Strong-field above-threshold photoemission from sharp metal tips

We present energy-resolved measurements of electron emission from sharp metal tips driven with low energy pulses from a few-cycle laser oscillator. We observe above-threshold photoemission with a photon order of up to 9. At a laser intensity of ～ 2 × 10(11) W/cm2 the suppression of the lowest order peak occurs, indicating the onset of strong-field effects. We also observe peak shifting linearly with intensity, with a slope of around -1.0 eV/(10(12) W/cm2). We attribute the magnitude of the laser field effects to field enhancement taking place at the tip's surface.     

Experimental aspects of angle-resolved photoemission from clean and adsorbate-covered metal surfaces

Instrumental techniques for the observation of angle-resolved photoelectron spectra from surfaces are reviewed. Results from the work of various groups on clean metals, particularly tungsten, copper, nickel and palladium, are described. Spectra induced by adsorption of hydrogen, oxygen, carbon monoxide and benzene on some of these metals are discussed.     

Observation of two-photon induced photoemission optogalvanic effect

Observation of laser induced two-photon photoemission optogalvanic (TPPOG) effect from tungsten electrode in a discharge cell using 564 nm radiation obtained from a pulsed dye laser is described. The magnitude of the POG signal is studied as a function of laser energy under various discharge parameters. Competition between one-photon and two-photon processes has been observed when nitrogen gas is used in the discharge cell.     

A resonant photoemission model for nickel metal

We present a model for the resonance near the 3p threshold in the photoemission associated with a static 3d pair-hole bound state in paramagnetic nickel. We show that, following Auger processes, the strong intra-atomic screening of the pair hole by the 4s?4p conduction band electrons is essential to explain experimental photoemission data not only in copper but also in nickel. From the energy relaxation of the conduction band states we estimate the bare Coulomb interaction between two d-holes in nickel.     

Quantum size effect in the photoemission intensity emitted from thin metal films

Photoemission spectra are calculated for thin films of Ag on Au(111) using the method of Green's function matching. For very low photon energies (ω ≲ 10 eV) the peak intensities of the thin film states oscillate with the film thickness. These oscillations are caused by the relaxation of the k_⊥-conservation of the optical excitation in photoemission.     

Observation of a superconducting gap in boron-doped diamond by laser-excited photoemission spectroscopy

We investigate the temperature (T)-dependent low-energy electronic structure of a boron-doped diamond thin film using ultrahigh resolution laser-excited photoemission spectroscopy. We observe a clear shift of the leading edge below T=11 K, indicative of a superconducting gap opening (Delta approximately 0.78 meV at T=4.5 K). The gap feature is significantly broad and a well-defined quasiparticle peak is lacking even at the lowest temperature of measurement (=4.5 K). We discuss our results in terms of disorder effects on the normal state transport and superconductivity in this system.     

Observation of a warped helical spin texture in Bi2Se3 from circular dichroism angle-resolved photoemission spectroscopy

A differential coupling of topological surface states to left- versus right-circularly polarized light is the basis of many optospintronics applications of topological insulators. Here we report direct evidence of circular dichroism from the surface states of Bi(2)Se(3) using laser-based time-of-flight angle-resolved photoemission spectroscopy. By employing a novel sample rotational analysis, we resolve unusual modulations in the circular dichroism photoemission pattern as a function of both energy and momentum, which perfectly mimic the predicted but hitherto unobserved three-dimensional warped spin texture of the surface states. By developing a microscopic theory of photoemission from topological surface states, we show that this correlation is a natural consequence of spin-orbit coupling. These results suggest that our technique may be a powerful probe of the spin texture of spin-orbit coupled materials in general.     

Lineshape studies of the X-ray photoemission of small metal clusters

It is shown that the lineshape asymmetry of the X-ray photoelectron spectra of the Pt(4f) and Pd(3d) core-levels in small metal clusters decreases with the size of the cluster. Such changes are related to the changes of the density of states of the valence electrons and the effective core-hole potential at the Fermi level. The decrease of the asymmetry leads to a shift of the line position toward a higher binding energy (about half of the observed shift) as governed by the first moment rule.     

Directional photoemission from tungsten

Photoemission energy distribution spectra have been scanned for photoelectrons emitted normal to two different crystal faces of tungsten, namely (100) and (110). The energy distributions and the derivative spectra show large differences between emission from the two faces. Whereas strong surface state emission is observed from the (100) face, no surface states are associated with the (110) face. The spectra show features characteristic of direct and also nondirect transitions, but these features appear very different for the two faces. Using existing band structure calculations, the spectral structure is assigned to electronic transitions between bands along the Δ axis for the (100) face and the Σ axis for the (110) face. The experiments show that directional photoemission spectra taken normal to a crystal surface make it possible to investigate electronic energy bands along symmetry directions in the Brillouin zone.     

Interferometric control of spin-polarized electron populations at a metal surface observed by multiphoton photoemission

In an interferometric pump-probe experiment, we demonstrate the phase tuning of the spin polarization of photoelectrons emitted in a three-photon process from Cu(001). A phase shift of pi between delayed ultrafast circularly polarized light pulses can switch the spin polarization from +/-20% to -/+40%. In the delay regime of overlapping pulses, we show the dominating role of optical interference effects in determining the spin polarization. For longer delays, we detect the influence of the coherent material response, manifested in both the final state electron population as well as the final state spin polarization.     

The application of simple selection rules to photoemission from clean and adsorbate covered metal single crystals

The selection rules governing the uv-photoemission process from clean and adsorbate covered surfaces, as a consequence of the symmetry properties of the system, are discussed. It is shown that the symmetries of the initial states giving rise to features in the angle-resolved photoemission spectra can be readily derived from photon polarization dependent measurements. Symmetry based selection rules are also applied to the problem of adsorbate molecule orientation for the case of adsorbate covered surfaces.     

Splitting of core-level lines in photoemission spectra of transition metal compounds

The multiplet structure of core-electron binding energies in Cr 3s, Cr 3p and Cr 2p levels of CrCl₃ and CrBr₃ compounds has been investigated by X-ray photoelectron spectroscopy. The Cr 3s levels show a doublet splitting (about 4.3?eV) for the main emission in both halides. Satellites features are observed in Cr 3s, 3p and 2p levels at higher binding energies. In the Cr 3s spectrum, the main emission is assigned to unscreened intra-atomic multiplet splittings with correlation-induced satellites. The Cr 3p and 2p spectra can be better explained by the multiplet splitting arising from the interaction between valence band 3d electrons and core p holes in the crystal field. Final state screening (charge-transfer) effects do not play a major role in Cr 3s main emission nor do they affect the satellite structures in a relevant way. This explains why the Cr 3s exchange splitting in chromium halides is proportional to the local magnetic moment.     

Some polarisation effects in photoemission from a cubic crystal

The energy distribution spectra of photoelectrons emitted normal to the (100) face of tungsten were investigated with respect to their polarisation dependence. The results are compatible with the assumption that surface photoemission vanishes if the exciting light is polarised parallel to the emitting surface. This is demonstrated for the examples of emission from a high density of states at the surface, a surface resonance, and an adsorbate induced level. The measurements also show that emission following direct optical excitation in the bulk is polarisation sensitive, supporting the idea of an anisotropic excitation along a star of k-vectors in the bulk. Some indication is also given of emission in ‘secondary cones’.     

Spin polarized photoemission from NiMnSb

Spin polarized photoemission experiments are reported on NiMnSb which has been predicted to be a halfmetallic ferromagnet. The 100% polarization near photothreshold has not been observed. If an energy gap in the minority electron density-of-states should exist, it will be smaller than 0.5 eV.