In order to understand the origin of features in RA spectra it is useful to compare experimental data with theoretically simulated data. Theoretical simulation of experimental RAS data from first principles is not yet possible. However there have
been several attempts to link experiment with theoretical simulations of RA spectra of the Au(110) surface.
Mochán et al. [45] developed a theoretical model to describe the electromagnetic response of Ag(110) and Au(110) surfaces. They compared the theoretical data to RA spectra of Au(110) in ambient conditions. The model was based on surface-local-field effects (SLFE) and takes into account the contributions from d and s-p electrons as well as the lattice geometry of the surface. The theoretical results showed only a slight agreement with the experimental results. This however demonstrated that RAS is sensitive to surface effects of cubic crystals. A further model based on SLFE was later compared to RA spectra of Au(110) in UHV conditions [45]. In this work Hansen et al. [45] modelled data based on the reconstructed (12) and the (11) Au(110) surface. The modelled data was then compared to the experimental RA spectrum of the Au(110) (12) reconstructed surface. The (12) model was shown to be closer to the experimental data than the (11) model. However this again was not a convincing match. The theoretical data for the (12) and (11) show an increase in intensity of a predicted feature at ~2.5 eV during the transition from (11) to the (12) surface. This feature of the (11) to (12) phase transition has been observed in experimental RAS studies of the Au(110) surface [14, 47-49]. A combined RAS and STM study of Au(110) by Mazine et al. [50] did not observe this increase at 2.5 eV, actually observing a decrease in intensity from (11) to (12). The Authors [50] produced the first RA spectra of Au(110) under electrochemical conditions and induced the surface reconstruction through potential control, allowing them to produce RA spectra they associated with the (11), (13) and the (12) surface structure. The RA spectrum of the (12) was described as the spectrum of a intermediate poorly reconstructed surface which consists of (13) domains. A second paper by the same Authors published in 2002 [51] also reported a decrease in intensity at 2.5 eV during the transition from the (11) to (12) reconstructed Au(110) surface. Mazine et al. [51] again produced very similar RA spectra which they associated with the (11), (12) and (13) reconstructed Au(110) surfaces. They presented this data as the ‘optical fingerprint’ of the (13) and (12) structures, although the RA spectra look very similar to the RA spectra the Authors first reported in earlier work [50].
Through more recent RAS experiments on the Au(110) surface, it has been shown that once the (13) surface has been adopted it is difficult to observe a pure (12) reconstructed surface structure. A pure (12) surface has consistently been observed with RAS after going from a (11) surface. Mazine at al. [50,51] held the crystal at -0.6 V in a (13) reconstruction before their experiments and this could account for disagreements in the labelling of the RAS of a (12) reconstructed Au(110) surface. The work of Sheridan et al. [49] produced RA spectra of the Au(110) (11) and (12) surface reconstructions in the electrochemical environment, which were in disagreement with the work of Mazine et al. [50,51]. It was suggested by Weightman et al. [52] that the inconsistency in the identification of the RA spectra for the Au(110) surface reconstructions arise from the differing morphology in the Au(110) surfaces used in each experiment. Recent experiments have shown a more detailed signature of the Au(110) surface reconstructions [1]. These are significantly different to the ones first reported by Sheridan et al. [49]. This can be attributed to differences in crystal preparation, improvements to the RAS equipment used and a different crystal supplier.
The work of Sheridan et al. [49] utilised UHV techniques LEED and STM in order to correlate changes in surface structure to observed changes in RA spectra. A wide negative feature, found in the spectral region of 3.7 eV to 4.5 eV of the RA spectrum, was shown to be particularly sensitive to surface roughness. The intensity of this feature was greatest after prolonged annealing, which produced a clear (12) LEED pattern and STM images that showed many monatomic steps oriented along the [1
1 0] direction in the surface. The intensity of this feature decreased after a shortened annealing process. The LEED pattern of this surface showed a clear (12) reconstructed surface but the STM images showed a reduced number of monatomic steps [49]. The theoretical work of Xu [43] on the electronic structure of the Au(110) surface allowed Sheridan et al. [49] to associate features on the RA spectrum of Au(110) with transitions between surface states. The RA profile in the region 1.5 eV to 2.5 eV was attributed to contributions from a surface state and so is expected to be sensitive to the surface and electronic structure [41]. The 2.5 eV peak observed on the RA spectrum of Au(110) was associated with a transition at the Γ point between an occupied surface resonance of odd symmetry arising from d states
of yz character to an even symmetry surface state of predominantly p character at ~ 0.3 eV above the Fermi energy. The transition from a state of even symmetry and sp character to a surface state of odd symmetry and d character derived at ~ 1.6 eV above the Fermi energy, was attributed to the feature at 3.5 eV on the RA spectrum [49]. The fact that the states involved in this transition have significant contributions from the second layer of atoms means the higher energy feature found at 3.5 eV lacks surface sensitivity in comparison to the 2.5 eV feature.
