The influence of the stationary electric field upon the resonance   absorption of microwaves in HeII
  The interaction between electromagnetic microwaves (40-200 GHz) and superfluid helium in a stationary electric field has been investigated experimentally. It is found that the narrow line of resonance absorption at the roton frequency is split in the electric field into two symmetric lines. The splitting magnitude increases almost linearly with the electric field, which suggests a linear Stark effect. The results obtained point of orientational polarizability and dipole moment (10^(-34)C*m) in HeII. It is shown that the spectral line profile consists of two parts - a narrow line of resonance absorption (or induced radiation when superfluid stream are generated) and a broad background. The background with agrees well with the latest neutron data for the roton line. 
A.S. Rybalko, S.P. Rubets, E.Ya. Rudavskii, V.A. Tikhiy, R.   Golovachenko, V.N. Derkach, S.I. Tarapov