Surface plasmon assisted photoemission from Au nanoparticles on graphite

The resonant multiple excitation of collective modes in metallic nanoparticles using ultrashort laser pulses leads to an enhanced multiphoton photoemission from the particles. This effect is here demonstrated for the surface-plasmon resonance of Au nanoparticles on graphite. The shape of the photoemission spectra is explained by multiphoton photo-assisted thermionic emission from the nanoparticles and resonant emission via the image-potential state on graphite. Tuning the photon energy between 1.7 eV and 3.2 eV allows the identification of an enhancement of the photoemission yield at 2.1±0.1-eV photon energy that is attributed to the resonant excitation of the surface plasmon in the Au nanoparticles. This identification of the surface-plasmon excitation in this energy range is also supported by electron energy loss spectroscopy. Received: 8 August 2001 / Revised version: 13 September 2001 / Published online: 10 October 2001     

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     

Electron dynamics in supported metal nanoparticles: relaxation and charge transfer studied by time-resolved photoemission

Resonant excitation of the plasmon polariton in supported nanoparticles leads to a strong enhancement of the multiphoton photoemission yield and consequently, the total yield is dominated by the emission from the nanoparticles although they cover only a minor fraction of the surface. This allows investigation of the electron dynamics in supported nanoparticles, directly in the time domain. Here, Ag nanoparticles grown on graphite are used to demonstrate that the transient shape of the photoemission spectrum in time-resolved two-color multiphoton photoemission spectroscopy, reveals the electron relaxation within the nanoparticle, and the dynamic charge transfer between substrate and nanoparticle. The photoemission spectra map the transient electron energy distribution and exhibit a transient shift that is attributed to a dynamic charging of the nanoparticle. The comparison with model calculations comprising the full relaxation cascade in the nanoparticle and substrate, shows that the dynamic charge transfer accounts for almost half of the total deposited energy in the nanoparticle. PACS 78.67.Bf; 73.22.-f; 67.40.Fd; 78.47.+p     

Nonlinear Excitation of Tryptophan Emission Enhanced by Silver Nanoparticles

We measured and analyzed the behavior of the fluorescence of tryptophan water solutions with and without silver nanoparticles, excited by one, two and three photon processes. Two different colloids with silver nanoparticles with distinct diameters (0.65 nm and 9 nm) were used in the experiments. Fluorescence quenching was observed with one and two photon excitation. However, upon three-photon excitation, significant fluorescence enhancement was observed in the colloid. In this case excitation of the amino acid is assisted by the nonlinear absorption of infrared light by the silver nanoparticles. In this paper we are proposing a new way to explore metallic nanoparticles to enhance autofluorescence of biomolecules.     

Phase propagation of localized surface plasmons probed by time-resolved photoemission electron microscopy

In combining time-resolved two-photon photoemission (TR-2PPE) and photoemission electron microscopy (PEEM) the ultra-fast dynamics of collective electron excitations in silver nanoparticles (localized surface plasmons – LSPs) is probed at fs and nm resolution. Here we demonstrate that the sampling of the LSP dynamics by means of time-resolved PEEM enables detailed insight into the propagation processes associated with these excitations. In phase-integrated as well as phase-resolved measurements we observe spatio-temporal modulations in the photoemission yield from a single nanoparticle. These modulations are assigned to local variations in the electric near field as a result of the phase propagation of a plasmonic excitation through the particle. Furthermore, the control of the phase between the fs pump and probe laser pulses used for these experiments can be utilized for an external manipulation of the nanoscale electric near-field distribution at these particles. PACS 78.47.+p; 78.67.Bf; 79.60.-i; 73.20.Mf     

Interatomic internal photoemission peaks in (Auger) electron spectra

Internal photoemission (IPE) features appearing in (Auger) electron spectra have two distinguishing properties: (1) they obey optical selection rules and (2) IPE intensities increase rapidly with incident excitation electron energy. IPE and Auger electrons from elemental materials have similar energies, giving complicated mixed spectra; however, core-level interatomic IPE peaks (from compounds) can appear by themselves, as shown here using SiO ₂. The IPE process has high photon utilization efficiency, and core interatomic IPE shows promise for certain specialized applications discussed in the text.     

