the high-resolution infrared spectrum of CCH⁺ (³Π) recorded in the range 3066-3184 cm^-1 by means of leak-out spectroscopy. This spectral range covers the fundamental of the CH stretching mode and a highly excited bending vibrational mode. Based on this data (385 ro-vibrational lines), accurate spectroscopic descriptions of the ground and the two vibrationally excited states of CCH⁺ were obtained. Besides the band origins, spin-orbit coupling constants, rotational constants, centrifugal distortion constants, and Λ-doubling constants for the ground and excited vibrational states have been derived. This effective Hamiltonian analysis allowed a search for pure rotational lines of CCH⁺ in its electronic and vibrational ground state using a two-color millimeterwave-infrared scheme. We observed all rotational transitions from J″ = 2 up to J″ = 6 within the Ω = 2 lowest-energy fine-structure component with resolved hyperfine splittings. This data has already guided the first detection of CCH⁺ in space toward the Orion Bar photodissociation region and has the potential to support further astronomical searches for CCH⁺ either through radio or infrared spectroscopy, for example, with the James Webb Space Telescope.