Angle-resolved Two-photon Photoemission Research Articles

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.

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.

Coverage-dependent adsorption geometry of octithiophene on Au(111)

The adsorption behavior of α-octithiophene (8T) on the Au(111) surface as a function of 8T coverage has been studied with low-temperature scanning tunneling microscopy, high resolution electron energy loss spectroscopy as well as with angle-resolved two-photon photoemission and ultraviolet photoemission spectroscopy. In the sub-monolayer regime 8T adopts a flat-lying adsorption geometry. Upon reaching the monolayer coverage the orientation of 8T molecules changes towards a tilted configuration, with the long molecular axis parallel to the surface plane, facilitating attractive intermolecular π-π-interactions. The photoemission intensity from the highest occupied molecular orbitals (HOMO and HOMO - 1) possesses a strong dependence on the adsorption geometry due to the direction of the involved transition dipole moment for the respective photoemission process. The change in molecular orientation as a function of coverage in the first molecular layer mirrors the delicate balance between intermolecular and molecule/substrate interactions. Fine tuning of these interactions opens up the possibility to control the molecular structure and accordingly the desirable functionality.

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.

The observation of unoccupied quantum-well states in Bi thin film grown on Si(111) by two-photon photoemission spectroscopy

We have performed angle-resolved two-photon photoemission (AR-2PPE) measurements of Bi thin film with the thickness of 12 bilayers grown on Si(111) surface. From the excitation photon-energy dependence of AR-2PPE spectra, we have obtained the energy dispersion of unoccupied electronic states in the energy range between 2.0 and 3.0 eV above the EF. The observed electronic states are attributed to the unoccupied quantum-well states due to quantization of the 6 p-derived bulk bands along the Γ-T direction.

Band structure effects in above threshold photoemission

We have performed an angle-resolved two-photon and three-photon photoemission study of the Ag(111) surface employing ultrashort laser pulses as the excitation source. We show the presence of multi-photon resonances between electronic states at selected points of the Brillouin zone which appear in the high-order photoemission spectral region. We observe clear signatures of electronic band structure effects of the Ag crystal in above-threshold photoemission (ATP) processes, identifying two types of transitions, which either proceed via non-resonant multi-photon excitation from an occupied initial state, or involve a real intermediate state located above the vacuum level of the solid directly influencing the ATP process. For this latter class of phenomena, we suggest that electron populations created by incoherent processes give a contribution to the multi-photon transition, possibly allowing us to trace the transmission of photoexcited electrons through the crystal.

Resonance and localization effects at a dipolar organic semiconductor interface

The image state manifold of the dipolar organic semiconductor vanadyl naphthalocyanine (VONc) on highly oriented pyrolytic graphite is investigated by angle-resolved two-photon photoemission (AR-TPPE) spectroscopy in the 0-1 monolayer regimes. Interfacial charge-transfer from the image potential state of clean graphite populates a near-resonant VONc anion level, identifiable by the graphite image potential state by its distinct momentum dispersion obtained from AR-TPPE. This affinity level is subject to depolarization by the neighboring molecules, resulting in stabilization of this state with coverage. Near a coverage of one monolayer, a hybrid image potential/anion state is also formed, showing progressive localization with coverage. Intensities for all these features develop rather differently with molecular coverage, pointing towards the different types of charge-transfer interactions at play at this interface.

Communication: Momentum-resolved quantum interference in optically excited surface states

Surface states play essential roles in condensed matter physics, e.g., as model two-dimensional (2D) electron gases and as the basis for topological insulators. Here, we demonstrate quantum interference in the optical excitation of 2D surface states using the model system of C(60)/Au(111). These surface states are transiently populated and probed in a femtosecond time- and angle-resolved two-photon photoemission experiment. We observe quantum interference within the excited populations of these surface states as a function of parallel momentum vector. Such quantum interference in momentum space may allow one to control 2D transport properties by optical fields.

Electronic states of Cu(110) investigated with angle-resolved two-photon photoemission spectroscopy

Occupied and unoccupied electronic states of Cu(110) were investigated with angle-resolved two-photon photoemission spectroscopy with an energy resolution of 30 meV. The high energy resolution enables the individual $n$ $=$ 1 and 2 image-potential-derived structures to be distinguished. The $n$ $=$ 1 structure was 0.85 eV below ${E}_{\mathrm{vac}}$ at $\mathrm{\ensuremath{\Gamma}\ifmmode \bar{}\else \={}\fi{}}$. The effective mass of the electron for the structure was the same for both dispersions along the $\mathrm{\ensuremath{\Gamma}\ifmmode \bar{}\else \={}\fi{}}\mathrm{X\ifmmode \bar{}\else \={}\fi{}}$ and $\mathrm{\ensuremath{\Gamma}\ifmmode \bar{}\else \={}\fi{}}\mathrm{Y\ifmmode \bar{}\else \={}\fi{}}$directions within the experimental error. The $n$ $=$ 2 structure appeared in the ${L}_{{2}^{\ensuremath{'}}}$-${L}_{1}$ gap along the $\mathrm{\ensuremath{\Gamma}\ifmmode \bar{}\else \={}\fi{}}\mathrm{Y\ifmmode \bar{}\else \={}\fi{}}$ direction, and it did not appear at $\mathrm{\ensuremath{\Gamma}\ifmmode \bar{}\else \={}\fi{}}$, or along the $\mathrm{\ensuremath{\Gamma}\ifmmode \bar{}\else \={}\fi{}}\mathrm{X\ifmmode \bar{}\else \={}\fi{}}$ direction, which means that the $n$ $=$ 2 structure was strongly affected by the bulk band. In this paper, other two-photon photoemission structures and dispersions are compared with results reported in the previous literature.

