Angular Resolved Photoelectron Spectra Research Articles

Phase-of-the-Phase Electron Momentum Spectroscopy on Single Metal Atoms in Helium Nanodroplets.

Magnesium atoms fully embedded in helium nanodroplets are exposed to two-color laser pulses, which trigger multiphoton above-threshold ionization (ATI). This allows exemplary study of the contribution of a dense, neutral, and finite medium on single electron propagation. The angular-resolved photoelectron spectra show striking differences with respect to results obtained on free atoms. Scattering of the individual Mg photoelectrons, when traversing the neutral helium environment, causes the angular distribution to become almost isotropic. Furthermore, the appearance of higher-energy electrons is observed, indicating the impact of the droplet on the concerted emission process. Phase-of-the-phase spectroscopy, however, reveals a marked loss in the 2ω-ω phase dependence of the electron signal. Taking into account sideband formation on a quantitative level, a Monte Carlo simulation which includes laser-assisted electron scattering can reproduce the experimental spectra and give insights into the strong-field-induced electron emission from disordered systems.

Direct Imaging of Transient Fano Resonances inN2Using Time-, Energy-, and Angular-Resolved Photoelectron Spectroscopy

Autoionizing Rydberg states of molecular N_{2} are studied using time-, energy-, and angular-resolved photoelectron spectroscopy. A femtosecond extreme ultraviolet pulse with a photon energy of 17.5eV excites the resonance and a subsequent IR pulse ionizes the molecule before the autoionization takes place. The angular-resolved photoelectron spectra depend on pump-probe time delay and allow for the distinguishing of two electronic states contributing to the resonance. The lifetime of one of the contributions is determined to be 14±1 fs, while the lifetime of the other appears to be significantly shorter than the time resolution of the experiment. These observations suggest that the Rydberg states in this energy region are influenced by the effect of interference stabilization and merge into a complex resonance.

Interference stabilization of autoionizing states in molecular N2 studied by time- and angular-resolved photoelectron spectroscopy.

An autoionizing resonance in molecular N2 is excited by an ultrashort XUV pulse and probed by a subsequent weak IR pulse, which ionizes the contributing Rydberg states. Time- and angular-resolved photoelectron spectra recorded with a velocity map imaging spectrometer reveal two electronic contributions with different angular distributions. One of them has an exponential decay rate of 20 ± 5 fs, while the other one is shorter than 10 fs. This observation is interpreted as a manifestation of interference stabilization involving the two overlapping discrete Rydberg states. A formalism of interference stabilization for molecular ionization is developed and applied to describe the autoionizing resonance. The results of calculations suggest, that the effect of the interference stabilization is facilitated by rotationally-induced couplings of electronic states with different symmetry.

Control of electronic dynamics visualized by angularly resolved photoelectron spectra: A dynamical simulation with an IR pump and XUV attosecond-pulse-train probe

A dynamical simulation via a coupled-equation scheme that includes the ionization continua and field-induced effects describes a pump-probe experiment that monitors ultrafast electronic dynamics in LiH. The ionizing XUV attosecond pulse train that is included in the simulation is used as a frequency filter. By tuning the time interval between the attosecond pulses of the train to a beating frequency of the wave packet induced by the IR pump pulse we characterize the changing spatial localization from one end of the molecule to the other, reflecting the interferences of the nonstationary electronic density.

Angle-resolved photoelectron spectra of metal cluster anions within a many-body-theory approach

A consistent many-body theory based on the jellium model is applied for the description of angular resolved photoelectron spectra of metal clusters anions. The results of calculations demonstrate the dominant role of the many-body effects in the formation of angular distributions of photoelectrons emitted from sodium clusters and are in a good agreement with recent experimental data. The concrete comparison of theory and experiment has been performed for the photoionization of Na${}_{7}$${}^{\ensuremath{-}}$ and Na${}_{19}$${}^{\ensuremath{-}}$ anions being characterized by the entirely closed shells of delocalized electrons.

Strong-field ionization of atoms and molecules: The two-term saddle-point method

We derive an analytical formula for the ionization rate of neutral atoms and molecules in a strong monochromatic field. Our model is based on the strong-field approximation with transition amplitudes calculated by an extended saddle-point method. We show that the present two-term saddle-point method reproduces even complicated structures in angular resolved photoelectron spectra.

Calculated spin polarization and magnetic dichroism of photoelectron diffraction spectra for magnetite below the Verwey temperature.

Spin- and angular-resolved photoelectron spectra are calculated for magnetite below the Verwey temperature. The results are compared for the Mizoguchi model, the Zuo model, and an assumed nonmagnetic ${\mathrm{Fe}}^{3+}$${\mathrm{Fe}}_{2}^{2.5+}$ ${\mathrm{O}}_{4}$ configuration. Due to an occurring symmetry breaking at the Verwey transition, the effect of the magnetization can be clearly separated from the spectra; further effects are due to charge ordering. Furthermore, the charge ordering models are compared by a separation of the spectra into contributions from ferrous and ferric ions.

Calculated spin and angular resolved photoelectron diffraction spectra for magnetite

Spin and angular resolved photoelectron spectra for magnetite at room temperature are calculated, first neglecting, then including the magnetisation. Using the occurring symmetry breaking, the effect of the magnetisation can be clearly separated from the spectra. Second, the surface relaxation is shown to be subject to an investigation by means of PED and SPED.

