Bulk plasmon induced ion neutralization near metal surfaces

A novel mechanism is proposed for ion neutralization near metal surfaces, whereby a bulk plasmon is emitted during the electron capture, induced by the presence of the external ion which does not penetrate the metal. In a semiclassical picture of this mechanism the electrons increase their velocity in the field of the ion until they surpass the threshold velocity for collective excitation, emitting the plasmon and getting bound to the ion.Primary evaluations of bulk plasmon transition rates for He⁺ interacting with Al surfaces indicate that very close to the image plane the bulk collective channel might become more efficient than the surface plasmon mode to neutralize the ion.     

Resonance and composition effects on the Raman scattering from silver-gold alloy clusters

Low-frequency Raman scattering experiments have been performed on thin films consisting of pure gold or gold-silver alloy clusters embedded in alumina matrix. It is clearly shown that the quadrupolar vibrational modes are observed by Raman scattering because of the effect of resonance with the excitation of the electronic surface dipolar plasmon. This is due to the strong coupling between the collective electronic dipolar excitation and the quadrupolar vibrational modes. This effect of resonance does not exist with the core electron excitations. The mixing of the conduction electron dipolar excitation (surface plasmon) with the core electrons leads to the quenching of the resonant Raman scattering. Received 16 November 2000     

Nonlocal screening of plasmons in graphene by semiconducting and metallic substrates: first-principles calculations

We investigate the role of substrates on the collective excitations of graphene by using a first-principles implementation of the density response function within the random-phase approximation. Specifically, we consider graphene adsorbed on SiC(0001) and Al(111) as representative examples of a semiconducting and metallic substrate. On SiC(0001), the long wavelength π plasmons are significantly damped although their energies remain almost unaltered. On Al(111), the long wavelength π plasmons are completely quenched due to the coupling to the metal surface plasmon. The strong damping of the plasmon excitations occurs despite the fact that the single-particle band structure of graphene is completely unaffected by the substrates illustrating the nonlocal nature of the effect.     

Els study of alkali metal adsorption on Ni(100)

Electron energy-loss spectra on clean and alkali-covered Ni(100) surfaces have been measured. The changes in the characteristic Ni(100) loss spectra in the presence of Na, K and Cs are interpreted as a result of large charge transfer to Ni. Alkali metal losses due to core-level excitation and plasmon excitations are also discussed.     

Plasmaschwingungen in Al,Mg, Li,Na und K angeregt durch schnelle Elektronen

We report on energy loss experiments with 50 keV electrons transmitted through thin films of the metals Al, Mg, Li, Na, and K. The valence electrons of these materials behave like a gas of free electrons to a good approximation and, in particular, give rise to significant collective effects. Since these substances oxidize rapidly they were evaporated and studied in ultra high vacuum. The apparatus and the measurement technique are described. Measurements of loss spectra for different electron scattering angles yield the volume plasmon dispersion curve and the dependence of the damping of the volume plasmon on the wave vector. New experimental values of the energy and the half width of the plasma oscillations and the coefficients of dispersion and damping are given. In addition, the results of some measurements on surface plasmons are presented. There are significant deviations from the predictions of the simple electron gas model.     

Extraordinary light transmission through a metal film perforated by a subwavelength hole array

It is shown that, depending on the incident wave frequency and the system geometry, the extraordinary transmission of light through a metal film perforated by an array of subwavelength holes can be described by one of the three mechanisms: the “transparency window” in the metal, excitation of the Fabry–Perot resonance of a collective mode produced by the hybridization of evanescence modes of the holes and surface plasmons, and excitation of a plasmon on the rear boundary of the film. The excitation of a plasmon resonance on the front boundary of the metal film does not make any substantial contribution to the transmission coefficient, although introduces a contribution to the reflection coefficient.     

End and central plasmon resonances in linear atomic chains

The existence and nature of end and central plasmon resonances in a linear atomic chain, the 1D analog to surface and bulk plasmons in 2D metals, has been predicted by ab initio time-dependent density functional theory. Length dependence of the absorption spectra shows the emergence and development of collectivity of these resonances. It converges to a single resonance in the longitudinal mode, and two transverse resonances, which are localized at the ends and center of the atom chains. These collective modes bridge the gaps, in concept and scale, between the collective excitation of atomic physics and nanoplasmonics. It also outlines a route to atomic-scale engineering of collective excitations.     

Plasmonic band gaps and trapped plasmons on nanostructured metal surfaces

Nanostructured metal surfaces comprised of periodically arranged spherical voids are grown by electrochemical deposition through a self-assembled template. Detailed measurements of the angle- and orientation-dependent reflectivity reveal the spectral dispersion, from which we identify the presence of both delocalized Bragg and localized Mie plasmons. These couple strongly producing bonding and antibonding mixed plasmons with anomalous dispersion properties. Appropriate plasmon engineering of the void morphology selects the plasmon spatial and spectral positions, allowing these plasmonic crystal films to be optimized for a wide range of sensing applications.     

