Microwave Spectroscopy of Condensed Helium at the Roton Frequency

A spectral absorption line of electromagnetic radiation in the frequency range 40-200 GHz and at temperatures 1.4-2.75 K is measured in liquid helium. It is found that the narrow line of resonance absorption near the roton frequency does exist against a wide pedestal. The results obtained are compared with the data on the roton spectrum obtained in neutron scattering experiments. The possible reason for narrow absorption peak appearance is analyzed and the analogy between the observed phenomenon and Mössbauer effect is considered.