Rotational Structure of molecules in Quantum 4He and 3He Liquids

It is shown that the drastic distinction in the rotational structure of molecules in He-4 and He-3 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 He-3 is determined by particle-hole excitations, whereas the small density of phonon excitations results only in the small broadening of levels in He-4.