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).