Onset of Rotational Decoupling for a Molecular Ion Solvated in Helium: From Tags to Rings and Shells

Little is known about how rotating molecular ions interact with multiple He4 atoms and how this relates to microscopic superfluidity. Here, we use infrared spectroscopy to investigate He4N H3O+ complexes and find that H3O+ undergoes dramatic changes in rotational behavior as He4 atoms are added. We present evidence of clear rotational decoupling of the ion core from the surrounding helium for N>3, with sudden changes in rotational constants at N=6 and 12. In sharp contrast to studies on small neutral molecules microsolvated in helium, accompanying path integral simulations show that an incipient superfluid effect is not needed to account for these findings.