Mechanisms of high-order perturbative photoemission from Cu(001)

We observed high-order 2- to 4-photon photoemission and above threshold photoemission (ATP) processes with 3.07 eV light from the Cu(001) surface. The intensity of 3-photon photoemission via excitation through the n = 1 image potential state significantly exceeded that of the 2-photon process. The ATP occurs either via single photon transitions from the image potential resonances above the vacuum level or by multiphoton transitions from image potential states below the vacuum level. The experimental ratio of the m- to (m + 1)-photon process yields is sensitive to the electronic band structure of the solid.     

Self-consistent electronic structure calculation for nitrogen chemisorbed on copper (100)

A fully self-consistent calculation of the electronic structure of a N covered Cu slab is reported. The density of states shows changes relative to a clean Cu slab that reproduce the changes observed in photoemission spectra when N is chemisorbed on Cu(100). Thus initial state information is correctly mirrored in the photoemission spectra in this case. The changes are due to the presence of the N 2p-bands, and to significant alteration of the surface Cu 3d-bands by the chemisorption of N. The localization in space of the chemisorption bond and the importance of self-consistency are demonstrated.     

Scattering of Cu(1 0 0) image state electrons from single Cu adatoms and vacancies: A comparative study

A theoretical study of the electron dynamics in image potential states on Cu(1 0 0) surfaces with different types of defects (Cu adatoms and Cu vacancies) is presented for low defect density at the surface. A wave packet propagation approach is employed for the electron scattering calculations, where the defect induced potentials are obtained from an ab initio density functional study. Scattering of the image state electron by a defect induces inter-band and intra-band transitions leading, respectively, to the population decay and to the dephasing of the image states. Comparison of the respective effects of adatoms and vacancies shows that Cu adatoms are much more efficient in inducing population decay and dephasing of the image potential states. Present results for the case of Cu adatoms are compared with available time-resolved two-photon photoemission data.     

Time-resolved two-color interferometric photoemission of image-potential states on Cu(100)

We describe an interferometric time-resolved photoemission technique that makes it possible to simultaneously observe the decay of optical induced polarizations and populations at surfaces in a two-color excitation scheme. In this scheme initially unoccupied electronic surface states are coherently excited by the interaction of laser pulses with frequency ω_a and the two-photon polarization which is induced by laser pulses with frequency ω_a/2. Interference is observed by changing the delay between both laser pulses using an actively stabilized two-color Mach–Zehnder interferometer. We demonstrate this technique for excitation of the n=1 image-potential state on a Cu(100) surface. PACS 78.47.+p; 79.60.Bm; 73.20.-r; 82.53.Kp; 42.50.Md     

Momentum-resolved lifetimes of image-potential States on cu(100)

The dependence of the inelastic lifetime of electrons in the image-potential states of Cu(100) on their momentum parallel to the surface has been studied experimentally by means of time- and angle-resolved two-photon photoemission and theoretically by evaluating the electron self-energy within the GW approximation. The pronounced decrease of the n = 1 lifetime from 40 fs at normal emission (k(parallel) = 0) to 20 fs for k(parallel) = 0.33 A(-1) cannot be accounted for by interband decay processes to bulk states. We show that intraband transitions within the image-state band give a contribution to this decrease comparable in magnitude with the interband channel.     

Influence of Ar,Kr, and Xe layers on the energies and lifetimes of image-potential states on Cu(100)

The influence of well-ordered adlayers of Ar, Kr, and Xe on the energetic and dynamical properties of image-potential states on Cu(100) has been investigated in a comprehensive study using time-resolved two-photon photoemission (2PPE). The effect of these insulating films varies systematically with the electron affinity EA of the condensed rare gases and with the film thickness. For the electron-repulsive Ar layers (EA=-0.25 eV), a strong lifetime increase of the n=1 state from 40 fs on clean Cu(100) to as much as 10 ps at a coverage of 5 monolayers is observed. For Kr and Xe layers (EA=+0.3 and +0.5 eV, respectively), decoupling from the metal is less efficient. These layers exhibit quantum-well-like resonances of the n=2 state as a function of layer thickness. The energies of the series of states depend characteristically on the affinity level and the dielectric constant of the films. A microscopic model is developed that includes the discrete atomic structure of the adsorbate layers. It is capable of describing the experimental results to a high degree of quantitative agreement. PACS 78.47.+p; 73.20.At; 77.55.+f     

Time-resolved photoimaging of image-potential states in carbon nanotubes

The first experimental evidence for the existence of image-potential states in carbon nanotubes is presented. The observed features constitute a new class of surface image states due to their quantized centrifugal motion. Measurements of binding energies and the temporal evolution of image state electrons were performed using femtosecond time-resolved photoemission. The associated lifetimes are found to be significantly longer than those of n=1 image state on graphite, indicating a substantial difference in electron decay dynamics between tubular and planar graphene sheets.     

