Millimeter waveband spectroscopy of liquid He II

Low temperature experiments on the interaction of electromagnetic field with liquid helium have produced a number of interesting and unexpected results that have yet to be explained in a conventional manner. One of such effects is the resonance absorption and radiation of electromagnetic waves in superfluid helium at a frequency f corresponding to the roton gap of the energy spectrum, ε = Δ/ħ. For Δ =8.65 K, which corresponds to a temperature of the order of 1.4 K, f = 2πω̃ ≈ 180.3 GHz. The temperature dependence of this absorption near the temperature of transition to superfluid state coincides precisely with temperature dependence of the roton gap obtained in neutron scattering experiments. Since at this frequency the photon momentum p_pt = 3.8 × 10³ cm^-1 is many orders of magnitude smaller than the roton momentum p_r = 1.9 × 10⁸ cm^-1, the question of how the momentum conservation law can be obeyed in a such process must be addressed.