Image-potential States Research Articles

Unoccupied electronic band structure of the semi-metallic Bi(111) surface probed with two-photon photoemission

While many photoemission studies have dealt with both the bulk band structure and various surface states and resonances, the unoccupied electronic structure above the Fermi level of the Bi(111) surface has not yet been measured directly although understanding of this model semi-metal is of great interest for topological insulators, spintronics, and related fields. We use angle-resolved two-photon photoemission to directly investigate the occupied and unoccupied $p$ bands of Bi, including the bulk hole pocket at the $T$ point, as well as the image potential states and surface states of Bi(111).

高エネルギー・高波数分解能二光子光電子分光：鏡像準位の純位相緩和測定

High energy-resolved (ΔE∼9.5 meV) two-photon photoemission was used to study the electron dynamics of the image potential states on the clean Cu(001) surface. We constructed a system consisting of a picosecond Ti:sapphire laser system and a hemispherical analyzer equipped with a two-dimensional electron detector to achieve high energy resolution. The image states of Cu(001) were clearly resolved up to n＝5, and the energy positions of these states were determined up to n＝7. Linewidth analysis yielded pure dephasing rate for the lowest image states.

Appearance of the minoritydz2surface state and disappearance of the image-potential state: Criteria for clean Fe(001)

The unoccupied surface electronic structure of clean and oxidized Fe(001) was studied with spin-resolved inverse photoemission and target current spectroscopy. For the clean surface, we detected a ${d}_{{z}^{2}}$ surface state with minority spin character just above the Fermi level, while the image-potential surface state disappears. The opposite is observed for the ordered $p$($1\ifmmode\times\else\texttimes\fi{}1$)O/Fe(001) surface: the ${d}_{{z}^{2}}$-type surface state is quenched, while the image-potential state shows up as a pronounced feature. This behavior indicates enhanced surface reflectivity at the oxidized surface. The appearance and disappearance of specific unoccupied surface states prove to be decisive criteria for a clean Fe(001) surface. In addition, enhanced spin asymmetry in the unoccupied states is observed for the oxidized surface. Our results have implications for the use of clean and oxidized Fe(001) films as spin-polarization detectors.

Lifetimes of electronic excitations in unoccupied surface states and the image potential states on Pd(110)

The contribution of inelastic electron-electron scattering to the decay rate of excitations in the surface states and first two image potential states at the Ȳ point on the surface is calculated in the GW approximation, and the quasi-momentum dependence of the corresponding contribution for the surface states is analyzed. The mechanisms of electron scattering in these states are studied, and the temperature dependence of the excitation lifetime is analyzed with allowance for the contribution of the electron-phonon interaction calculated earlier.

Molecular-Scale and Wide-Energy-Range Tunneling Spectroscopy on Self-Assembled Monolayers of Alkanethiol Molecules

The electronic properties of alkanethiol self-assembled monolayers (alkanethiolate SAMs) associated with their molecular-scale geometry are investigated using scanning tunneling microscopy and spectroscopy (STM/STS). We have selectively formed the three types of alkanethiolate SAMs with standing-up, lying-down, and lattice-gas phases by precise thermal annealing of the SAMs which are conventionally prepared by depositing alkanethiol molecules onto Au(111) surface in solution. The empty and filled states of each SAM are evaluated over a wide energy range covering 6 eV above/below the Fermi level (E(F)) using two types of STS on the basis of tunneling current-voltage and distance-voltage measurements. Electronic states originating from rigid covalent bonds between the thiol group and substrate surface are observed near E(F) in the standing-up and lying-down phases but not in the lattice-gas phase. These states contribute to electrical conduction in the tunneling junction at a low bias voltage. At a higher energy, a highly conductive state stemming from the alkyl chain and an image potential state (IPS) formed in a vacuum gap appear in all phases. The IPS shifts toward a higher energy through the change in the geometry of the SAM from the standing-up phase to the lattice-gas phase through the lying-down phase. This is explained by the increasing work function of alkanethiolate/Au(111) with decreasing density of surface molecules.

