Chemistry Reference
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This assignment is corroborated by IR spectra taken at higher Au exposure
(Fig.
24b
, spectra 3-5, blue lines). In addition to the low-frequency bands discussed
before, new blue-shifted lines emerge in the frequency range around 2060 cm
1
that shifts to 2070 cm
1
upon annealing to 100 K. For the highest coverage of
0.1 ML (Fig.
24b
, spectra 6-8, red lines), the spectrum is dominated by a band at
2074 cm
1
, while the strongly red-shifted bands have disappeared. Annealing the
system to 100 K induces again a blue shift of the line to 2097 cm
1
. Inspection of
the two data sets shown in Fig.
24b
, spectra 3-5 and spectra 6-8 reveals that neither
the intensity nor the line shape of the Au-related bands changes when the CO
coverage is increased from 15 to 100% of the saturation value. Annealing the
systems to 100 K, on the other hand, leads to a blue shift of the lines that reaches
15 cm
1
for the bands at 2060 (Fig.
24b
, spectrum 5) and 2080 cm
1
(Fig.
24b
,
spectrum8). At 100K, COdesorbs from theMgOfilmas evident from the reduction of
the MgO-related CO-stretch bands between 2150 and 2180 cm
1
.Ingeneral,theCO
stretching frequencies experience a decreasing red shift with higher Au exposure and
increasing cluster size, a phenomenon that can be rationalized in the following way.
Assuming that each Au aggregate nucleates on a single color center and takes up one
electron, the density of excess charges decreases in the larger particles, which reduces
the amount of
-back-donation into the CO and hence the red shift of the stretch mode.
A similar conclusion was drawn from CO adsorption experiments on charged
gas-phase clusters, where the CO-stretch [
104
] mode was found to increase when
going fromAu
(2050 cm
1
)toAu
+
clusters (2150 cm
1
). This simple picture needs,
however, to be modified in view of the earlier discussion. CO binds to the
low-coordinated sites of the charged Au aggregates, which accommodate the extra
electrons as well. The amount of
π
-back-donation now depends on the ability of the
cluster to distribute the charge away from the CO adsorption site to lower the Pauli
repulsion with the CO. Naturally, this ability diminishes with decreasing particle size,
causing the charge transfer into the CO 2
π
π
*toincrease.
Based on these considerations, the experimental IR bands may be assigned. The
bands above 2060 cm
1
that shift in an almost continuous fashion with Au exposure
are produced by particles containing a few tens of atoms, whose charge density
changes only gradually with size. The quasi-discrete bands at 1990 and 2010 cm
1
,
on the other hand, are characteristic for ultrasmall clusters such as dimers. A direct
assignment of these bands is difficult, as not only the size of the aggregate but also
the nature of the oxide defect underneath determines the vibrational response.
According to DFT calculations, Au dimers bound to F
0
-centers interact only weakly
with CO and produce a red shift of the stretching frequency of 194 cm
1
, whereas
on F
+
-centers the binding is sizable and the red shift is smaller (158 cm
1
).
However, also negatively charged Au
3
and Au
4
clusters [
100
] produce a red
175 cm
1
. The experimental CO bands at 1990 and 2010 cm
1
shift of
are
therefore compatible with several ultrasmall Au clusters.
In summary, whereas, on ultrathin films, the ad-particles charge up due to an
electron transfer from the support, the charging on thicker films is realized by an
electron donation from defects. In both cases, the excess electrons give rise to unusual
