Millimeter waveband spectroscopy of liquid He II

Low temperature experiments [1–4] 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, e = Δ/ħ. For Δ =8.65 K, which corresponds to a temperature of the order of 1.4 K, f = 2πω ≈ 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 [5, 6]. Since at this frequency the photon momentum p pt = 3.8 × 10 3 cm −1 is many orders of magnitude smaller than the roton momentum p r = 1.9 × 10 8 cm −1 , the question of how the momentum conservation law can be obeyed in a such process must be addressed.