It is shown that well-known chemical effects that result in a large enhancement of the positive secondary ion emission from metal surfaces coated with electronegative adsorbates can be explained in terms of the electron-tunneling model. The presence of adsorbed particles on a surface induces an electrostatic potential that causes a shift in the electronic level of the emitted atom, in addition to the shift caused by the conventional imagelike potential. This additional shift results in an increase in the effective neutralization distance and a decrease in the electron tunneling rate. As a consequence there is a drastic decrease in the neutralization probability of secondary ions.

• Received 4 December 1992

DOI:https://doi.org/10.1103/PhysRevB.47.4139