The energy dependence of photoelectron peak intensities,Part I: Experimental methods

With the increasing popularity of synchrotron radiation as a tunable photon source for photoemission spectroscopy, the significance of photoelectron peak variation with photon energy is becoming apparent. This variation is due to a number of intrinsic factors (excitation cross section, photon and electron reflection and refraction, inelastic processes, etc.) as well as instrumental factors (monochromator transmission and spectral purity, electron analyzer transmission and resolution, etc.). In this paper we develop methods for factoring out these contributions with the goal of determining the excitation cross section from a peak area measurement. Parts II and III will deal with a range of cross section measurements, and the use of relative peak area information to determine stoichiometry vs. probe depth.     

Resonant photoemission spectroscopy of Cu(InGa)Se2 materials for solar cells

The electron structure of CuIn_{1 ? x} Ga _x Se₂ single crystals is determined via resonant photoemis-sion and the main regularities of its transformation upon varying concentration x from 0 to 1 are established. The dependence of the shape of valence band spectra on the photon energy is studied. Integral photoemission intensities are shown to be determined by atomic photoionization cross sections. Processes of the direct and two-step creation of photoelectrons accompanying photoemission and the participation of internal states in the spectra of electrons from valence bands are studied. Two-hole final states in photoemission are obtained upon threshold excitation of the Cu 2p level. The strong interaction of holes leads to the multiplet splitting of these states. Partial densities of the components’ states are determined using the energy dependence of atomic photoionization cross sections.     

Pure and Sb-doped SnO 2 nanoparticles studied by photoelectron spectroscopy

We describe photoemission results from pure and Sb-doped SnO₂ nanoparticles deposited on gold substrates. Photoelectron spectra with synchrotron radiation were recorded for Sn 3d, Sb 3d and O 1s core levels and valence bands in the 500-1200 eV energy range. For pure SnO₂ nanoparticles the surface is terminated by an oxygen rich layer with no obvious surface environment for Sn. When doped n-type with 9.1% or 16.7% Sb, dopant atoms are concentrated near the surface of the nanoparticles. The valence state of the dopant atoms is predominantly Sb^V. Plasmon satellite features are also observed in core level photoemission spectra and their intensity relative to the main peak increases with increasing photon energy. Received 30 November 2000     

Resonant valence band satellite in palladium

Normal incidence photoemission has been used to investigate the existence of a valence band satellite as in the case of Ni. It is observed at 8.5 eV below the Fermi level and has an intensity which is only measurable when a photon energy is reached which corresponds to the threshold for excitation of an electron from the 4p levels to the Fermi level.     

Photoemission from laser excited semiconductors: Dynamic response of the electronic structure in Si

Using angle-resolved photoemission in coincidence with a pulsed Nd:Yag laser beam (hν_L = 2.33 eV), we have studied the dynamic rearrangement of the electronic states in Si in the presence of a large number of electron hole pairs created by laser photon excitation. We observed considerable changes in the valence band structure, a renormalization of the bandgap, and a satellite peak in the 2p core level emission due to more effective core hole screening.     

Charging mechanisms of trapped element-selectively excited nanoparticles exposed to soft x rays

Charging mechanisms of trapped, element-selectively excited free SiO2 nanoparticles by soft x rays are reported. The absolute charge state of the particles is measured and the electron emission probability is derived. Changes in electron emission processes as a function of photon energy and particle charge are obtained from the charging current. This allows us to distinguish contributions from primary photoelectrons, Auger electrons, and secondary electrons. Processes leading to no change in charge state after absorption of x-ray photons are identified. O 1s-excited SiO2 particles of low charge state indicate that the charging current follows the inner-shell absorption. In contrast, highly charged SiO2 nanoparticles are efficiently charged by resonant Auger processes, whereas direct photoemission and normal Auger processes do not contribute to changes in particle charge. These results are discussed in terms of an electrostatic model.     