The RA spectrum of a clean Au(110) (12) surface is shown in figure 3.7. The spectrum is characterised by a sequence of features, a positive feature between 1.5 eV and 2.3 eV, large negative region between 2.5 eV and 3.5 eV with two prominent negative peaks at these two energies and a positive feature between 4.0 eV and 5.0 eV. While the origin of some of these features have been discussed in terms of transitions between states in the electronic structure of the surface, the positive feature at 4.0 eV has been attributed to the presence of monatomic steps in the surface [49]. In a further study by Martin et al. [53] the effect of Ar bombardment on the surface morphology of Au(110) was studied by monitoring changes in the RA spectra and STM images as a function of increasing Ar bombardment. Martin et al. [53] showed large (12) reconstructed terraces terminated by long monatomic steps aligned along the [1
1 0] direction on the clean Au(110) surface. The surface morphology after successive Ar ion bombardment was observed using STM. The surface had changed from well ordered terraces of (12) reconstruction and continuous monatomic steps to a rippled morphology of elevated islands and valley regions [53]. Figure 3.7 below shows the STM image and the corresponding RA spectrum of the clean Au(110) surface in UHV, reproduced from [53].
Figure 3.7: (a) Contrast STM image of annealed Au(110) surface in UHV showing monatomic steps and large terraces associated with the (12) reconstruction.
Figure 3.7: (b) RA spectrum associated with the crystal in a).
The removal of the step structure as a result of Ar ion bombardment has also been reported in RAS studies of Cu(110) [54]. A similar effect was observed in RA spectra of the Au(110) surface: a large change was observed in the region of 3.0 eV and 5.0 eV after Ar ion bombardment, which was attributed to the changes in surface roughness of the Au(110) surface as a result of Ar ion bombardment. The authors [53] found a slight intensity decrease in the 2.5 eV peak, however the feature retained its definition, even after 36 minutes of bombardment and so is considered insensitive to surface roughness and the slight decrease in intensity was attributed to the loss of (12) reconstruction in some regions of the surface, due to the Ar ion bombardment, which is in agreement with the other studies of the (12) to (11) transition [14,47-49].
40 nm
The effect of temperature on the Au(110) surface has also been monitored using RAS firstly by Stahrenberg et al. [55] and more recently by Martin et al. [56]. RA spectra were taken between the temperature range 300 K to 800 K [55] and 300 K to 1000 K [56], the RA profile was characterised by features at 1.8 eV, 2.52 eV, 3.52 eV and 4.50 eV and these features were monitored as a function of temperature (figure 3.8). The initial RA spectrum produced for a clean Au(110) (12) reconstructed surface is in good agreement with earlier work [49-53]. The feature at 1.8 eV appears to be independent of temperature as it remains unchanged for temperatures up to 1000 K. The feature at 2.5 eV in the RA spectrum on the other hand does change, the intensity of the feature decreases and the peak broadens with increasing temperature, increasing the temperature above 580 K results in a slight positional peak shift of this feature to higher energy. When the temperature is increased beyond TR ~ 800 K the feature at 2.5 eV is so broad and its intensity is so
weak that it is difficult to distinguish from the background RA profile. The smooth decrease in RA intensity between 1.5 eV and 2.5 eV as the temperature is increased is consistent with the gradual increase in surface disorder during the phase transition from (12) to (11) structure [56]. The features at 3.5 eV and 4.5 eV change substantially as a result of increased temperature, both peaks shift to lower photon energies as the temperature was increased above 300 K. In addition to this positional shift both features also broaden and the similarity in behaviour of the two features suggested that they are closely related.
Figure 3.8: RA spectra of Au(110) as a function of temperature. Reproduced from [56].
Martin et al. [56] analysed the results of the temperature dependent RA spectra using a model derived by Russow et al. [57] and found good agreement with a thermovariation spectroscopy study of Au, Ag and Cu which showed that the main features contributing to the bulk dielectric, εb, for Au are from interband transition in
the vicinity of the L point of the Brillouin zone (BZ) [56]. The consistency of the thermovariation spectroscopy study [58] with the variation in temperature and RA profile changes [56] allowed the 3.5 eV and 4.5 eV features on the RA spectra to be assigned with specific transitions, the EF Lu1 and the L’2 Lu1 respectively [56], thus establishing that the region between 2.5 eV and 4.5 eV on the RA spectrum of Au(110) is derived from contributions from surface modified bulk state transitions.