Work function dependent neutralization of low-energy noble gas ions

The work function dependence of the neutralization of low-energy He(+), Ne(+), and Ar(+) ions was studied by determining the neutralization probability of ions scattered from submonolayer coverages of Ba on W(110) and Re(0001) substrates. At high work functions (>3.5 eV), it was found that the dominant neutralization mechanism for noble gas ions with initial energy between 2 and 5 keV scattering from Ba is collision-induced neutralization. The neutralization probability for this mechanism was found to be insensitive to work function changes. We argue that collision-induced neutralization is also the dominant charge transfer process for scattering from other earth-alkali and alkali elements in this energy range, although at lower energies it is expected that Auger neutralization will become important. At work functions below roughly 3.5 eV, resonant neutralization to the first excited level of the noble gas ions occurs in addition to the charge transfer processes operating at high work functions. We show that the additional neutralization at low work functions can be described using resonant charge exchange theory. Due to resonant neutralization, the neutralization probability for noble gas ions increases exponentially with decreasing work function.