254
13 Introduction
geometry and band structures, are interdependent. Furthermore, Bartos et al. [27, 34]
found that corresponding theoretical I-E curves could be obtained in good agreement
with experimental data from the densely packed Cu(111) surface when the anisotropy
of the electron attenuation (inelastic potential) was taken into account. Therefore,
VLEED gives information about the anisotropy of the SPB.
Unfortunately, little progress had been made until 1995 [35] in the structure determination of surfaces with large unit meshes or surfaces with adsorbed molecules
using VLEED. No attempts had even been given using VLEED to simultaneously
determine the bond geometry, energy states, the three-dimensional (3D) SPB and
the bond forming dynamics of systems with chemisorbed oxygen for that time being
[7, 8]. Situation was changed when a multi-atom and multiple diffraction code was
developed to deal with the dynamics of oxygen chemisorption on Cu(001) surface
[35].
In the multiple scattering calculation code [35], a complex unit cell containing five
atoms treats the SPB as an additional layer of interference. However, the parameter
space used to describe the elastic ReV(z) and the inelastic ImV(z, E) part of the SPB
was too large. At least nine variables in a one-dimensional system were required
to describe the spatial variation and energy dependence of the SPB [37]. If the
strongly correlated SPB parameters were treated independently the one-dimensional
approximation did not work for the chemisorbed surface besides the information
miss on the localized nature of the valence states and the anisotropy of the SPB.
Such an approximation worked fairly well for the clean Cu(001) surface but it
faced severe difficulties in decoding the VLEED data from the chemisorbed OCu(001) surface. Spectra near the <11> azimuth of the O-added Cu(001) surface
could not be fitted despite various structure models and SPB parameters being used
[35]. Three-dimensional SPB attributes are very important for the adsorbate-involved
system, which is local on atomic scale in many properties [37].
As a matter of fact, correct SPB parameters must depend on energy E, lateral
momentum K // , and surface atomic coordinates (x, y). However, the primary effect
of the 3D terms in the SPB is to generate off-diagonal matrix elements in the SPB
scattering matrix, which leads to too much longer computer running. One way to
include the 3D effects in a simpler manner is to use a hybridized one-dimensional
model that incorporates some of the more important 3D features. As the multiple
scattering is often less important in the VLEED energies, the hybridization of onedimensional model with 3D contribution is acceptable.
When the high-energy photons, X-ray or hard ultraviolet ionizing radiation, slam
into the sample in vacuum chamber, they evict some of its electrons, launching
them out of the material. Detectors then count these homeless electrons and measure
their energies and directions of travelling. UPS, which examines electrons ejected
from valence shells, is more suited to establishing the bonding characteristics and
the details of valence shell electronic structures of substances on the surface. Its
usefulness is its ability to reveal which orbitals of the adsorbate are involved in the
bond to the substrate.
Consequently, if photoelectrons ejected from filled levels will have kinetic energies (KEs) that fall within this structured region, the observed intensity will be a
13 Introduction
geometry and band structures, are interdependent. Furthermore, Bartos et al. [27, 34]
found that corresponding theoretical I-E curves could be obtained in good agreement
with experimental data from the densely packed Cu(111) surface when the anisotropy
of the electron attenuation (inelastic potential) was taken into account. Therefore,
VLEED gives information about the anisotropy of the SPB.
Unfortunately, little progress had been made until 1995 [35] in the structure determination of surfaces with large unit meshes or surfaces with adsorbed molecules
using VLEED. No attempts had even been given using VLEED to simultaneously
determine the bond geometry, energy states, the three-dimensional (3D) SPB and
the bond forming dynamics of systems with chemisorbed oxygen for that time being
[7, 8]. Situation was changed when a multi-atom and multiple diffraction code was
developed to deal with the dynamics of oxygen chemisorption on Cu(001) surface
[35].
In the multiple scattering calculation code [35], a complex unit cell containing five
atoms treats the SPB as an additional layer of interference. However, the parameter
space used to describe the elastic ReV(z) and the inelastic ImV(z, E) part of the SPB
was too large. At least nine variables in a one-dimensional system were required
to describe the spatial variation and energy dependence of the SPB [37]. If the
strongly correlated SPB parameters were treated independently the one-dimensional
approximation did not work for the chemisorbed surface besides the information
miss on the localized nature of the valence states and the anisotropy of the SPB.
Such an approximation worked fairly well for the clean Cu(001) surface but it
faced severe difficulties in decoding the VLEED data from the chemisorbed OCu(001) surface. Spectra near the <11> azimuth of the O-added Cu(001) surface
could not be fitted despite various structure models and SPB parameters being used
[35]. Three-dimensional SPB attributes are very important for the adsorbate-involved
system, which is local on atomic scale in many properties [37].
As a matter of fact, correct SPB parameters must depend on energy E, lateral
momentum K // , and surface atomic coordinates (x, y). However, the primary effect
of the 3D terms in the SPB is to generate off-diagonal matrix elements in the SPB
scattering matrix, which leads to too much longer computer running. One way to
include the 3D effects in a simpler manner is to use a hybridized one-dimensional
model that incorporates some of the more important 3D features. As the multiple
scattering is often less important in the VLEED energies, the hybridization of onedimensional model with 3D contribution is acceptable.
When the high-energy photons, X-ray or hard ultraviolet ionizing radiation, slam
into the sample in vacuum chamber, they evict some of its electrons, launching
them out of the material. Detectors then count these homeless electrons and measure
their energies and directions of travelling. UPS, which examines electrons ejected
from valence shells, is more suited to establishing the bonding characteristics and
the details of valence shell electronic structures of substances on the surface. Its
usefulness is its ability to reveal which orbitals of the adsorbate are involved in the
bond to the substrate.
Consequently, if photoelectrons ejected from filled levels will have kinetic energies (KEs) that fall within this structured region, the observed intensity will be a