Acoustic Surface Plasmon on Cu(111) as an Excitation in the Mid-Infrared Range

Abstract

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...

Direct observation of many-body charge density oscillations in a two-dimensional electron gas

“…31), in the considered 2D electron system the collective-excitation mode has a linear dispersion o pl q ð Þ $ q due to screening by the underlying three-dimensional electron system 25 . This mode is the aforementioned ASP, which was predicted to exist at metal surfaces with a partly occupied surface-state band 25,26 and was recently observed on Be 32 , Cu 33,34 and Au 35,36 surfaces by electron-energy-loss spectroscopy.…”

The influence of electron confinement, quantum size effects, and film morphology on the dispersion and the damping of plasmonic modes in Ag and Au thin films

“…Shockley surface states existing at the (1 1 1) surfaces of noble metals [56,149,[366][367][368][369][370][371][372][373][374] support this collective excitation, which owes its existence to the spatial coexistence of a 2DEG with a 3D electron gas. In Ag the excitation is ascribed to the screening of the Shockley surface states electrons by bulk electrons [367,375], while recent results have indicated that in both Au [150] and Cu [376] the opposite phenomenon occurs: slow bulk electrons are screened by the faster surface state electrons. Both types of interplay lead to an acoustic (sound-like) dispersion relation, giving the excitation its name ASP.…”

Photoemission footprints of extrinsic plasmarons

“…Later on Electron Energy Loss Spectroscopy experiments have confirmed the presence of the ASP mode at the Be(0001) [5,6] and noble metal surfaces [7][8][9][10][11][12][13][14] in good agreement with calculations and graphene adsorbed on metal substrates [15][16][17][18][19][20][21][22]. In cases when surface localized quantum well states are formed, e.g.…”