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 ${\nu }_3$ (${}^2\mathrm{\Pi }$ $\leftarrow$ ${}^2\mathrm{\Pi }$) 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 $\mathrm{\Lambda }$-doubling components of this open-shell molecule are mostly well resolved, leading to a slight refinement of the spectroscopic parameters.