We
investigate gas-phase reactions of free Ag
n
Ce+ and Ag
n
Sm+ clusters
with oxygen molecules to explore s–d,
s–f,
and d–f electron interactions in the finite size regime; a
Ce atom has a 5d electron as well as a 4f electron, whereas a Sm atom
has six 4f electrons without 5d electrons. In the reaction of Ag
n
Ce+ (n = 3–20),
the Ce atom located on the cluster surface provides an active site
except for n = 15 and 16, as inferred from the composition
of the reaction products with oxygen bound to the Ce atom as well
as from their relatively high reactivity. The extremely low reactivity
for n = 15 and 16 is due to encapsulation of the
Ce atom by Ag atoms. The minimum reactivity observed at n = 16 suggests that a closed electronic shell with 18 valence electrons
is formed with a delocalized Ce 5d electron, while the localized Ce
4f electron does not contribute to the shell closure. As for Ag
n
Sm+ (n = 1–18),
encapsulation of the Sm atom was observed for n ≥
15. The lower reactivity at n = 17 than at n = 16 and 18 implies that an 18-valence-electron shell
closure is formed with s electrons from Ag and Sm atoms; Sm 4f electrons
are not involved in the shell closure as in the case of Ag
n
Ce+. The present results suggest that
the 4f electrons tend to localize on the lanthanoid atom, whereas
the 5d electron delocalizes to contribute to the electron shell closure.
Small copper-oxide cluster cations, namely, CuOAr+ and Cu2O2+, are studied by mass spectrometry and X-ray absorption spectroscopy (XAS) to investigate the oxidation state of copper atoms. The XAS in Cu L3-region revealed that the oxidation number of Cu is almost +2.1 and +2.3 for CuOAr+ and Cu2O2+, respectively, which are consistent with the natural charges obtained by quantum chemical calculation. It was also revealed that the binding energy of Ar to CuO+ is unexpectedly high to form CuOAr+ dominantly as a result of a significant amount of charge transfer to the Ar atom.
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