Electronic surface states of liquid helium

Electrons at the surface of liquid He4 are potentially valuable probes of both static and dynamic properties of the interface. Two species of surface state are discussed. One of these is localized just above the interface, weakly bound by the image force. The existence of this state has been confirmed by direct spectroscopic observation. Measurements of parallel field mobility and life-time on the surface are discussed and compared with predictions. The other surface state is an electron bubble held below the interface with an applied field. A resonance experiment in this geometry determines an effective mass different from the bulk liquid value. Studies of field emission from the bubbles lead to an evaluation of other bubble properties.