Electron counting is a concept that often governs properties of molecules, clusters, and complexes. Here we explore silver clusters doped with a transition-metal atom, where it has been an issue...
We
report reactions of gas-phase free silver cluster cations, Ag
n
+ (n = 3–18),
with nitric oxide molecules, which was studied by kinetics measurements
using an ion trap. Ag
n
O(NO2)
m−1
+ and Ag
n
(NO2)
m
+ were observed as major products after multiple reactions.
The reaction pathway to form these product ions was identified by
fitting the data to rate equations for n ≤
15, except for inert n = 3 and 5. Two different reaction
mechanisms were found for the formation of these products depending
on cluster size; pseudo-first-order rate constants of each step of
elementary reactions were obtained. First, as found for n = 4, 6, and 9, Ag
n
O+ is formed
by a reaction with two NO molecules, which is followed by a release
of neutral N2O. A further reaction of Ag
n
O+ with another NO molecule produces Ag
n
NO2
+. Ag
n
(NO2)
m
+ (m ≥ 1) is thus successively formed via an intermediate,
Ag
n
O(NO2)
m−1
+. This is analogous to the reaction of
NO on silver surfaces to produce NO2. Second, both Ag
n
NO2
+ and Ag
n
O+ are formed concurrently, as found for n = 7, 8, 10, 11, 12, and 15; Ag
n
O+ does not act as an intermediate for Ag
n
NO2
+. Ag
n
O(NO2)
m−1
+ and Ag
n
(NO2)
m
+ (m ≥ 2) are
formed by successive addition of NO2 to Ag
n
O+ and Ag
n
NO2
+, respectively. It is speculated that the
successive addition of NO2 proceeds via disproportionation,
i.e., three NO molecules are converted to NO2 and N2O. The reaction pathways of n = 13 and 14
are explained equally well by the two mechanisms. The overall reaction
rate coefficients exhibit an odd–even alternation; the higher
reactivity for even values of n is due to an odd
number of valence electrons.
We present a novel high-repetition-rate photoelectron imaging (PEI) apparatus for exploring electronic structures of metal cluster anions. A continuous beam of mass-selected metal cluster anions, generated by a magnetron-sputtering cluster-ion source coupled with a quadrupole mass filter, is chopped into sub-megahertz ion bunches using a high-voltage pulser. The quasi-continuous anion beam is introduced into a PEI spectrometer, where the anions are photodetached using a 404 nm (3.07 eV) continuous-wave laser diode. As a demonstration, we acquire photoelectron images for size-selected Ag cluster anions, Ag N− ( N = 3, 7, 14), and show that each image can be obtained in a short accumulation time (50 s) with a kinetic energy resolution (Δ E/ E) of 4% at E = 1.77 eV. The quasi-continuous PEI technique enables high-count-rate, space-charge-free acquisition of photoelectron spectra and angular distributions not only from size-selected metal cluster anions but also from anions prepared by other continuous ion sources, such as electrospray ionization.
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.
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