Little is known about how rotating molecular ions interact with multiple ${}^4\mathrm{He}$ atoms and how this relates to microscopic superfluidity. Here, we use infrared spectroscopy to investigate ${{}^4\mathrm{He}}_N\cdots {\mathrm{H}}_3{\mathrm{O}}^{+}$ complexes and find that ${\mathrm{H}}_3{\mathrm{O}}^{+}$ undergoes dramatic changes in rotational behavior as ${}^4\mathrm{He}$ 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.

• Received 21 July 2022
• Revised 5 December 2022
• Accepted 18 January 2023

DOI:https://doi.org/10.1103/PhysRevLett.130.083001