Vibrational spectroscopy of H2He+ and D2He+

Abstract Vibrational modes of the relatively strongly bound H2He+ molecular ion and its deuterated congener D2He+ are investigated by low-resolution multi-photon photodissociation spectroscopy, using a combination of a 4 K cryogenic ion-trap machine and the free-electron-laser FELIX. The band origins obtained are fully explained by accurate variational calculations of the rovibrational states of H2He+ and D2He+ based on the three-dimensional potential energy surface of Koner et al. [Phys. Chem. Chem. Phys. 21 (2019) 24976]. Results from second-order vibrational perturbation theory, based on a linear H–H–He equilibrium structure, agree well with those of the variational calculations for energies up to about 1300 cm-1. This suggests that H2He+ and D2He+ may either be considered as linear triatomic molecules with a degenerate bending mode, or as Van der Waals complexes with a strongly hindered rotation of He around the H 2 + and D 2 + subunits. The variational calculations show that in states close to the dissociation limit, 1794 and 1852 cm-1 for para- and ortho-H2He+, respectively, the angular internal motion becomes delocalized. The low-resolution experiments corroborate the linear structure of the ions and identify the bright IR-active HH-stretch fundamental in H2He+ at about 1840 cm-1 and the DD-stretch fundamental in D2He+ at about 1309 cm-1, both with an uncertainty of 0.5%, in good agreement with the calculations. The experiments also confirm the H 2 + –He bend and stretch fundamentals calculated at 632 and 732 cm-1 and the D 2 + –He bend and stretch fundamentals at 473 and 641 cm-1, respectively.