Photocurrents from cesium, flowing through gaseous 3 He or 4 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 3 He grows on the Cs and forms a tunnel barrier; but for 4 He, the film is too thin to form a tunnel barrier below liquid-vapor coexistence. This is because 4 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. 3 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 3 He has a potential energy of 1.0 eV. We find that the thickness d of a helium film is given by C3 /d 3 =� kBT lnp /p0 for films thicker than approximately three monolayers.