High-resolution ro-vibrational and rotational spectroscopy of deuteronated formaldehyde, H₂COD

Among the deuterated forms of protonated formaldehyde, astrochemical models predict H₂COD⁺ as the most abundant isotopologue in prestellar cores. However, its observation is currently hampered by the lack of high-resolution molecular data. In this work, both the ro-vibrational and rotational spectra of H₂COD⁺ were investigated in a 4 K cryogenic ion trap instrument employing the leak-out spectroscopy method. Ro-vibrational transitions associated with the ν₁ fundamental band (antisymmetric C-H stretching) were detected in the region between 3095 and 3155 cm^-1 and assigned with the support of high-level quantum-chemical calculations. Preliminary analysis of the ro-vibrational lines enabled us to predict the rotational spectrum of the ground vibrational state with high accuracy. Subsequently, 36 pure rotational transitions were measured using a rotational-vibrational double-resonance scheme. The simultaneous analysis of both ro-vibrational and rotational data yielded highly accurate ground state spectroscopic parameters that can be used for future radio astronomical searches of H₂COD⁺ in the interstellar medium. Finally, these ground state rotational constants, together with those of three additional isotopologues, were employed to derive an almost complete semi-experimental equilibrium structure for protonated formaldehyde.