High-resolution spectroscopy of the ?3 antisymmetric C-H stretch of C2H2+ using leak-out action spectroscopy

The antisymmetric C-H stretching vibration v(3) ((2)? ? (2)?) of ionised acetylene, C2H2+, 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 ?-doubling components of this open-shell molecule are mostly well resolved, leading to a slight refinement of the spectroscopic parameters.