High resolution rovibrational and rotational spectroscopy of H₂CCCH⁺

The rovibrational spectrum of the molecular ion H2CCCH+ was investigated in a 4 K cryogenic ion trap instrument employing the leak-out spectroscopy method. Transitions within the fundamental ν1 (C–H stretch) and the combination band ν3+ν5 (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 H2CCCH+ in the interstellar medium, as recently reported (Silva et al., Astron. Astrophys. 676, L1, 2023).