Rotational Structure of Molecules in QuantumH4eandH3eLiquids

It is shown that the drastic distinction in the rotational structure of molecules in ${}^4\mathrm{He}$ and ${}^3\mathrm{He}$ liquids, observed by Grebenev, Toennies, and Vilesov [Science 279, 2083 (1998)], is due to the difference in the spectral density of excitations regardless of the hydrodynamic properties. The large width of rotational levels in ${}^3\mathrm{He}$ is determined by particle-hole excitations, whereas the small density of phonon excitations results only in the small broadening of levels in ${}^4\mathrm{He}$.