Adsorbed on the nickel ͑111͒ surface, hexagonal boron nitride ͑h-BN͒ forms a commensurate monolayer, which retains much of the band structure of layered bulk h-BN, specifically the wide band gap. This article explores its unoccupied band structure with femtosecond time-resolved two-photon photoelectron spectroscopy in a two-color setup, and presents spectra, dispersion, and cross-correlation measurements. Two distinct, dispersive intermediate states are observed at the origin of the surface Brillouin zone ⌫: a à -related interface state at 1.51 eV above the Fermi level, which forms the conduction band, and an image-potential state at 0.65 eV below the vacuum level. With 260 fs for the image state and 110 fs for the interface state the lifetimes of the two unoccupied states are remarkably high due to the very small overlap of the corresponding wave functions with the nickel bulk states.

(2008). 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. Physical Review B, 77(8) AbstractAccurate 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 eV photon energy for excitation. The surface state dispersion was found to exhibit dispersion corresponding to 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 p-orbitals at the step edges. The results corroborate the general model that on vicinals with an average terrace size below a given length, surface states behave as de-localized states, and that the corresponding wavefunctions extent over several terraces.Moreover, the low electron kinetic energies encountered in such angle-resolved experiments with laser excitation translate into high momentum resolution. 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 multi-photon 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.

We have mapped the electronic bands of the ''quasi-(5ϫ5)'' monolayer structure of Cu on Si͑111͒ near the Fermi energy at high angular and energy resolution. Although this unusual system does not exhibit a true long-range periodicity, well-defined bands and sharp Fermi surface contours are observed. The fundamental Fermi surface inside the first surface Brillouin zone has the shape of a hexagon and possesses strong nesting features. However, no relation to the unusual translational symmetry could be established. Strong umklapp bands and Fermi surface contours are observed with umklapp vectors that correspond to the reciprocal lattice of the slightly expanded and rotated discommensurate Cu 2 Si layer, and to that of the quasi-(5ϫ5) domain pattern formed by the regular dislocation network. We find thus a one-to-one correspondence between the electronic bands and the complex structure model that has been established by Zegenhagen et al. ͓Phys. Status Solidi B 204, 587 ͑1997͔͒. This suggests that the two-dimensional electron gas formed on this surface does not induce the formation of the discommensurate structure but rather takes a spectator role.

High power, high uniformity strip electron gun design, simulation and performance Rev.We present the design and performance tests of a miniaturized pulsed low-energy electron gun. Electrons photoemitted from a gold cathode are accelerated over a distance of 75 m and then collimated by a microchannel plate. According to calculations, this novel concept will allow the time spread of the electron pulses to be kept below 5 ps for kinetic energies as low as 100 eV. The achievement of a minimum angular beam divergence ͑Ϸ1°͒ along with an energy resolution of 1.1 eV has to be paid for by low signal intensities. We demonstrate the performance of the gun and the high electron-beam coherence by presenting low-energy-electron diffraction images taken from a submonolayer of lead adsorbed on the germanium ͑111͒ surface. We anticipate that this electron gun will open up new possibilities for following structural changes on solid surfaces in real time.

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