Electronic band dispersion and pseudogap in quasicrystals: Angular-resolved photoemission studies on icosahedral Al70Pd21.5Mn8.5.

Angular-resolved photoelectron spectra from the fivefold surface of single-grain icosahedral ${\mathrm{Al}}_{70}$${\mathrm{Pd}}_{21.5}$${\mathrm{Mn}}_{8.5}$ exhibit a quasiperiodic dispersion of 300 meV at 2.3 eV binding energy. Low energy electron diffraction studies confirm quasicrystalline order at the surface. A distinct pseudogap feature is observed with a density of states near the Fermi level decreasing as a square-root power law.

Influence of the core hole interaction on the dichroism and spin polarization of the 3s angular resolved photoelectron spectra of transition metals in the surface region

Core photoelectron diffraction is considered to investigate the geometrical structure of surface layers. Due to the chemical shift of the core level the spectrum can be split off into a bulk and a surface contribution. Besides this chemical shift there is an additional shift due to the influence of the core hole potential and of the core hole spin. This shift includes an exchange splitting and a configuration interaction which changes the angular momentum character of the final state. The influence of this configuration interaction on the shape of the angular resolved photoelectron spectrum is calculated for the 3s spectrum of Fe. It is shown that the difference is more pronounced in the circular dichroism of the spectrum than in the total spectrum. The dichroism of the spin-up and spin-down spectrum, respectively, is a measure for the strength of the configuration interaction. Thus, it allows to make a distinction between the chemical shift at the surface and the influence of the core hole charge.

Investigation of Na 2p53s3p resonances using angular resolved photoelectron spectroscopy of laser-aligned sodium atoms

With a combination of laser and synchrotron radiation the photoionization process of sodium atoms in the region of the autoionizing resonances Na 2p53s3p was investigated. The well defined polarization of both radiation fields was used to prepare the atoms by laser-pumping in aligned valence excited states and study the angular distribution of the photoelectrons which were emitted after the 2p to 3s excitation by synchrotron radiation. An analysis of the angular resolved photoelectron spectra and ab initio calculations for the 2p53s3p resonances are presented.

Angular resolved photoemission studies of pyridine on Pt(111)—Evidence for an α-pyridyl species

HeI angular resolved photoelectron spectra (ARUPS) of pyridine adsorption at room temperature on Pt(111) are very similar to those previously reported for adsorption on Ir(111) but are very different from those for adsorption on Cu(110). The angular variations suggest a ring orientation approximately perpendicular to the metal surfaces in all three systems, and it is argued that this information, supported by independent NEXAFS results for the Cu(110) and Pt(111) systems, is best interpreted in terms of the adsorbed species on both Pt(111) and Ir(111) being α -pyridyl. An interpretation of ESDIAD results for the Ir(111) system is proposed.

Angular resolved UPS and XPS spectra of benzenethiol adsorbed on Cu(111) at 300 °K

Angular resolved photoelectron spectra using Hei (21.2 eV) radiation have been obtained for benzenethiol (phenyl mercaptan) chemisorbed on Cu(111), as well as for the clean surface. The correlation with the gas-phase spectra suggests that the benzenethiol bonds to copper by way of sulfur-containing orbitals rather than carbon π ring orbitals. The angular dependence of the different bands formed in the photoelectron spectra indicates that the benzene ring is oriented perpendicular or near perpendicular to the surface. XPS analysis, with Al–Kα radiation, of the Cu 2p, C 1s, and S 2p orbitals confirms the stoichiometry as well as the surface coverage. The extent of surface coverage supports the argument against benzenethiol lying flat on the copper surface.

Assignment of the d-orbitals involved in the chemisorption bond of CO on Re(0001)

Angular resolved photoelectron spectra and thermal desorption spectra show that CO is molecularly adsorbed at room temperature on the Re(0001) surface. The observed d-band photoemission peaks are individually reduced in intensity after CO chemisorption. This behavior is discussed in terms of a bonding model. Structures in the photoelectron spectra which are most strongly attenuated are mainly derived from atomic orbitals of symmetry d xz and d yz . Thus these orbitals are involved in the chemisorption bond. This interpretation of the photoemission data together with the observation of an increase in work function and a diffuse LEED pattern leads to the conclusion that CO is bonded in bridge or hollow sites up to the saturation coverage of the α 2 state in the thermal desorption spectra.

Angular resolved photoemission from PbS (100) for 16.85 eV and 21.22 eV excitation energy

Angular resolved photoelectron spectra are presented for the narrow-gap semiconductor PbS. Two different photon energies and two different azimuthal orientations of the (100) surface are employed. The experimental results provide a further test for the validity of the one-dimensional density of states approach in the case of the lead chalcogenides. The experimental binding energies of 10 three-dimensional critical points are compared with the prediction of two independent band structure calculations.

Angular-resolved uv photoemission and the band structure of GeS

Angular-resolved photoelectron spectra (ARPES) are presented for GeS using 21-eV excitation energy. From the measurements that cover a wide range of angles along the two principal directions in the basal plane of the orthorhombic layered compound GeS $E$ vs ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{\ensuremath{\parallel}}$ curves were constructed. The band structure of GeS was calculated utilizing the empirical pseudopotential method. The $E$ vs ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{\ensuremath{\parallel}}$ curves can almost completely be understood in terms of this band structure under the assumption that only peaks in the one-dimensional density of states along ${k}_{\ensuremath{\perp}}$ contribute significantly to the ARPES spectrum.