13.1 LEED, VLEED, STM/S, and PES
253
close-packed surfaces of transition metals [9–16] and ZnO(0001) surface [17]. The
scattering from light adsorbates is often stronger, even comparable to the host substrate at very low energies, and this can lead to knowledge of the adsorbate positions
[18] as well as the valence states of individual surface atoms with the assistance of
an appropriate structural model [7, 8, 19].
In fact, bond formation modifies the electronic structures of both the negative ions
of the light adsorbate and the host valence states [20]. Moreover, in VLEED calculations, fewer beams and fewer phase shifts are considered and spectral peaks occur
with a greater density in the energy range. Hence, computation time is considerably
reduced. Furnished with a proper calculation code and suitable structural models, the
high-resolution VLEED I-E spectra can be analyzed for comprehensive information
about the behavior of atoms and electrons at a surface simultaneously.
There are two kinds of characteristic features on a measured VLEED I-E spectrum.
One is the Rydberg series that relate to the interference or resonance effect due to the
SPB [21]. The Rydberg features come from the interference between the measured
beam and the pre-emergent beams that reflect repeatedly between the substrate lattice
and the SPB [1]. The Rydberg series are used to be known as “threshold effects”, or
“SPB resonance or interference” [22], since they converge from below the emergence
thresholds of diffracted beams.
The other kind of sharp features comes from the band gap Bragg diffraction at
the Brillouin zone boundaries [23]. The positions of the narrow, sharp, solitary and
violent peaks rely both on the crystal geometry of the surface and on the incident and
azimuth angles of the electron beams [24, 25]. The sharp peaks converge into the
thresholds of emerging new beams inside the crystal, as found by many researchers
from the O-Ru(001) [26], Cu(111) [27, 28], and Ni(111) and the ZnO(0001) [17]
surfaces. These sharp features result from the energy-band structure. The troughs
relate to the intensity variation caused by the wave-like energy dependence of the
additional sextets in VLEED patterns [29].
The relationship between the band structure and the elastic reflection coefficient
in VLEED can be obtained through the matching method [30, 31]. The matching
between the vacuum wave function with the superposition of Bloch waves excited in
the solid determines the elastic reflection coefficient. The energy location of a trough
in the spectrum corresponds to the rapid change of reflection, coinciding with the
location of the band-structure critical positions at the boundaries of Brillouin zones
[32, 33]. Therefore, it is possible to determine the connection between the VLEED
profiles and the band structure by directly measuring the energy positions of the
critical points.
Many researchers have devoted their efforts to using VLEED to characterize the
energy states above vacuum level of the surfaces—unoccupied states. Strocov et al.
[10–13] demonstrated that VLEED measurements are ideally suited for accurate
determination of the desired upper states. Knowledge about the excited states could be
substantially improved than could the band mapping by photoelectron spectroscopy.
From the band structure point of view, VLEED is very sensitive to modifications of
the upper band formed by the variations of geometric structure, which encourages
one to use VLEED as a tool to investigate which way these two entities, i.e., atomic
253
close-packed surfaces of transition metals [9–16] and ZnO(0001) surface [17]. The
scattering from light adsorbates is often stronger, even comparable to the host substrate at very low energies, and this can lead to knowledge of the adsorbate positions
[18] as well as the valence states of individual surface atoms with the assistance of
an appropriate structural model [7, 8, 19].
In fact, bond formation modifies the electronic structures of both the negative ions
of the light adsorbate and the host valence states [20]. Moreover, in VLEED calculations, fewer beams and fewer phase shifts are considered and spectral peaks occur
with a greater density in the energy range. Hence, computation time is considerably
reduced. Furnished with a proper calculation code and suitable structural models, the
high-resolution VLEED I-E spectra can be analyzed for comprehensive information
about the behavior of atoms and electrons at a surface simultaneously.
There are two kinds of characteristic features on a measured VLEED I-E spectrum.
One is the Rydberg series that relate to the interference or resonance effect due to the
SPB [21]. The Rydberg features come from the interference between the measured
beam and the pre-emergent beams that reflect repeatedly between the substrate lattice
and the SPB [1]. The Rydberg series are used to be known as “threshold effects”, or
“SPB resonance or interference” [22], since they converge from below the emergence
thresholds of diffracted beams.
The other kind of sharp features comes from the band gap Bragg diffraction at
the Brillouin zone boundaries [23]. The positions of the narrow, sharp, solitary and
violent peaks rely both on the crystal geometry of the surface and on the incident and
azimuth angles of the electron beams [24, 25]. The sharp peaks converge into the
thresholds of emerging new beams inside the crystal, as found by many researchers
from the O-Ru(001) [26], Cu(111) [27, 28], and Ni(111) and the ZnO(0001) [17]
surfaces. These sharp features result from the energy-band structure. The troughs
relate to the intensity variation caused by the wave-like energy dependence of the
additional sextets in VLEED patterns [29].
The relationship between the band structure and the elastic reflection coefficient
in VLEED can be obtained through the matching method [30, 31]. The matching
between the vacuum wave function with the superposition of Bloch waves excited in
the solid determines the elastic reflection coefficient. The energy location of a trough
in the spectrum corresponds to the rapid change of reflection, coinciding with the
location of the band-structure critical positions at the boundaries of Brillouin zones
[32, 33]. Therefore, it is possible to determine the connection between the VLEED
profiles and the band structure by directly measuring the energy positions of the
critical points.
Many researchers have devoted their efforts to using VLEED to characterize the
energy states above vacuum level of the surfaces—unoccupied states. Strocov et al.
[10–13] demonstrated that VLEED measurements are ideally suited for accurate
determination of the desired upper states. Knowledge about the excited states could be
substantially improved than could the band mapping by photoelectron spectroscopy.
From the band structure point of view, VLEED is very sensitive to modifications of
the upper band formed by the variations of geometric structure, which encourages
one to use VLEED as a tool to investigate which way these two entities, i.e., atomic