Spectroscopy of image-potential states by two-photon photoemission

Unoccupied electronic states in solids and at solid surfaces are usually studied by inverse photoemission. An alternative method is two-photon photoemission. It is superior in resolution but limited to states of sufficiently long lifetime below the vacuum level. So far this method has mainly been applied to image-potential states on metal surfaces. On Ag(111) and Cu(111) a narrow surface state below the Fermi level serves as the initial state, which results in a pronounced resonance in the two-photon photoemission. Ni(111) shows similar results. In the resonance the image-potential state is so highly populated that electron-electron interaction leads to an Auger-type process. Nevertheless, the system is not so greatly disturbed as to show deviations from the one-photon photoemission results concerning the occupied states. Ag(100) and Cu(100) have a smooth continuum of initial states. Consequently, no resonance occurs. The binding energy does not depend on the material but changes with surface orientation: it is about 0.80 eV at the (111) surfaces and about 0.55 eV at the (100) surfaces. The effective mass is free electron like except on Ag(111), where it is 30% heavier. The lifetime on Ag(100) is about 20 fs. The agreement with theory is excellent in some cases and only fair in others.     

Study of chemisorption on copper low-index and stepped surfaces

Adsorption of CHCl₃, O₂, and hydrocarbons has been studied on Cu(111) and stepped surfaces using LEED, AES, and UPS at room temperature. We find that ordered Cl overlayers form upon Cu(111), Cu[3(111) × (100)], and Cu[5(111) × (100)] surfaces upon exposure to CHCl₃. Exposure to O₂ results in rearrangement of the Cu[5(111) × (100)] surface to hill-and-valley regions with large (111) areas, whereas Cu[2(111) × (100)] is stable for the same exposure. The photoemission spectra show new energy levels due to C1 above and below the Cu d band region and a small splitting of the halogen p orbitals. Effects consistent with interaction with the Cu d band are observed. Similar effects are observed with oxygen adsorption. The initial rate of Cl or O₂ chemisorption as measured by photoemission is proportional to the density of steps on these surfaces. Apparently, structural effects play an important role in chemisorption on metals (such as copper) with low density of states at the Fermi energy.     

Magnetic phase transition in Co/Cu/Ni/Cu(100) and Co/Fe/Ni/Cu(100)

Magnetic phase transitions in coupled magnetic sandwiches of Cu/Co/Cu/Ni/Cu(100) and Cu/Co/Fe/Ni/Cu(100) are investigated by photoemission electron microscopy. Element-specific magnetic domains are taken at room temperature to reveal the critical thickness at which the magnetic phase transition occurs. The results show that a coupled magnetic sandwich undergoes three types of magnetic phase transitions depending on the two ferromagnetic films' thickness. A phase diagram is constructed and explained in the process of constructing Monte Carlo simulations, which corroborate the experimental results.     

Bichromatic two-photon photoemission spectroscopy of image potential states on Ag(100)

Bichromatic two-photon photoemission spectroscopy (Bi2PPES) leads to an increased signal-to-noise ratio compared to conventional two-photon photoemission spectroscopy and therefore allows the observation of the first four image potential states as well as a lineshape analysis with improved accuracy on Ag(100). The n=2 image state with a measured linewidth of 37 meV FWHM is the narrowest unoccupied structure measured on any solid surface so far. The intrinsic linewidths of the first two image states were determined as 21±4 meV and 5±5 meV, respectively, in reasonable agreement with theoretical calculations. Disordered adsorption of oxygen on Ag(100) leads to a linewidth broadening of the first and — to a lesser extent — of the second image potential state. A quantitative analysis of the broadening suggests that the underlying mechanism is lifetime shortening due to scattering of the image state electrons by the adsorbate atoms.     

Surface state like bulk band emission in the photoelectron energy spectra of Cu(111)

Using a relativistically calculated band structure the presence in Cu(111) UV photoemission spectra of a prominent 3d peak, previously ascribed to a Tamm type surface state, is explained in terms of bulk band transitions.     

Observation of band renormalization effects in hole-doped high-Tc Superconductors

We report a systematic high-resolution angle-resolved photoemission spectroscopy on high-T(c) superconductors Bi(2)Sr(2)Ca(n-1)Cu(n)O(2n+4) (n=1-3) to study the origin of many-body interactions responsible for superconductivity. For n=2 and 3, a sudden change in the energy dispersion, so called "kink", becomes pronounced on approaching (pi,0) in the superconducting state, while a kink appears only around the nodal direction in the normal state. For n=1, the kink shows no significant temperature dependence even across T(c). This could suggest that the coupling of electrons with Q=(pi,pi) magnetic mode is dominant in the superconducting state for multilayered cuprates, while the interactions at the normal state and that of single-layered cuprates have a different origin.     

