Direct Evidence of Electron Tunneling in the Ionization of Sputtered Atoms

Direct evidence has been obtained for electron tunneling between sputtered ${\mathrm{Cs}}^{+}$ ions and the solid surfaces from which they originate. Strong neutralization of the ${\mathrm{Cs}}^{+}$ ions is observed whenever the tunneling channel is opened by changing the surface work function so that a crossing of the Fermi level by the Cs $6s$ level occurs. A tunneling model which takes the spatial dependences of the position and the width of the atomic level into account explains the data successfully.