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Fig. 28 (a) CO-IRAS spectra (
top
) and Au 4f XPS (
bottom
) spectrum from 0.05 ML Au deposited
at 100 K onto MgO
hydr
.(b) Same as (a) taken after annealing of the samples to 600 K
argument cannot be conclusively substantiated, direct evidence for the second one
can be obtained with the help of spectroscopic methods.
Infrared spectroscopy using CO as a probe molecule has in the past been shown
to provide valuable information about the charge state of gold particles [
92
]. While
the stretching frequency of CO adsorbed on neutral Au particles is typically found
in a narrow spectral region around 2100 cm
1
, the presence of positively (nega-
tively) charged Au particles results in a shift to higher (lower) wave number as a
result of reduced (increased) back-donation into the antibonding 2
* orbital of CO
as has been discussed in the previous section. We noted that for CO adsorbed on
neutral, single gold atoms on the MgO(001) surface, a
ʽ
(CO) of 1852 cm
1
was
observed as a result of internal charge reorganization in the MgO-Au-CO adsorp-
tion complex (see Sect.
3
above, Fig.
28a
)[
93
]. This very particular characteristic
represents an easily accessible spectroscopic indicator for the presence of single Au
atoms on MgO and its presence or absence provides information about the nucle-
ation of Au on modified MgO surfaces, such as the hydroxylated one investigated
here. Indeed, this signal cannot be recovered for gold deposited at 100 K onto
MgO
hydr
, which provides an initial evidence for the different nucleation of gold on
this surface relative to clean MgO(001). More specific information about the state
of gold nucleated on MgO
hydr
is obtained by looking into the spectral region around
π
