Plasma resonance of the Wigner solid on the free surface of normal and superfluid 3He

The Wigner solid on liquid He-3 is used as a sensitive probe for the study of the surface dynamics. We have experimentally investigated the behavior of optical coupled plasmon-ripplon (CPR) resonances for temperatures down to 0.4 mK. We have used a linewidth of CPR resonance Delta f for getting tau = (2 pi Delta f)(-1),the collision time in the scattering of surface electrons from the surface roughness. The collision time is in good agreement with single-electron-ripplon scattering theory down to 30 mK, while below 30 mK the collision time deviates from the theory, and at 1 mK experimental value of tau is one order of magnitude longer than the theoretically predicted one.