Absolute phase effect in ultrafast optical responses of metal nanostructures

We predict that nonlinear ultrafast electron photoemission by strong optical fields and, potentially, other nonlinear optical responses of metal nanostructures significantly depend on the absolute (carrier–envelope) phase of excitation pulses. Strong enhancement of the local optical fields produces these responses at excitation intensities lower by order(s) of magnitude than for known systems. Prospective applications include control of ultrafast electron emission and electron injection into nanosystems. A wider class of prospective applications is the determination of the absolute phase of pulses emitted by lasers and atoms, molecules, and condensed matter at relatively low intensities. PACS  78.67.-n; 78.47.+p; 79.60.Jv; 73.20.Mf     

Layer-resolved photoelectron diffraction from Si(0 0 1) and GaAs(0 0 1)

Photoelectron diffraction in the layer-resolved mode brings more detailed information about local atomic arrangement than is obtained in the standard mode. This is demonstrated in crystals with diamond and zinc-blende structures, both for unpolarized photon excitation as well as for circularly polarized excitation. The full angular distributions of photoemission intensities are evaluated for large atomic clusters representing ideally truncated surfaces of Si(0 0 1) and GaAs(0 0 1). Highly structured layer-resolved patterns enable a more detailed understanding of the standard mode outcomes. Photoelectron intensities from atomic layers placed at different depths under the crystal surface provide direct evidence about electron attenuation and its anisotropy in crystals.     

Laser induced threshold photoemission magnetic circular dichroism and its application to photoelectron microscope

This work enlightens the threshold photoemission magnetic circular dichroism (MCD) and its adaption on photoemission electron microscopy (PEEM) using lasers. MCD is a simple and efficient way to investigate magnetic properties since it does not need any spin analyzers with low efficiency, and thus the MCD related techniques have developed to observe magnetic domains. Usually, MCD in a total yield measurement in the valence band with weak spin–orbit coupling (SOC) excited by low photon energy (hν≤ 6 eV) does not compete with the X-ray magnetic circular dichroism (XMCD) with strong SOC. XMCD PEEM observation of magnetic domains has been successfully established while MCD PEEM derived from valence bands has not been. However, using angle and energy resolved photoelectron, valence band MCD provides large asymmetry similar to that by XMCD. Threshold measurement of photoelectron in a total electron yield procedure can take advantage of the measurement of photoelectrons with a limited angle and energy mode. This restriction of the photoelectron makes the threshold MCD technique an efficient way to get magnetic information and gives more than 10% asymmetry for Ni/Cu(0 0 1), which is comparable to that obtained by angle resolved photoemission. Thus the threshold MCD technique is a suitable method to observe magnetic domains by PEEM. For threshold MCD, incident angle dependence and high sensitivity to out-of-plane magnetized films compared with in-plane ones are discussed. Ultrashort pulse lasers make it feasible to measure two photon photoemission MCD combined with PEEM, where resonant excitation has a possibility to enhance dichroic asymmetry. Recent results for valence band magnetic dichroism PEEM are presented.     

Photoemission of molecular adsorbates

In photoemission the observed changes in band intensities as a function of light polarization, electron emission direction, and photon energy can be used to deduce information, for example, about geometry, two-dimensional band structures, intermolecular interactions and chemical reactivity of molecular adsorbates. To characterize the ion states of molecular adsorbates in the so-called inner valence electron region we discuss results from autoionization spectroscopy as a method that yields complementary information to photoemission in this spectral region. We use results on CO, N₂, and CO₂ adsorbates to discuss the various aspects of photoemission of molecular adsorbates.     

Angular resolved studies of the photoemission bands from carbon monoxide adsorbed on palladium single crystals,and the assignment of these bands

The photoemission from CO adsorbed on a Pd(111) surface has been studied as a function of electron exit angle and of photon incidence angle; some studies have also been made using Pd(100) and Pd(110). The variations of intensity of the two photoemission bands support the sequence of levels 1π > 5θ ? 4θ for adsorbed CO. The adsorption on some other metals is discussed briefly.     