Temperature dependence of the dynamics of the first image-potential state on Ag(111)

The temperature dependence of the dynamics of electrons in the $n=1$ image potential state on the Ag(111) surface has been investigated by means of time-resolved two-photon photoemission spectroscopy and many-body calculations. We show that the decay rate of electrons in this state grows linearly with temperature. The thermal shortening of the lifetime is caused by the increase of the electron-electron scattering rate, due to deeper penetration of the image state wave function into the bulk metal at higher temperature. The electron-phonon scattering in this state is found to be small.

Quantum-well states with image state character for Pb overlayers on Cu(111)

We study theoretically the quantum well states (QWSs) localized in Pb overlayers on Cu(111) surface. Particular emphasis is given to the states with energies close to the vacuum level. Inclusion of the long-range image potential tail into the model potential description of the system allows us to show the effect of hybridization between QWSs and image potential states (ISs). The particle-in-a-box energy sequence characteristic for QWSs evolves into the Rydberg series converging towards the vacuum level. The electron density of the corresponding states is partially moved from inside the metal overlayer into the vacuum. The decay rates due to the inelastic electron-electron scattering decrease with increasing energy, opposite to ``conventional'' QWSs and similar to the ISs. Many-body and wave packet propagation calculations of the inelastic decay rates are supplemented by simple analysis based on the phase accumulation model and wave-function penetration approximation. This allows an analytical description of the dependence of the QWS/ISs hybridization on different parameters and, in particular, on the overlayer thickness.

Momentum-resolved lifetimes of image-potential states on Ag(001)

Binding energies and decay rates of image potential states at an Ag(001) surface have been investigated with time-resolved two-photon photoemission. For the first four image potential states the binding energies are determined to be 561, 170, 72 and 37meV with respect to the vacuum level. Lifetimes of the first three states are extracted to be 57, 165 and 380fs at k∥=0. With increasing parallel momentum k∥ the lifetime of the n=1 state decreases such that the inverse lifetime scales proportional to the kinetic energy within the parallel motion with 34meV/eV. As light source for the pump-probe photoemission experiments a novel all fiber based femtosecond laser system with a repetition rate of 1.5MHz has been used.

Digital Image Correlation with Self-Adaptive Gaussian Windows

A novel subpixel registration algorithm with Gaussian windows is put forward for accurate deformation measurement in digital image correlation technique. Based on speckle image quality and potential deformation states, this algorithm can automatically minimize the influence of subset sizes by self-adaptively tuning the Gaussian window shapes with the aid of a so-called weighted sum-of-squared difference correlation criterion. Numerical results of synthetic speckle images undergoing in-plane sinusoidal displacement fields demonstrate that the proposed algorithm can significantly improve displacement and strain measurement accuracy especially in the case with relatively large deformation.

Tubular image states: General formulation and properties for metallic and nonmetallic nanotubes

In this work we study the existence of image potential states around nanotubes by considering the interaction between a stationary charged particle and a polarizable cylinder of infinite length. For metallic nanotubes and wires we obtain the eigenstates and eigenenergies corresponding to nonzero angular momentum states, localized at a relatively long distance from the surface of the tube. We study the scaling properties of the system in order to define general conditions for which image states can be supported. We also analyze the case of nonmetallic carbon nanotubes, for which we do not obtain bound states.

Momentum-resolved electron dynamics of image-potential states on Cu and Ag surfaces

The dependence of the inelastic lifetime of electrons in the first $n=1$ image-potential state of clean and rare-gas covered Ag(111), Cu(111), and Cu(100) surfaces on their momentum parallel to the surface has been studied experimentally by means of time- and angle-resolved two-photon photoemission spectroscopy (2PPE) and theoretically by calculations based on the many-body theory within the self-energy formalism. Similar to the previously studied clean Cu(100) surface, the theoretical results are in excellent agreement with the experiment findings for Cu(111). For Ag(111), the theory overestimates the decay rate and its momentum dependence, which is attributed to the neglect of surface plasmon excitations. With increasing parallel momentum, the $n=1$ state shifts out of the projected bulk band gap on both surfaces and turns into an image-potential resonance. This opens an additional decay channel by resonant electron transfer into the bulk, which is theoretically treated by the application of the wave packet propagation approach. The expected stronger increase of the decay rate upon crossing the edge of the band gap, however, is not observed in the experiment. The decoupling of the image-potential states from the metal surface upon adsorption of rare-gas layers results in a decrease of the decay rate as well as of its momentum dependence by a similar factor, which can be successfully explained by the change of interband and intraband contributions to the total decay rate.

