The morphology of gold surfaces plays a major role in many domains of contemporary research. Infrared (IR) spectroscopy in combination with carbon monoxide (CO) as a probe adsorbate is able to sensitively monitor differences in the morphology of gold surfaces on an atomic level if CO adsorption on the various surfaces is clarified.Our investigation comprises the first study of CO adsorption on Au(111) under welldefined ultra-high vacuum conditions at 30 K. We find that CO adsorbs on Au(111) in atop geometry as has been reported before for a variety of gold surfaces but a significantly higher frequency of the internal CO stretching vibration is observed confirming results from recent theoretical studies. Furthermore, the presence of a submonolayer amount of gold adatoms on the Au(111) surface results in the properties of gold surfaces towards CO adsorption at higher temperatures known from literature.Step-wise annealing of these atomically rough surfaces leads to a gradual transition between the literature case and the behavior observed on the pristine Au(111) surface at 30 K.

We report on an investigation of the acoustic surface plasmon on Cu(111), an electronic excitation in the infrared range related to the Shockley surface state. As shown here by high-resolution electron energy loss spectroscopy, it contributes together with other low-energetic electronic transitions to a broad excitation feature. Our analysis is similar to that recently reported for the Au(111) surface and clarifies that the group velocity of the acoustic surface plasmon is slower than the Fermi velocity of the surface state. The acoustic surface plasmon thus overlaps with the electron−hole pair continuum and may therefore influence adsorption processes as well. ■ INTRODUCTIONElectron dynamics govern adsorption processes and chemical reactions at surfaces. Understanding of these dynamics is therefore an important precondition to control chemical and physical processes on surfaces. 1 It is already well-known that electron−hole (e−h) pair excitations on metal surfaces even affect adsorption, as can be seen from the broadening of vibrational lines of adsorbates. 2−11 In 2004, Silkin et al. showed that in addition to these single particle excitations, a collective excitation with similarly low energy can be expected to exist on certain metal surfaces. 12−14 Prerequisite for its occurrence is the spatial coexistence of a partially filled two-dimensional Shockley surface state (SS) band with the three-dimensional bulk continuum of electrons. While initially, the excitation was ascribed to screening of the SS electrons by the majority of faster bulk electrons, 12,14,15 new results suggest that at least for Au(111) the deviation from free electron-like behavior causes slow bulk electrons to be screened by the now faster SS electrons which gives rise to the excitation. 16 Both types of interplay have theoretically been shown to lead to an acoustic (sound-like) dispersion relation (linear and zero intercept with energy axis), giving the excitation its name acoustic surface plasmon, ASP. Due to its low energy and linear dispersion, such an excitation is expected to play an important role in a variety of exciting phenomena ranging from superconductivity unto the optical properties of nanostructures, as emphasized in refs 12, 17, and 18. Furthermore, due to boundaries on an imperfect surface, standing wave patterns might exist.We highlight here also the importance of the ASP in its role as an additional nonradiative decay channel. This comes into play, e.g., in biochemical sensing applications: While surface plasmon-enhanced fluorescence spectroscopy relies on the one hand on the confinement and enhancement of the electromagnetic field in the vicinity of metal surfaces due to the presence of surface plasmon polaritons, 19 it is on the other hand well-known that the conventional surface plasmon itself affects the quantum yield by competing against the fluorescence. 20 The ASP presents itself as a probable candidate to contribute to this quenching. Another example for the importance of the ASP is surface photochemistry mediated...

With mid-infrared spectroscopy at room temperature, we investigated the adsorption of oxygen on the clean, single-crystalline Cu(110) surface resulting in the well-known (2 × 1)O–Cu(110) added-row reconstruction. We observed an anisotropic change of broadband reflectance which corresponds to an anisotropic surface resistance change. The resistance change is more pronounced by a factor of 7 for the plane of light incidence parallel to the [11̅0] direction. However, even perpendicular to this direction, a small but significant change is observed. A qualitative explanation of the anisotropic baseline shift can be given within existing theory, but for a quantitative description anisotropic electronic scattering of the bulk is crucial. Our results may be relevant for the optical behavior of nanocrystallites.

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