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The absorption of 40–200GHz electromagnetic waves in liquid helium at temperatures 1.4–2.8K is investigated experimentally. The spectrum of oscillations of the “whispering gallery” modes of a dielectric disk-shaped resonator immersed in liquid helium is recorded. Superfluid flows in HeII is produced artificially using two “heat guns.” A narrow microwave absorption line has been detected at a frequency corresponding to the excitation energy of a single roton. It has been determined that the character of the resonance line changes radically as the velocity vs of the superfluid flow increases: microwave absorption was replaced by induced microwave emission, which was seen as a sharp intensification of the high frequency signal. The effect can be explained qualitatively on the basis of a two-level model of HeII. An abrupt change in the velocity of the superfluid flow is observed with continual increase of the power delivered to the heat gun. This could attest to quantization of vs. It is shown that the superfluid flow has a low-frequency effect on a microwave which outwardly is manifested as modulation of microwave oscillations. Possible reasons for this effect are analyzed.

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The amplitude-frequency characteristic of the spectral line of electromagnetic absorption in liquid helium is measured in the frequency range 40–200GHz at temperatures in the interval 1.4–2.75K. It is found that in the roton frequency region a narrow resonance absorption line on a broad pedestal is observed. The results are compared with data on the roton spectrum found in neutron scattering experiments in liquid helium. The narrow line is due to the creation of a single roton. It is shown that the momentum conservation law is satisfied on account of the transfer of momentum to the superfluid component. The analogy of this effect with the Mössbauer effect is pointed out.

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The stability and oscillatory properties of superfluid ring flow arising around the cylindrical surface of a disk-shaped dielectric resonator immersed in liquid helium are studied experimentally. The velocity of superfluid flow is controlled with special heat guns, placed in He II and generating counterflows of the normal and superfluid components, directed along the tangent to the cylindrical surface of the resonator. In the experiment the amplitude of the microwave signal passing through the resonator is measured and the effect of the phase and dynamic states of the liquid on the signal amplitude is studied. It is found that periodic oscillations of the signal are observed in the He II state, and each period starts with a sharp spike of the amplitude. It is proposed that this behavior signifies instability of superfluid ring flow due to a change in the number of circulation quanta of the superfluid velocity over a very short time. Another effect is due to the appearance of new periodically repeating resonance peaks, together with the ordinary whispering gallery modes, in the system. The effect is absent above λ point and is attributed to characteristic oscillatory modes of the undamped superfluid flow. Possible reasons for the observed features are analyzed.