Superfluid Hydrodynamic Model for the Enhanced Moments of Inertia of Molecules in Liquid ${}^4\mathrm{He}$

We present a superfluid hydrodynamic model for the increase in moment of inertia, $\Delta I$, of molecules rotating in liquid ${}^4\mathrm{He}$. The static inhomogeneous He density around each molecule (calculated using the Orsay-Paris liquid ${}^4\mathrm{He}$ density functional) is assumed to adiabatically follow the rotation of the molecule. We find that the $\Delta I$ values created by the viscousless and irrotational flow are in good agreement with the observed increases for several molecules [OCS, $(\mathrm{HCN}{)}_2$, HCCCN, and $\mathrm{HCCCH}{}_3$]. For HCN and HCCH, our model substantially overestimates $\Delta I$. This is likely to result from a (partial) breakdown of the adiabatic following approximation.