Collective excitations and quantum size effects on the surfaces of Pb(111) films: An experimental study

Pb(111) film is a special system that exhibits strong quantum size effects in many electronic properties. The collective excitations, i.e., plasmons, in Pb(111) films are also expected to show signatures of the quantum size effect. Here, using high-resolution electron energy loss spectroscopy, we measured the plasmons on the surface of Pb(111) films with different film thicknesses and analyzed the plasmon dispersions. One surface plasmon branch exhibits prominent damping in the small momentum range, which can be attributed to the interaction between the top and bottom interfaces of the Pb(111)films. With the film thickness increasing, the critical momentum characterizing the damping in Pb(111) films decays not only much slower in Pb(111) films than in other metal films, and even in films with the thickness up to 40 monolayers the damping still exists. The slow decay of the surface plasmon damping, manifesting the strong quantum size effect in Pb(111) films, might be related to the strong nesting of the Fermi surface along the(111) direction.     

Theory of the intersurfacedband plasmon

The dispersion relation of the intersurfacedband plasmon, i.e., collective excitation of the surface electrons of the clean or adsorbed surfaces, is investigated by the self-consistent field approach. The dispersion is predicted to start with the linear term in the wave number parallel to the surface, which will be helpful to distinguish the plasmon from the individual excitations. The sign of the dispersion and the depolarization shift depend on the polarization of the interband transition. The contribution of the plasmon to the electron energy loss cross section is given.     

Foundations of Plasmonics

Plasma physics is a very mature field, studied extensively for well over a century. The cross-disciplinary field of plasmonics (electromagnetics of metallic nanostructures), on the other hand, with its potential for an extraordinary light control through novel class of materials and the resulting applications, has become very fashionable only recently. Inevitably, as a result of this rapid development, the deep connections with the mother discipline, the plasma physics, have sometimes been overlooked. The goal of this work is to review some of these basic connections, which are relevant, and ultimately helpful for researchers in the new field. We focus on the solid-state structured plasmas and address the issue of classical versus quantum treatments. We discuss the little known subtleties of the surface plasmons at metallic surfaces (e.g. multipole plasmons) and their consequences on plasmonics of the textured metallic films. Plasmonics of nanoparticles has been preceded by studies of plasma effects in metallic clusters and semiconducting quantum dots (QDs). In this context, we discuss the little known connection between the Mie resonance in metallic particles and the collective resonance in wide parabolic quantum wells (QWs) and QDs. Researchers dealing with plasmonics of thin films can benefit from earlier studies of plasmons in the semiconductor modulation doped heterojunctions and QWs, with its rich spectrum of intersubband and two-dimensional plasmons. In non-equilibrium plasmonic systems, generation of plasmons can be stimulated, leading to the exciting possibility of the plasmon instability. Extraordinarily complex is the plasmonics of carbon nanotubes and graphene, with its numerous van Hove, one- and three-dimensional plasmons, and we discuss how the plasmonics of metamaterials can benefit from this complexity. Finally, we discuss a few applications, which could directly benefit from plasmonics, including medical and the novel class of solar cells.     

Plasmon loss structure in synchrotron radiation photoemission from Mg films

Synchrotron radiation excitation has been used to vary the kinetic energy of electrons photoemitted from the 2p core level of polycrystalline magnesium films and the energy-dependence of the accompanying plasmon loss structure has been observed. Difficulties associated with background subtraction are discussed, and it is shown that the method of background subtraction profoundly affects the conclusions drawn from such an experiment on the relative importance of plasmon and single particle excitation mechanisms for electron scattering. Very thin films of Mg have also been studied in an attempt to separate the intrinsic and extrinsic modes of plasmon creation. Although the plasmon loss structure is dependent on film thickness implying a considerable contribution from extrinsic processes, we are not able to observe plasmons excited in the thin film by electrons originating in the substrate core level. The implications of these results are discussed and further experiments suggested.     

The influence of wire shape on surface plasmon mode distribution

The influence of the nanowire shape on the excitation of surface plasmon polaritons at metallic nanowire arrays is studied numerically. For a system of silver nanowires housed on a polymer substrate, nanowires with rectangular and elliptical cross sections are compared. It was found that in the case of rectangular nanowires the excitation efficiency is higher for surface plasmons at the polymer–metal interface than for surface plasmons at the air–metal interface. Conversely, in the case of elliptical nanowires the air–metal plasmon modes are stronger. Further, it is noted that the nanowire shape directly influences the position of the surface plasmon resonance.     

