Nonadiabatic quasiparticle dynamics in time resolved electron spectroscopies of surface bands

Surface localized electronic states constitute electronic environment for a variety of physical and chemical phenomena taking place on surfaces. Various processes of model catalytic reactions may be influenced or mediated by hot electrons and holes excited in quasi-two-dimensional bands occurring on a large number of metal surfaces. Here we discuss several important aspects of nonadiabatic dynamics of these excitations which may affect the measurements of surface electronic properties by ultrafast electron spectroscopies.     

损耗对表面等离子体激元压缩态的影响

表面等离子体激元是金属表面电子集体振荡,它以波的形式在金属和介质之间的界面中传播.近期Huck等证明等离子体激元可以处在压缩态,本文利用量子光学的热库理论,研究金波导损耗对表面等离子体激元压缩态的影响,并对Huck等的实验结果给与理论解释. 关键词： 表面等离子体激元 压缩态 热库理论     

Local 2PPE-yield enhancement in a defined periodic silver nanodisk array

Well-prepared periodic arrays of silver nanoparticles are investigated by means of linear and non-linear photoemission electron microscopy. The structures show homogeneous photoemission for UV excitation in the linear photoemission regime whereas striking inhomogeneities are mapped in the case of the nonlinear (2 photon) excitation using ultrashort 400 nm laser pulses. A detailed analysis enables to assign these inhomogeneities to defect induced electron momentum transfer processes only effective for the 2 photon excitation process. We propose this mechanism to be of relevance for the appearance of so-called hot spots in nonlinear photoemission as identified in other 2PPE studies in the past. Furthermore, the complementarity between all-optical studies and nonlinear photoemission studies of localized surface plasmons in nanoparticles is discussed.     

Energieverlustmessungen von schnellen Elektronen an Oberflächen von Ga,In, Al und Si

The excitation of surface plasmons on liquid indium, gallium and aluminum surfaces as well as on silicon cleavage faces is studied by reflection of 10 keV electrons at grazing incidence (80° up to 89°). Due to the high excitation probability of surface plasmons at these large angles of incidence multiples of surface plasmons are observed. (Except on Si.) The Poisson distribution of the intensity of these energy losses is verified quantitatively as well as the dependence on (cos)^–1. The value of the excitation probability indicates that at grazing incidence the electrons are reflected just at the surface without penetrating into the volume. This is further demonstrated by the lack of volume plasmons. The surface loss positions in the solid and the liquid state show the calculated differences due to the density difference of both phases     

Transverse electron focusing as a way of studying surface crystallography

Conduction electrons represent a unique probe for studying surfaces (and interfaces) due to their extremely low excitation energies and to the fact that they impinge upon the surface from inside the sample. Focusing of conduction electrons by means of a transverse homogeneous magnetic field—transverse electron focusing (TEF)—provides a means for probing the atomic structure and composition of surfaces and interfaces, including both regular and irregular roughness. This article explains what TEF is, reviews what has been learned from it about surface structure, and describes what can be learned in the future.     

Impact of electron–hole pair excitation on the kinetics of exoergic processes at metal surfaces

The effect of energy disposal to metal surfaces, via electron–hole (e–h) pair excitation, on the rate of exoergic processes such as adsorption and adatom recombination, has been investigated. The modeling is based on the mean field rate equations and allows one to couple e–h pair excitation with either recombination or stimulated-desorption processes within a unique kinetic scheme. The energy distribution function of the metal electrons and the number of electrons available for detection as “chemicurrent” have been computed and compared to experimental results on chemicurrent yield for the H–Cu system. The kinetic model can also be applied to interpret experimental data on H(D)-adatom abstraction and recombination and on adsorption stimulated desorption of CO from metal surfaces. Rate coefficients of these desorption processes are proper for non-equilibrium energy distribution functions of the adlayer, which depend on the flux of adsorbing species and correlate to the surface electron density as modified by the adsorbate.     

Surface Plasmon Mediated Controllable Spin‐Resolved Transmission in Meta‐Hole Structures

Chiral responses are optical responses involving circular polarizations. Controlling the chiral response in a flexible way is very important in optical manipulations. Chiral metamaterials have thus drawn enormous interest due to their flexible designing feature. However, most of the previous studies are mainly realized by designing the structure of the individual meta‐atom. Meanwhile, to enhance the response, complex design and fabrication processes are typically required. Here, by introducing spin‐dependent propagating surface plasmons and spin‐selective interference, giant spin‐resolved transmission is achieved in a simple meta‐hole structure. In this interaction process, spin‐orbital angular momentum conversion plays an essential role. By controlling the phase difference between the interference components, controllable spin‐resolved transmission is achieved. Furthermore, such method can also be applied to realize spin‐resolved excitation of surface plasmons. The proposed controlling strategy offers a versatile platform for a variety of promising applications, such as polarization control, asymmetric transmission, surface plasmon excitation, and on‐chip chiral manipulation.     

