Transverse Optical Resonance of Wigner Solid as a Sensitive Probe of Liquid Helium Surface

The formation of the Wigner Solid in the sheet of Surface Electrons (SE) on liquid helium leads to change in the plasmon dispersion relation of the system. As a result the coupled phonon-ripplon (CPR) modes appear in the spectrum of SE. The transverse optical (TO) CPR mode is scarcely dispersive, and consequently convenient for precise determination of the resonance frequency. In our experiments we have shown that the frequency of TO CPR resonance can be used to obtain the absolute value of liquid helium surface tension. The measurements have been gained for pure 3 He and for weak solution 3 He (~0.5 ppm) in 4 He. The modification of weak 4 He- 3 He mixture surface tension in respect of pure 4 He data was observed. This behaviour can be explained by the filling of the Andreevʹʹs levels with 3 He atoms that results in the formation of the 2D Fermi-liquid on the surface of 4 He. The TO CPR resonance of SE can be proposed as a sensitive and convenient method for study of surface properties of liquid 4 He, 3 He and their mixtures.