Collective excitations of the polaron-gas

The dispersion of the collective excitations of a polaron-gas is calculated. It is found that the interaction between the L.O. phonons and the plasmons induces a relatively strong wave vector dependence in the ω⁺, ω^- modes. This dispersion is primarily a consequence of the interaction of the L.O. phonons with the resonant plasmon branch in the pair excitation spectrum.     

The secondary-electron yield of air-exposed metal surfaces

The secondary-electron-yield (SEY) variation of atomically clean metal surfaces due to air exposure and during subsequent heat treatments is described. As an example SEY results are presented for the case of a sputter-deposited Nb thin film. Corresponding variations in the surface chemical composition have been monitored using AES and SSIMS. On the basis of these results, and of previously obtained SEY results for metals and metal oxides, the origin of the SEY variations is discussed. The SEY increase, which is generally observed during long-lasting air exposure of clean metals, is mainly caused by the adsorption of an airborne carbonaceous contamination layer. The estimated value of about three for the maximum SEY of this layer is higher than that of all pure metals. Only in some cases can the air-formed oxide contribute to the air-exposure-induced SEY increase, while many oxides have a lower SEY than their parent metals. From the experimental data it can also be excluded that the SEY increase during air exposure is mainly due to an increased secondary-electron escape probability. Received: 17 June 2002 / Accepted: 25 June 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Fax: +41-22/767-9150, Email: Christian.Scheuerlein@cern.ch     

Electronic excitations and their relaxations in clean and oxidized zirconium surfaces measured in eels

Electron energy loss Spectroscopy on clean and oxidized Zr surfaces shows features related to collective and single electron excitations. The 4p–4d transition resembles that encountered in isolated atoms. On oxidized surfaces, the interatomic transitions dominate. At the low kinetic energy side, spectra contain contributions due to the decay of plasmons, inter- and intra-atomic Auger processes, and autoionization emission.     

Photoemission from multiply excited surface plasmons in Ag nanoparticles

Two-photon photoemission spectroscopy using femtosecond laser pulses is used to investigate the excitation and decay mechanisms of the surface plasmon resonance in Ag nanoparticles grown on graphite. The resonant excitation of this collective excitation leads to a two-orders-of-magnitude-enhanced two-photon photoemission yield from a graphite surface with Ag nanoparticles compared to the yield from pure graphite. From the shape of the photoemission spectra, the polarization dependence of the photoemission yield and the excitation probabilities for different excitation pathways we conclude that excitation with 400-nm femtosecond laser pulses leads to the coherent multiple excitation of the surface plasmon in the Ag nanoparticles. This multiply excited plasmon mode can decay via the coupling to a single-particle excitation leading to the emission of an electron if its final state is located in the continuum. The surface plasmon in metallic nanoparticles is a model system to investigate collective excitations in multiphoton processes. Received: 26 June 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Plasmon response of a quantum-confined electron gas probed by core-level photoemission

We demonstrate the existence of quantized "bulk" plasmons in ultrathin magnesium films on Si(111) by analyzing plasmon-loss satellites in core-level photoemission spectra, recorded as a function of the film thickness d. Remarkably, the plasmon energy is shown to vary as 1/d2 all the way down to three atomic layers. The loss spectra are dominated by the n=1 and n=2 normal modes, consistent with the excitation of plasmons involving quantized electronic subbands. With decreasing film thickness, spectral weight is gradually transferred from the plasmon modes to the low-energy single-particle excitations. These results represent striking manifestations of the role of quantum confinement on plasmon resonances in precisely controlled nanostructures.     

Plasmons in a planar graphene superlattice

Plasmon collective excitations are studied in a planar graphene superlattice formed by periodically alternating regions of gapless graphene and of its gapped modification. The plasmon dispersion law is determined both for the quasi-one-dimensional case (the Fermi level is located within the minigap) and for the quasi-two-dimensional case (the Fermi level is located within the miniband). The problem concerning the absorption of modulated electromagnetic radiation at the excitation of plasmons is also considered.     

Dynamics and stability of nanostructures on metal surfaces

Ultrathin metal films consisting of two-dimensional clusters are typically unstable: the cluster ensemble has the tendency to reduce its total free energy via Ostwald ripening or dynamic coalescence of mobile clusters. In this paper we give an overview of recent model experiments addressing these coarsening mechanisms. The experiments have been performed using STM on ensembles consisting of adatom or vacancy clusters with typical diameters in the nanometer range on fcc(111)-metal surfaces. Agreement with and deviations from conventional theories are discussed. Received: 29 March 1999 / Accepted: 17 August 1999 / Published online: 30 September 1999