High Resolution Rovibrational and Rotational Spectroscopy of H${}_2$CCCH${}^{+}$

The rovibrational spectrum of the molecular ion H${}_2$CCCH${}^{+}$ was investigated in a 4~K cryogenic ion trap instrument employing the leak-out spectroscopy method. Transitions within the fundamental ${\nu }_1$ (C-H stretch) and the combination band ${\nu }_3$+${\nu }_5$ (C-C stretches) were detected, the search aided by high level quantum chemical calculations. The analysis of the rovibrational measurements enabled us to predict the rotational structure of the ground state. Using a rotational-vibrational double-resonance scheme, 14 pure rotational transitions were measured. This, in turn, led to the radio astronomical detection of H${}_2$CCCH${}^{+}$ in the interstellar medium, as recently reported (Silva et al., Astron. Astrophys. 676, L1, 2023).