Two-Photon Photoemission Study of the Coverage-Dependent Electronic Structure of Chemisorbed Alkali Atoms on a Ag(111) Surface

We report a systematic investigation of the electronic structure of chemisorbed alkali atoms (Li-Cs) on a Ag(111) surface by two-photon photoemission spectroscopy. Angle-resolved two-photon photoemission spectra are obtained for 0-0.1 monolayer coverage of alkali atoms. The interfacial electronic structure as a function of periodic properties and the coverage of alkali atoms is observed and interpreted assuming ionic adsorbate/substrate interaction. The energy of the alkali atom σ-resonance at the limit of zero coverage is primarily determined by the image charge interaction, whereas at finite alkali atom coverages, it follows the formation of a dipolar surface field. The coverage- and angle-dependent two-photon photoemission spectra provide information on the photoinduced charge-transfer excitation of adsorbates on metal surfaces. This work complements the previous work on alkali/Cu(111) chemisorption [Phys. Rev. B 2008, 78, 085419].

Quantum confinement and anisotropy in thin-film molecular semiconductors

Unoccupied electronic states are probed in epitaxial films of metal phthalocyanines on Ag(111) using angle-resolved two-photon photoemission spectroscopy. Quantum confinement of an image state wave function results in band folding and the opening of a band gap. Angle-resolved photoemission intensity measurements resolve the probability density of the wave function, and are able to identify the symmetry of each band. These results are compared with predictions from a two-dimensional scattering model, and they point to the importance of the spatial extent and symmetry of functional groups in predicting and controlling electronic structure on surfaces and in molecular thin films.

Electronic structure and electron dynamics at an organic molecule/metal interface: interface states of tetra-tert-butyl-imine/Au(111)

Time- and angle-resolved two-photon photoemission (2PPE) spectroscopies have been used to investigated the electronic structure, electron dynamics and localization at the interface between tetra-tert-butyl imine (TBI) and Au(111). At a TBI coverage of one monolayer (ML), the two highest occupied molecular orbitals, HOMO and HOMO-1, are observed at an energy of −1.9 and −2.6 eV below the Fermi level (EF), respectively, and coincide with the d-band features of the Au substrate. In the unoccupied electronic structure, the lowest unoccupied molecular orbital (LUMO) has been observed at 1.6 eV with respect to EF. In addition, two delocalized states that arise from the modified image potential at the TBI/metal interface have been identified. Their binding energies depend strongly on the adsorption structure of the TBI adlayer, which is coverage dependent in the submonolayer (⩽1 ML) regime. Thus the binding energy of the lower interface state (IS) shifts from 3.5 eV at 1.0 ML to 4.0 eV at 0.5 ML, which is accompanied by a pronounced decrease in its lifetime from 100 fs to below 10 fs. This is a result of differences in the wave function overlap with electronic states of the Au(111) substrate at different binding energies. This study shows that in order to fully understand the electronic structure of organic adsorbates at metal surfaces, not only adsorbate- and substrate-induced electronic states have to be considered but also ISs, which are the result of a potential formed by the interaction between the adsorbate and the substrate.

Ultrafast Magnon Generation in an Fe Film on Cu(100)

We report on a combined experimental and theoretical study of the spin-dependent relaxation processes in the electron system of an iron film on Cu(100). Spin-, time-, energy- and angle-resolved two-photon photoemission shows a strong characteristic dependence of the lifetime of photoexcited electrons on their spin and energy. Abinitio calculations as well as a many-body treatment corroborate that the observed properties are determined by relaxation processes involving magnon emission. Thereby we demonstrate that magnon emission by hot electrons occurs on the femtosecond time scale and thus provides a significant source of ultrafast spin-flip processes. Furthermore, engineering of the magnon spectrum paves the way for tuning the dynamic properties of magnetic materials.

