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

Photocurrents from cesium, flowing through gaseous He or He 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 He grows on the Cs and forms a tunnel barrier; but for He, the film is too thin to form a tunnel barrier below liquid-vapor coexistence. This is because He 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. He 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 He has a potential energy of 1.0 eV. We find that the thickness d of a helium film is given by C3 /d=−kBT ln p /p0 for films thicker than approximately three monolayers.