Charge Separation at the Molecular Monolayer Surface: Observation and Control of the Dynamics.

Charge separation dynamics relevant to an electron transfer have been revealed by time- and angle-resolved two-photon photoemission spectroscopy for an n-alkanethiolate self-assembled monolayer (SAM) on a Au(111) surface fabricated by a chemical-wet process. The electron was photoexcited into an image potential state located at 3.7 eV above the Fermi level (EF), and it survived well for more than 100 ps on dodecanethiolate (C12)-SAM. The degree of electron separation is precisely controlled by selecting the length of the alkyl chain (C10-C18). We have also evaluated molecular conductivity at the specific electron energy of EF + 3.7 eV. The tunneling decay parameter, β, was fitted by β90K = 0.097 Å(-1) and βRT = 0.13 Å(-1). These values were one order smaller than that at around EF by conventional contact probe methods.

Two-dimensional band dispersion and momentum-resolved lifetime of the image-potential state on graphite studied by angle-resolved multiphoton photoemission spectroscopy

We studied the image-potential state (IPS) on a highly oriented pyrolitic graphite surface by means of angle-resolved multiphoton photoemission spectroscopy. The free-electron--like dispersion of IPSs with $n$ = 1, 2, and 3 is observed for the first time with high energy and momentum resolution. From the dependence of the spectral intensity on the excitation density and comparisons with the energy diagram, it is suggested that IPSs are populated by the ${k}_{/\phantom{\rule{-0.16em}{0ex}}/}$-conserved transition from the \ensuremath{\sigma}-band and the indirect transition from the \ensuremath{\pi}-band. The linear dependence of the intrinsic line width on the energy suggests a contribution of intraband relaxation within the IPS band.

Image Potential State Mediated Excitation at Rubrene/Graphite Interface

We report an electronic excitation of adsorbed molecules mediated by the image potential state (IPS) on the substrate. Two-photon photoemission (2PPE) spectroscopy for rubrene films formed on a highly oriented pyrolytic graphite (HOPG) surface reveals a prominently enhanced peak due to an unoccupied molecular level (denoted as Ln), which is resonantly excited from the highest occupied molecular orbital (HOMO) derived level. The enhancement of the Ln peak becomes less significant at the coverage higher than 1 monolayer (ML), where the IPS peak on the substrate disappears. The resonance enhancement is moderate with s-polarization, by which the transition to IPS is completely suppressed. We ascribe that the excitation of the Ln level is mediated by the IPS on HOPG. Though the IPS wave function extends outside the molecules, it interacts with the unoccupied molecular orbital at the edges of molecular islands, causing the strong resonance enhancement of the Ln level.

Lateral Quantization of Two-Dimensional Electron States by Embedded Ag Nanocrystals

We show that quantization of image-potential state (IS) electrons above the surface of nanostructures can be experimentally achieved by Ag nanocrystals that appear as stacking-fault tetrahedrons (SFTs) at Ag(111) surfaces. By means of cryogenic scanning tunneling spectroscopy, the n=1 IS of the Ag(111) surface is revealed to split up in discrete energy levels, which is accompanied by the formation of pronounced standing wave patterns that directly reflect the eigenstates of the SFT surface. The IS confinement behavior is compared to that of the surface state electrons in the SFT surface and can be directly linked to the particle-in-a-box model. ISs provide a novel playground for investigating quantum size effects and defect-induced scattering above nanostructured surfaces.

