Chemistry Reference
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Fig. 14 IR-MPD spectra
for oxygen complexes of
neutral gold clusters. The
observed absorptions
correspond to superoxo
moieties (O
2
; ~1050 cm
1
)
and physisorbed O
2
(O
2
0
; ~1550 cm
1
).
Reprinted with permission
from Woodham
et al. [
12
]. Copyright 2013
American Chemical Society
mentioned earlier, the reactivity for the anions is dominated by the even-sized
clusters whereas mass peaks corresponding to both even and odd-sized cluster
complexes are observed in the neutral mass spectrum. The ionisation energies (IEs)
of gold clusters tend to be high, and therefore the 7.9 eV photons do not efficiently
ionise all the clusters with a single photon. As such the ionisation cross sections have
a marked effect on the observed distribution. Unfortunately these cross sections are
not known for the different species and so no definitive conclusions concerning the
reactivity can be drawn from these mass spectra. Those species which can be ionised,
however, can have spectroscopic probes applied to them. Figure
14
shows the
IR-MPD spectra for such complexes of neutral gold clusters [
12
].
For Au
10
(O
2
)
1-2
and Au
12
(O
2
)
1-3
, no bands are observable in the range of
1,000-1,700 cm
1
. This indicates that either the oxygen dissociates upon binding
to the gold clusters, resulting in the formation of a gold oxide which is not expected
to possess a vibrational fundamental at these frequencies, or the oxygen is
sufficiently weakly adsorbed to the cluster that the formally IR-inactive O-O
stretch is not sufficiently perturbed to become IR-active. As yet no evidence exists
to distinguish the two scenarios.
The other complexes show vibration fundamentals in two ranges, one
corresponding to a superoxo moiety between 1,000 and 1,100 cm
1
and is seen