Enhancement of ultrafast nonlinear optical response of silicon nanocrystals by boron-doping

Nonlinear optical responses of boron (B)-doped silicon nanocrystals (Si-ncs) embedded in borosilicate glass were studied by z-scan and optical Kerr gate methods under femtosecond excitation at 780 nm. The nonlinear refractive index (n(2)) and the two photon absorption coefficients (β) of B-doped Si-ncs were found to be 3 times enhanced, compared to those of intrinsic Si-ncs. The response time was faster than 100 fs even at 5 K. The origin of the large nonlinear optical response was discussed, based on the experimental data of n(2), electron spin resonance spectra, and linear absorption spectra.     

基于深紫外激光-光发射电子显微技术的高分辨率磁畴成像

基于磁二色效应的光发射电子显微镜磁成像技术是研究薄膜磁畴结构的一种重要研究手段,具有空间分辨率高、可实时成像以及对表面信息敏感等优点.以全固态深紫外激光(波长为177.3 nm;能量为7.0 eV)为激发光源的光发射电子显微技术相比于传统的光发射电子显微镜磁成像技术(以同步辐射光源或汞灯为激发源),摆脱了大型同步辐射光源的限制;同时又解决了当前阈激发研究中由于激发光源能量低难以实现光电子直接激发的技术难题,在实验室条件下实现了高分辨磁成像.本文首先对最新搭建的深紫外激光-光发射电子显微镜系统做了简单介绍.然后结合超高真空分子束外延薄膜沉积技术,成功实现了L10-FePt垂直磁各向异性薄膜的磁畴观测,其空间分辨率高达43.2 nm,与利用X射线作为激发源的光发射电子显微镜磁成像技术处于同一量级,为后续开展高分辨磁成像提供了便利.最后,重点介绍了在该磁成像技术方面取得的一些最新研究成果:通过引入Cr的纳米"台阶",成功设计出FePt的(001)与(111)双取向外延薄膜;并在"台阶"区域使用线偏振态深紫外激光观测到了磁线二色衬度,其强度为圆二色衬度的4.6倍.上述研究结果表明:深紫外激光-光发射电子显微镜磁成像技术在磁性薄膜/多层膜体系磁畴观测方面具备了出色的分辨能力,通过超高真空系统与分子束外延薄膜制备系统相连接,可以实现高质量单晶外延薄膜制备、超高真空原位传输和高分辨磁畴成像三位一体的功能,为未来磁性薄膜材料的研究提供了重要手段.     

The influence of striction density perturbations on the excitation of plasma oscillations inside thermal inhomogeneities of plasma

In this paper, we continue our studies begun in [Izv. Vyssh. Uchebn. Zaved., Radiofiz.,41, No. 3, 270 (1998)].Calculating the coefficients and nonlinear phase shift in the equation for plasma-wave intensity introduced in the eralier paper, we have solved the problem of the influence of striction perturbations of the plasma density on the excitation of shortwave plasma oscillations by an electromagnetic wave; the above oscillations are captured in a volume of inhomogeneities, which are extended along the magnetic field and have reduced electron density that crosses the level of the upper-hybrid resonance. The dissipative processes of absorption and emission of plasma waves beyond the inhomogeneity are assumed to be weak. The variation of excitation and reflection of plasma waves from the resonance level due to deformation of the plasma- density profile is described. The band of effective generation of eigenmodes of captured oscillations as a function of the total wave-phase increment in an inhomogeneity is determined. The effect of penetration of the field of a high-power plasma wave into the non-transmittance region as a result of the striction expulsion of plasma is calculated. With allowance for the nonlinear phenomena in question, we estimated the heating of artificial inhomogeneities of thermal origin as a result of collisional absorption of the plasma oscillations excited in a volume of inhomogeneities under the action of a high-power radio wave. The materials of this paper were reported at the IIIrd International School on Space Plasma Physics. Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation, Russian Academy of Sciences, Troitsk, Moscow Region, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 10, pp. 1226–1247, October 1998.