High-resolution spectroscopy of the $\nu_3$ antisymmetric C-H stretch of C$_2$H$_2^+$ using leak-out action spectroscopy

The antisymmetric C-H stretching vibration $\nu_3$ ($^2\Pi$ $\leftarrow$ $^2\Pi$) 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 $\Lambda$-doubling components of this open-shell molecule are mostly well resolved, leading to a slight refinement of the spectroscopic parameters.