Trapping surface electrons on graphene layers and islands

We report the use of time- and angle-resolved two-photon photoemission to map the bound, unoccupied electronic structure of the weakly coupled graphene/Ir(111) system. The energy, dispersion, and lifetime of the lowest three image-potential states are measured. In addition, the weak interaction between Ir and graphene permits observation of resonant transitions from an unquenched Shockley-type surface state of the Ir substrate to graphene/Ir image-potential states. The image-potential-state lifetimes are comparable to those of mid-gap clean metal surfaces. Evidence of localization of the excited electrons on single-atom-layer graphene islands is provided by coverage-dependent measurements.

Particle-in-a-box confinement of electron gases in and above the surface of self-organized Co nanowires at Au(111) step edges

We report on confinement of the series of two-dimensional (2D) electron gases provided by image-potential states (ISs) above wire-like Co nanoparticles that are formed at Au(111) step edges upon Co atom deposition. Maps of the local density of states recorded by means of scanning tunneling microscopy reveal the formation of standing wave (SW) patterns at the Co nanowire surface caused by effective scattering at its boundaries. Due to the simple rectangular shape of the wire-like Co nanoparticles, SW patterns, which reflect the eigenfunctions of the particles, are unambiguously resolved. Quantitative comparison to the confinement of Co surface state electrons reveals that the IS electrons follow a similar particle-in-a-box type behavior. These findings open new perspectives for investigation of quantum size effects of 2D electron systems above nanostructured surfaces.

Time-Resolved Two-Photon Photoemission of Unoccupied Electronic States of Periodically Rippled Graphene on Ru(0001)

The unoccupied electronic states of epitaxially grown graphene on Ru(0001) have been explored by time- and angle-resolved two-photon photoemission. We identify a Ru derived resonance and a Ru/graphene interface state at 0.91 and 2.58eV above the Fermi level, as well as three image-potential derived states close to the vacuum level. The most strongly bound, short-lived, and least dispersing image-potential state is suggested to have some quantum-well character with a large amplitude below the graphene hills. The two other image-potential states are attributed to a series of slightly decoupled states. Their lifetimes and dispersions are indicative of electrons moving almost freely above the valley areas of the moiré superstructure of graphene.

Temperature Dependent Structural Phase Transition at the Perfluoropentacene/Ag(111) Interface

Monolayers of perfluoropentacene (PFP) on Ag(111) undergo a reversible structural transition from a crystalline (6 3) phase at low temperatures to a disordered phase at room temperature. Two-photon photoemission (2PPE) of the low temperature phase revealed a distinct signal of the first image-potential state (n = 1) of Ag(111), thus showing the presence of bare substrate regions between the crystalline PFP islands. At higher temperatures, the entire surface is covered by a disordered phase of diffusing molecules causing a complete suppression of the (n = 1) signal and the electron diffraction (LEED) pattern. X-ray absorption spectroscopy showed that neither a change of the weak chemical interaction nor a change of the molecular tilt angle occurs during this phase-transition. The quantitative analysis of the 2PPE signal and the LEED contrast yields a transition temperature of TC = 145 ( 5 K. The present experiment shows that desorption of excess multilayers leads to an effective coverage of less than a dense monolayer.

Dispersions of image potential states on surfaces of clean graphite and lead phthalocyanine film

Dispersions of image potential states on a graphite surface (denoted IPS1) and on 1 monolayer (ML) film (denoted IPS2) of lead phthalocyanine (PbPc) are investigated by the micro-spot angle-resolved two-photon photoemission (micro-AR-2PPE) spectroscopy. On the graphite surface, whole dispersions of the two members of IPS1 (n = 1 and 2) are observed. The n = 1 IPS1 peak is weakly visible at energy higher than the vacuum level. The effective mass of an electron in the n = 1 IPS1 becomes slightly light at the high momentum region, suggesting the interaction between the IPS1 and the unoccupied σ-band of graphite. On the PbPc film, the IPS2 band forms a band gap and back-folds at the boundary of the Brillouin zone. A 1-dimensional Kronig-Penny model is used to reproduce the effective mass and the shift of binding energy.