Isolation and spectroscopy of C₂H⁺ ions in helium droplets

Ethynylium (C2H+) may be an important intermediate in astrochemistrybut remains poorly studied by laboratory techniques due to its high reactivity. Immersion of single acetylene molecules in helium nanodroplets followed by electron impact ionization effectively generates and isolates C2H+ cations in an inert cryogenic environment, enabling infrared spectroscopic measurements. We find that in the C-H stretching range, the C2H+ spectrum appears as a triplet with splitting of 33.6 cm-1 and 37.5 cm-1. The splitting is temporarily assigned to the product of interactions between the C-H stretching mode and overtones of the bending mode.