Chemistry Reference
In-Depth Information
Fig. 10 Experimental and calculated topography, HOMO shape, and model structure for Au
3
to
Au
7
chains on alumina/NiAl(110). All images are 5.0
5.0 nm
2
. The HOMO-1 for the Au
7
is
shown in addition. Measured chain lengths are 9, 12, 15, and 22
Å
; calculated distances between
first and last chain atom amount to 5.3, 7.8, 10.5, and 15.5
. To compare theoretical and exp.
lengths, 2-3
Å
should be added to both sides of the chain to account for the diffusivity of the 1D
orbitals [
37
]
Å
the unoccupied 6p
z
-like QWS, the onset of which has been determined to 1.8 eV. As
the energy spacing between adjacent 6p
z
-levels is relatively small, the corresponding
dI/dV maps do not show the unperturbed symmetry of isolated QWS but always
result from a superposition of adjacent levels. In contrast to ultra-small aggregates,
filled QWS are detected in the longer Au chains as well as the tetramer displays two
dI/dV peaks below
E
F
, whereby the upper one at
1.1 V (the HOMO) exhibits three
electron-density maxima and two nodal planes along the chain axis (Fig.
10
). The
HOMO is therefore assigned to the third s-like QWS, being characterized by three
density maxima. Assuming double occupancy of all QWS below the Fermi level, the
total number of s-electrons in this ad-chain would be six. While four electrons are
provided by the incoming Au atoms, the other two originate from the NiAl support,
rendering the Au
4
chain twofold negatively charged. With the same arguments, the
Au
5
chain was found to contain three extra electrons, as its HOMO at
1.6 V exhibits
four intensity maxima and is thus assigned to the fourth QWS. Only five of the eight