Making metals transparent for white light by spoof surface plasmons

From first-principles computations we reveal that metallic gratings consisting of narrow slits may become transparent for extremely broad bandwidths under oblique incidence. This phenomenon can be explained by a concrete picture in which the incident wave drives free electrons on the conducting surfaces and part of the slit walls to form spoof surface plasmons (SSPs). The SSPs then propagate on the slit walls but are abruptly discontinued by the bottom edges to form oscillating charges that emit the transmitted wave. This picture explicitly demonstrates the conversion between light and SSPs and indicates clear guidelines for enhancing SSP excitation and propagation. Making structured metals transparent may lead to a variety of applications.     

Surface plasmon photodetectors and their applications

Surface plasmon photodetectors are of vigorous current interest. Such detectors typically combine a metallic structure that supports surface plasmons with a photodetection structure based on internal photoemission or electron‐hole pair creation. Detector architectures are highly varied, involving surface plasmons on planar metal waveguides, on metal gratings, on nano‐particles, ‐islands, or ‐antennas, or involving plasmon‐mediated transmission through one or many sub‐wavelength holes in a metal film. Properties inherent to surface plasmons, such as sub‐wavelength confinement and their ability to resonate on tiny metallic structures, are exploited to convey useful characteristics to detectors in addressing applications such as low‐noise high‐speed detection, single‐plasmon detection, near‐ and mid‐infrared imaging, photovoltaic solar energy conversion, and (bio)chemical sensing. The operating principles behind surface plasmon detectors are reviewed, the literature on the topic is surveyed, and avenues that appear promising are highlighted.     

Terahertz-pulse emission through laser excitation of surface plasmons in a metal grating

The second-order processes of optical-rectification and photoconduction are well known and widely used to produce ultrafast electromagnetic pulses in the terahertz frequency domain. We present a new form of rectification that relies on the excitation of surface plasmons in metal films deposited on a shallow grating. Multiphoton ionization and ponderomotive acceleration of electrons in the enhanced evanescent field of the surface plasmons results in a femtosecond current surge and emission of terahertz electromagnetic radiation. Using gold, this rectification process is third or higher-order in the incident field.     

Angular and energy dependences of the surface excitation parameter for electrons crossing a solid surface

When fast electrons cross a solid surface, surface plasmons may be generated. Surface plasmon excitations induced by electrons moving in the vacuum are generally characterized by the surface excitation parameter. This parameter was calculated for 200-1000 eV electrons crossing the surfaces of Au, Cu, Ag, Fe, Si, Ni, Pd, MgO and SiO₂ with various crossing angles. Such calculations were performed based on the dielectric response theory for both incident (from vacuum to solid) and escaping (from solid to vacuum) electrons. Calculated results showed that the surface excitation parameter increased with crossing angle but decreased with electron energy. This was due to the longer time for electron-surface interaction by glancing incident or escaping electrons and by slow moving electrons. The results were fitted very well to a simple formula, i.e. , where P_s is the surface excitation parameter, E is the electron energy, α is the angle between the electron trajectory and the surface normal, and a, b and c are material dependent constants.     

Photoluminescence enhancement from hybrid structures of metallic single-walled carbon nanotube/ZnO films

We investigated the dependence of localized surface plasmon resonance (LSPR) coupled photoluminescence (PL) emission on the density of a metallic single-walled carbon nanotube (m-SWCNT). The m-SWCNTs of various densities were deposited on top of ZnO films by spin coating and filtration transfer method to form the hybrid structures. We observed PL enhancement from ZnO films deposited with spin coated m-SWCNT, comparing with pure ZnO film. The m-SWCNT acts as absorbers for the light emitted due to SPR. After resonant excitation, hot electrons in m-SWCNT are created in high energy states, which can then transfer from the m-SWCNT to the conduction band of the ZnO films. We discuss the relationship between the hot electron flow generated by internal photoemission and LSPR.     

Excitation of surface plasmons in metal particles by fast electrons and x rays

The cross sections for excitation of surface plasmons by fast electrons and x rays are calculated within the random phase approximation for a spherical metal particle. The results are compared with those obtained within the hydrodynamic model of a free electron gas.     

