Charges near the free surface of liquid helium: collective effects

Two types of two-dimensional (2D) charged systems can appear near the free surface of liquid helium: surface electrons and surface ions [1]. The existence of these two systems has been predicted almost simultaneously (see Ref. [2] ). However, the electron variant of the problem has subsequently been investigated more thoroughly both theoretically and experimentally. The interest in 2D ion systems has increased significantly only recently because of certain specific features of their collective behaviour. This short review is intended to serve as a brief introduction to the current status of the problem of the 2D ion systems near the free surface of liquid helium. In addition, some qualitatively new trends in the study of the properties of 2D electron systems above helium are discussed.