A new effect of electromagnetic radiation generated by heat flow in superfluid helium has been detected experimentally. The generating heat flow was produced using hydrodynamic thermal guns. Electromagnetic radiation was registered with a dielectric disk resonator in the interval 1.4-2.17K. The power of the thermal gun heater varied within Q up to 1mW. A distinct signal of electromagnetic radiation was registered when the thermal gun was on and there was no signal at Q=0. The frequency of the generated electromagnetic radiation was measured with a Fabry-Perrot interferometer. It was about 180GHz at T=1.4K and decreased with a rising temperature practically following the temperature dependence of a roton gap. The detected signal was investigated as a function of the heat flow temperature, direction and power Q. The results obtained show that He II behaves as a two - level system in which the difference between the levels is equal to the roton energy.
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