Neutralization phenomena observed with secondary ions originating from inner‐shell excitation

Abstract

The observation of comparatively high yields of low-energy secondary ions, ejected from clean ion-bombarded solids, has been a long-standing miracle. The measured velocity dependence of the ionization probability is at variance with the predicted exponential velocity dependence. It has been proposed that the recorded high yields are due to electronic excitations initiated by the collision cascade. Here, the issue is addressed by exploring energy spectra of ions generated by inner shell electron promotion during energetic collisions. Using new experimental results and literature data, ionization probabilities were determined using two different approaches. The evaluated data suggest that doubly charged ions of Mg, Al and Si (and possibly, Ca) can escape neutralization, presumably as a consequence of Auger deexcitation taking place at sufficiently large distances from the surface. The ion fractions for Sc, Ti, V and Cr are in accordance with an exponential velocity dependence at energies >300 eV, but approach a constant level at low energies. Given the fact that the electronic-excitation model does not apply in the case of ionization by electron promotion, it is suggested that the unusually high ionization probabilities observed at low energies are due to suppressed neutralization. The idea is that in a small fraction of impact events the collision cascade may generate a very high perturbation of the surface ('dynamic randomization') so that a well-defined band structure may not exist, locally and for a short period of time. In these rare cases, the conduction electrons required for neutralization are not available.