Electron tunnelling through a quantifiable barrier of variable width

This is the first study of electron tunnelling through a quantifiable barrier of adjustable width. We find quantitative agreement between the measured and calculated tunnelling probability with no adjustable constants. The tunnel barrier is a thin film of 3 He on Cs 1 which it wets. We excite photoelectrons which have to tunnel through the barrier to escape. The image potential must be included in calculating the barrier and hence the tunnelling current. This has been a debatable point until now. We confirm that an electron has a potential of 1.0 eV in liquid 3He for short times before a bubble forms. We show that the thickness of the 3 He is given by thermodynamics for films of thickness at least down to 3 monolayers.