Electron tunneling through barriers of adjustable width : Role of the image potential and the wetting behavior of Cs by He

Abstract

Photocurrents from cesium, flowing through gaseous 3He or 4He and also through thin liquid helium films, are investigated as a function of the chemical potential of helium at T=1.33 K. At low pressures, the two isotopes behave similarly as the photocurrent is governed by scattering by the gas. At higher pressures, a film of 3He grows on the Cs and forms a tunnel barrier; but for 4He, the film is too thin to form a tunnel barrier below liquid-vapor coexistence. This is because 4He does not wet Cs at this temperature and the finite thickness needed to form a tunnel barrier is larger than the thickness of the thin-film state. 3He enables a continuously variable tunnel barrier thickness to be studied. We show that the image potential is important and confirm that an electron in liquid 3He has a potential energy of 1.0 eV. We find that the thickness d of a hlium film is given by for films thicker than approximately three monolayers.