252
13 Introduction
chemisorbed overlayers and the reconstructed clean surfaces. LEED pattern analysis
forms the basis for structure models of a chemisorption process and it is vital to an
understanding of catalysis and corrosion.
However, pattern analysis only tells us the relative size, the symmetry and the
orientation of the unit cell in the overlayer; it tells us little in quantity about the spacing
between the overlayer and the substrate. Spectra analysis can find the adsorption
site but the accuracy of bond lengths and surface layer spacings is often subject to
accuracy [5].
In contrast, LEED at very low energies (VLEED) below plasma excitation or
ionization (~15 eV below E F ) holds rich and profound information not only about
the crystal geometry of the outermost atomic layer of a crystallite but also the behavior
of surface electrons described using the real and the imaginary part of the surface
potential barrier (SPB). The conjunction of the SPB with the multiple scattering
dynamics gives rise to the interesting phenomena of surface states, surface resonance
or VLEED fine structures [2, 6].
Interaction between the incident electron beams and the surface electrons defines
the fine-structure features in the VLEED I-E spectra. The so-called surface electrons
tie closely to the positions and valence states of the atoms at the surface. VLEED
spectrum integrates the following information [7, 8]:
• diffraction from the ion cores of a very small number of surface atomic layers and
the number of diffraction layer depends on the incident beam energy,
• scattering and interference by the elastic SPB; and,
• attenuation by the imaginary SPB, or inelastic damping, because of energy
exchange with surface electrons and excitation of phonons.
The mechanism for VLEED is more complicated than that for the normal LEED
but it is much more comprehensively revealing. Because of the difficulties arising
from the effects of stray electronic and magnetic fields on slowly moving electrons,
VLEED technique needs to screen the stray field by installing Halmholtz coils [53].
Theory calculations usually do not extend to these low energies because of the
approximation limitation in the calculation package. The use of an optical potential independent of electron energy in convention is valid at higher energies but it
becomes unreliable at energies below that for the plasmon-excitation. Furthermore,
the spectral fine structures are sensitive to the shape of the SPB. In conventional
calculations the SPB scattering is treated in a very simple uniform fashion, which is
insufficient for the chemisorbed systems.
It seemed conventionally impossible to use LEED to simultaneously determine
the crystal geometry and the SPB as one was unable to identify the spectral feature
arising from the change of either atomic positions or from the shape of the SPB.
In circumstances where the shape of the SPB is unknown, the energy dependence
of the muffin-tin inner potential constant is unknown, and the spatial decay and
energy dependence of the inelastic potential is unknown, it would be hardly possible
to derive comprehensive information on the structural and energetic configurations
from the VLEED fine-structure features alone. Appropriate modeling including all
the contributions and their nature links in decoding the VLEED data is necessary.
However, VLEED possesses more advantages than LEED in obtaining information about the SPB and the energy band structure as demonstrated for several mostly
13 Introduction
chemisorbed overlayers and the reconstructed clean surfaces. LEED pattern analysis
forms the basis for structure models of a chemisorption process and it is vital to an
understanding of catalysis and corrosion.
However, pattern analysis only tells us the relative size, the symmetry and the
orientation of the unit cell in the overlayer; it tells us little in quantity about the spacing
between the overlayer and the substrate. Spectra analysis can find the adsorption
site but the accuracy of bond lengths and surface layer spacings is often subject to
accuracy [5].
In contrast, LEED at very low energies (VLEED) below plasma excitation or
ionization (~15 eV below E F ) holds rich and profound information not only about
the crystal geometry of the outermost atomic layer of a crystallite but also the behavior
of surface electrons described using the real and the imaginary part of the surface
potential barrier (SPB). The conjunction of the SPB with the multiple scattering
dynamics gives rise to the interesting phenomena of surface states, surface resonance
or VLEED fine structures [2, 6].
Interaction between the incident electron beams and the surface electrons defines
the fine-structure features in the VLEED I-E spectra. The so-called surface electrons
tie closely to the positions and valence states of the atoms at the surface. VLEED
spectrum integrates the following information [7, 8]:
• diffraction from the ion cores of a very small number of surface atomic layers and
the number of diffraction layer depends on the incident beam energy,
• scattering and interference by the elastic SPB; and,
• attenuation by the imaginary SPB, or inelastic damping, because of energy
exchange with surface electrons and excitation of phonons.
The mechanism for VLEED is more complicated than that for the normal LEED
but it is much more comprehensively revealing. Because of the difficulties arising
from the effects of stray electronic and magnetic fields on slowly moving electrons,
VLEED technique needs to screen the stray field by installing Halmholtz coils [53].
Theory calculations usually do not extend to these low energies because of the
approximation limitation in the calculation package. The use of an optical potential independent of electron energy in convention is valid at higher energies but it
becomes unreliable at energies below that for the plasmon-excitation. Furthermore,
the spectral fine structures are sensitive to the shape of the SPB. In conventional
calculations the SPB scattering is treated in a very simple uniform fashion, which is
insufficient for the chemisorbed systems.
It seemed conventionally impossible to use LEED to simultaneously determine
the crystal geometry and the SPB as one was unable to identify the spectral feature
arising from the change of either atomic positions or from the shape of the SPB.
In circumstances where the shape of the SPB is unknown, the energy dependence
of the muffin-tin inner potential constant is unknown, and the spatial decay and
energy dependence of the inelastic potential is unknown, it would be hardly possible
to derive comprehensive information on the structural and energetic configurations
from the VLEED fine-structure features alone. Appropriate modeling including all
the contributions and their nature links in decoding the VLEED data is necessary.
However, VLEED possesses more advantages than LEED in obtaining information about the SPB and the energy band structure as demonstrated for several mostly