丁胺包裹的CdSe量子点敏化的TiO2纳米晶薄膜电子转移机制

利用超快光谱技术系统研究了在丁胺包裹的CdSe量子点敏化的TiO₂纳米晶薄膜起始时刻界面间电子转移动力学。与之前的报道不同,该实验结果表明:CdSe量子点经过表面修饰后,两相电子注入机制--热电子和冷电子注入得以被证实,即:电子能分别从CdSe量子点导带中高的振动能级和导带底转移到TiO₂的导带。该机制详细描绘了电子在纳米界面间转移的图景。进一步研究发现:热电子注入的电子耦合强度(3.6±0.1 meV)比弛豫后的基态电子注入高两个数量级,基于Marcus理论,伴随着0.083 eV的重组能,冷电子注入的耦合强度值为~50 μeV。     

Silver nanoparticle coverage dependence of surface-enhanced Raman scattering

We report surface-enhanced Raman scattering (SERS) from 4-mercaptopyridine adsorbed on nanotextured silver surfaces as the coverage of silver is varied. The degree of surface enhancement is strongly dependent on silver coverage and correlated to the extinction of the surface at the Raman excitation wavelength, that extinction being determined by multiparticle surface plasmon resonances. The coverage dependence of the Raman intensity is consistent with signals being dominated by molecules at junctions inside nanoparticle aggregates where electromagnetic energy is localized into “hot spots” by interactions of the incident and scattered fields with the surface plasmons. The Raman intensity drops precipitously near the conductivity percolation threshold because these hot spots are destroyed when conducting paths allow plasmons to propagate. Our approach to substrate preparation provides clean surfaces with average enhancements ≥10⁷, an order of magnitude larger than typical for SERS. PACS 78.67.-n; 78.68.+m; 33.20.Fb     

Optically and thermally induced molecular switching processes at metal surfaces

Using light to control the switching of functional properties of surface-bound species is an attractive strategy for the development of new technologies with possible applications in molecular electronics and functional surfaces and interfaces. Molecular switches are promising systems for such a route, since they possess the ability to undergo reversible changes between different molecular states and accordingly molecular properties by excitation with light or other external stimuli. In this review, recent experiments on photo- and thermally induced molecular switching processes at noble metal surfaces utilizing two-photon photoemission and surface vibrational spectroscopies are reported. The investigated molecular switches can either undergo a trans-cis?isomerization or a ring opening-closure reaction. Two approaches concerning the connection of the switches to the surface are applied: physisorbed switches, i.e.?molecules in direct contact with the substrate, and surface-decoupled switches incorporated in self-assembled monolayers. Elementary processes in molecular switches at surfaces, such as excitation mechanisms in photoisomerization and kinetic parameters for thermally driven reactions, which are essential for a microscopic understanding of molecular switching at surfaces, are presented. This in turn is needed for designing an appropriate adsorbate-substrate system with the desired switchable functionality controlled by external stimuli.     

Chemical reactions on palladium surfaces studied with Pd-MOS structures

It is pointed out that metal-oxide-semiconductor structures can be used to stfidy catalytic reactions on metal surfaces (like Pd and Pt surfaces). The flatband voltage shift induced by hydrogen at the metal-oxide interface is a measure of the amount of hydrogen in the metal, which in turn reflects the chemical reactions on the surface. Experimental results on hydrogen in argon and in air detected with Pd-MOS structures are compared with absolute reaction rate theory. The agreement between theory and experiments is surprisingly good.     

Nonlocal dissociative chemistry of adsorbed molecules induced by localized electron injection into metal surfaces

We present a novel approach to surface chemistry studies using scanning tunneling microscopy (STM), where dissociation of molecules adsorbed on metal surfaces is induced nonlocally in a 10-100 nm radius around the STM tip by hot electrons that originate from the STM tip and transport on the surface. Nonlocal molecular excitation eliminates the influence of the STM tip on the outcome of the electron-induced chemical reaction. The spatial attenuation of the nonlocal reaction is used as a direct measure of hot-electron transport on the surface.     

On the role of electron-hole pair excitation in the kinetics of atom recombination at metal surfaces

The dynamics of energy relaxation of the adspecies in exoergic processes at metal surfaces has been modeled by means of the master equation approach. The effect of energy disposal to the solid via electron-hole (e-h) pair excitation on the rate of adatom recombination, has been investigated in the case of a discrete set of vibrational levels of the adspecies. The kinetics is solved, analytically, for two recombination channels and by taking into account two energy dissipation pathways of the adspecies. It is shown that dissipation pathways, characterized by a sizable energy transfer per scattering event, affect the kinetics leading to enhanced recombination rates. The kinetic model has been applied to describe experimental data on H(D)-adatom abstraction and recombination at metal surfaces. The rate coefficient of the process is shown to be proportional to the energy power transferred to the solid, owing to the reaction exothermicity, and correlates to the surface electron density.