Band structure dependence of hot-electron lifetimes in a Pb/Cu(111) quantum-well system

The band structure dependence of the inelastic lifetime of electrons is investigated in a Pb quantum-well system on a Cu(111) substrate with femtosecond time- and angle-resolved two-photon photoemission. For a single monolayer of Pb on Cu(111), we find an unoccupied quantum-well state with a free-electron-like parabolic dispersion around the $\overline{\ensuremath{\Gamma}}$ point and with negative dispersion for finite momentum. Our investigation of this state is a momentum-dependent study of hot-electron lifetimes in a quantum-well system. We demonstrate the importance of intrasubband-scattering processes for the decay of hot electrons at finite momentum. Furthermore, we find that the competition between intersubband- and intrasubband-scattering processes directly induces a momentum anisotropy in the hot-electron lifetimes. This momentum anisotropy is strongly dependent on the specific electronic band dispersion. We compare our findings with previous investigations of ultrafast electron dynamics in model-like surface-state systems and with ultrafast electron dynamics in Pb full-bulk material. Our findings suggest that the peculiar electronic structure of quantum-well systems can be used to tune ultrafast dynamical properties in metals.

時間・角度分解２光子光電子光法によるCu(111)表面電子状態の研究(II)

時間・角度分解２光子光電子光法によるCu(111)表面電子状態の研究(II)

Dynamics of bulk-to-surface electron transitions onSi(001)−(2×1)studied by time-resolved two-photon photoemission spectroscopy

The carrier dynamics on $\text{Si}(001)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$ have been studied by means of time- and angle-resolved two-photon photoemission spectroscopies at 291 K. Temporal changes in electron populations at the normally unoccupied surface band $({D}_{\text{down}})$ and at the bulk conduction-band minimum (CBM) have been probed simultaneously to characterize the pathways of bulk-to-surface electron transfer. The decay kinetics of the electron population at the bottom of ${D}_{\text{down}}$ and at the CBM, obtained at two different excitation wavelengths, where the bulk absorption coefficient differs significantly, show a strong dependence on wavelength. Analysis shows that the dependence of the population at the CBM is characterized by carrier diffusion away from the surface and by rapid surface recombination, with a surface-recombination velocity of $1.2\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }\text{cm}/\text{s}$. The surface-recombination pathway from the CBM to the high-lying state along the $\overline{\ensuremath{\Gamma}}\ensuremath{-}{\overline{J}}^{\ensuremath{'}}$ dispersive branch of the ${D}_{\text{down}}$ band has been directly probed by an angle-resolved technique. Quantitative comparisons of electron flows into the ${D}_{\text{down}}$ band and the decay kinetics of the population of the bottom of ${D}_{\text{down}}$ reveal an efficient quadratic decay channel of the surface electron-hole recombination.

30aRD-4 角度分解2光子光電子分光法によるグラファイト表面の鏡像準位の研究(30aRD 表面界面電子物性,領域9(表面・界面,結晶成長))

30aRD-4 角度分解2光子光電子分光法によるグラファイト表面の鏡像準位の研究(30aRD 表面界面電子物性,領域9(表面・界面,結晶成長))

Electron Lifetime in a Shockley-Type Metal-Organic Interface State

The lifetimes of electrons at the interface between 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) and Ag(111) have been studied by means of time- and angle-resolved two-photon photoemission. We observe a dispersing unoccupied state 0.6 eV above the Fermi level with an effective electron mass of 0.39m{e} at the Gamma[over ] point. The short lifetime of 54 fs is indicative of a large penetration of the wave function into the metal. Supported by model calculations this interface state is interpreted as predominantly arising from an upshift of the occupied Shockley surface state of the clean metal due to the interaction with the PTCDA overlayer.

The Ultrafast Dynamics of Image Potential State Electrons at the Dimethylsulfoxide/Ag(111) Interface

Angle-resolved two-photon photoemission was used to study the energy relaxation, population decay, and localization dynamics of image potential state (IPS) electrons in ultrathin films of dimethylsulfoxide (DMSO) on an Ag(111) substrate. Dynamic energy shifts of 50 ± 10 meV and 220 ± 10 meV were observed for n = 1 IPS electrons at one monolayer and two monolayer coverages of DMSO, respectively. The difference in energy shifts is attributed to rotational hindrance of the molecular dipole in the chemisorptive first monolayer. The finding confirms the proposed mechanism for the low differential capacitance of dimethylsulfoxide at noble metal interfaces in solution. A novel description of the IPS as a surface capacitance is presented to facilitate comparisons with electrochemical systems.

Photoemission momentum mapping and wave function analysis of surface and bulk states on flat Cu(111) and stepped Cu(443) surfaces: A two-photon photoemission study

Accurate momentum mapping of bulk and surface electronic states by angle-resolved two-photon photoemission is demonstrated on Cu(111) and one of its vicinal surfaces, Cu(443), using laser light of $3.08\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ photon energy for excitation. The surface state dispersion found agrees well with that expected from the periodic arrangement of terraces and monatomic steps on Cu(443). Polarization dependent data suggest that the state consists of out-of-plane ${p}_{z}$ orbitals like on the flat (111) copper surface, mixed with in-plane orbitals at the step edges. Maps of the Fermi surface taken from the vicinal surface are found to be in excellent agreement with conventional photoemission data and density-functional calculations. This proves that multiphoton photoemission can be used like direct one-photon photoemission as initial state spectroscopy with high energy and momentum resolution provided that no real intermediate states are involved in the excitation process.