Rotation of NH3+ and H3O+ ions in superfluid helium droplets

Infrared spectra of small molecules embedded in helium nanodroplets exhibit well-resolved rotational structure owing to the superfluid nature of the droplets and the weak interactions between the dopant molecules and surrounding helium atoms. In this work, we investigated the ν3 stretching bands of the light cationic rotors NH3+ and H3O+ in helium nanodroplets. The ν3 band of NH3+ displays a well-resolved rotational structure, whereas that of H3O+ is only partially resolved. The rotational constants associated with rotation about the C3 symmetry axis remain close to their gas-phase values. In contrast, the rotational constants corresponding to tumbling motion are reduced in helium to ∼60% of their free-molecule values. A substantially larger reduction was reported previously for the CH3+ ion. This difference is attributed to differences in the electronic structures of the ions, which govern their interactions with helium atoms. In addition, the NH3+ spectrum exhibits anomalies in both line positions and intensities, suggesting that the rotational dynamics of ions in superfluid helium may not be adequately described by the conventional free-molecule rotational Hamiltonian.