Mixed-valence correlations in charge-transferring atom-surface collisions

Motivated by experimental evidence for a mixed-valence state to occur in the neutralization of strontium ions on gold surfaces we analyze this type of charge-transferring atom-surface collision from a many-body theoretical point of view using quantum-kinetic equations together with a pseudo-particle representation for the electronic configurations of the atomic projectile. Particular attention is paid to the temperature dependence of the neutralization probability which--experimentally--seems to signal mixed-valence-type correlations affecting the charge-transfer between the gold surface and the strontium projectile. We also investigate the neutralization of magnesium ions on a gold surface which shows no evidence for a mixed-valence state. Whereas for magnesium excellent agreement between theory and experiment could be obtained, for strontium we could not reproduce the experimental data. Our results indicate mixed-valence correlations to be in principle present, but for the model mimicking most closely the experimental situation they are not strong enough to affect the neutralization process quantitatively.