The metal-atom chains on the Si(111) - 5 × 2 - Au surface represent an exceedingly interesting system for the understanding of one-dimensional electrical interconnects. While other metal-atom chain structures on silicon suffer from metal-to-insulator transitions, Si(111) - 5 × 2 - Au stays metallic at least down to 20 K as we have proven by the anisotropic absorption from localized plasmon polaritons in the infrared. A quantitative analysis of the infrared plasmonic signal done here for the first time yields valuable band structure information in agreement with the theoretically derived data. The experimental and theoretical results are consistently explained in the framework of the atomic geometry, electronic structure, and IR spectra of the recent Kwon-Kang model.

The molecular vibrations of ethene adsorbed on roughened Cu(111) surfaces have been investigated with high resolution electron energy loss spectroscopy and density-functional-theory calculations. The roughness was introduced by sputtering or evaporation of copper, respectively, on the cooled surface. We found stabilization of the ethene layer compared to ethene adsorbed on pristine Cu(111). Furthermore, two new vibrational features observed on the rough surface can be assigned to frustrated translations and rotations of the ethene molecule on surface defects and are indicative of a different binding on the rough surface.

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...