Chlorine adsorption on copper: III. Angle-resolved electron energy loss spectroscopy

Electronic excitations on Cu(001) and Cu(001)c(2 × 2)-Cl have been investigated by angle-resolved electron energy loss spectroscopy at an angular resolution of Δθ = ±1° and an energy resolution of ΔE = 60 meV. Primary energies in the 50–100 eV range were chosen and the specular reflection was studied for angles 35° ? θ ? 71° with respect to the surface normal. The results are summarized as follows: The specular Cu(001) spectra are compared to optical data and good agreement is found for the energetic position of direct transitions. The electronic losses observed for the ordered overlayer system may be interpreted by one-electron excitations from occupied surface bands (known from angle-resolved photoemission results) into an empty band with a minimum energy at 0.4 eV above the Fermi level.     

Scattering of image-potential-state electrons by steps on Cu(001)

Scanning tunneling spectroscopy (STS) reveals a distinct asymmetry in the scattering properties of an isolated step for the n=1 image-potential state on Cu(001). The elastic scattering probability for an electron traveling downstairs is determined from the strength of density oscillations in front of a step edge and is found to be approximately two times higher than for the opposite upstairs direction. A one-dimensional scattering model is extended to the case of asymmetric transmission and reflection coefficients. The calculations using the asymmetry measured by STS explain the dispersion and the decay rate of the n=1 band on Cu(119) measured by two-photon photoemission. In particular, the asymmetry of the decay rate can be described quantitatively with a minimum of adjustable parameters. While the results can also be transferred successfully to the Cu(1115) surface, the limit of applicability is reached for Cu(117) with a step separation of 3.5 nearest-neighbor distances. PACS 73.20.At; 79.60.Bm; 68.37.Ef; 72.10.Fk     

Cu(1 1 1) and Cu(0 0 1) surface electronic states. Comparison between theory and experiment

Recent advances in both the experimental resolution and in the computational capabilities motivate new studies of surface properties. In particular, a detailed comparison between theoretical and experimental data is expected to provide a better insight into surface and image states. In this work we present a joint effort analyzing such features of the Cu(1 1 1) and Cu(0 0 1) surfaces. The experiments are performed by using both linear and non-linear angle-resolved photoemission. From the theoretical point of view, we make use of the Green function embedding technique within density functional theory. We are able to account for the image states by suitably modifying the effective potential in the Kohn-Sham equation and the generalized boundary conditions on the Green function. Comprehensive theoretical and experimental results on the effective mass and the binding energy of the Shockley state and the first image states are reported.     

Theory of time-resolved two-photon photoemission from Cu(111): effect of Coulomb interactions among electrons

The effect of Coulomb interactions among electrons on the time-resolved two-photon photoemission spectra of Cu(111) is investigated by nonequilibrium perturbation theory. It is demonstrated that there are contributions to photoemission via an image state on the surface from processes involving scattering of photoexcited electrons and holes due to Coulomb interactions. As a result, the correlation trace (photoelectron intensity as a function of the pump-probe delay time) is affected by the lifetimes of the electron in the image state, the photoexcited electron and hole.     

The photon angle dependence of photoemission from a Cu(100) crystal

The symmetry properties of direct interband transitions are shown to have important consequences for angle-resolved u.v. photoemission. Predictions are made regarding the photon incidence angle dependence of Ar I (11.7 eV) excited photoemission from a Cu(100) single crystal surface, and these are shown to be in close agreement with the experimental results.     

Spin-dependent electron dynamics in front of a ferromagnetic surface

Exchange splitting and dynamics of image-potential states in front of a 3 monolayer iron film on Cu(100) have been studied with time-, energy-, and spin-resolved bichromatic two-photon photoemission. For the first image-potential state n=1 we observe an exchange splitting of 56 +/- 10 meV and spin-dependent lifetimes of 16 +/- 2 fs for majority-spin and of 11 +/- 2 fs for minority-spin electrons, respectively. The time-resolved studies of both the population and the linewidth of image-potential states manifest that at the magnetic surface not only inelastic but also quasielastic scattering processes are spin dependent.     

Subthreshold photoemission from copper nanoclusters on the SiO2 surface

Subthreshold photoemission from copper nanoclusters formed on the SiO₂ surface has been observed under irradiation of the surface by photons in the 3.1–6.5-eV energy range. The average size of copper nanoclusters on the silicon oxide surface is 250–500 nm. Besides the conventional photoemission from the filled Shockley surface state (SS), strong photoemission has been recorded at incident photon energies of 0.5 eV below the work function of the copper surface. This emission is assumed to be generated in direct electron transitions from the SS state to the unfilled electron surface states formed by the Coulomb image potential, followed by escape from these states into vacuum.