Unraveling Atomic and Electronic Surface Structure and Dynamics from Angular Photoelectron Distributions

Angle-resolved photoelectron spectroscopy (ARPES) is a powerful tool in solid state sciences. Beside the direct measurement of the energy-momentum dispersion relation, the angular distribution of the photoelectron current reveals the structural environment of the emitting atoms via photoelectron diffraction effects. Moreover, in the case of molecular layers, the angular distribution of emission from molecular orbitals can be directly related to their charge density distribution via so-called orbital tomography. In the present paper we summarize our efforts undertaken over the past 12 years to add the dimension of time to these two methods via pump-probe experiments with femtosecond resolution. We give a comprehensive introduction to standard ARPES and time-resolved two photon photoemission and then focus on our efforts towards time-resolved versions of photoelectron diffraction and orbital tomography. Both, optimization of experimental parameters and data acquisition procedures, as well as new numerical tools are needed in order to realize such challenging full stop missing after experiments

Unraveling Atomic and Electronic Surface Structure and Dynamics from Angular Photoelectron Distributions

Angle-resolved photoelectron spectroscopy (ARPES) is a powerful tool in solid state sciences. Beside the direct measurement of the energy-momentum dispersion relation, the angular distribution of the photoelectron current reveals the structural environment of the emitting atoms via photoelectron diffraction effects. Moreover, in the case of molecular layers, the angular distribution of emission from molecular orbitals can be directly related to their charge density distribution via so-called orbital tomography. In the present paper we summarize our efforts undertaken over the past 12 years to add the dimension of time to these two methods via pump-probe experiments with femtosecond resolution. We give a comprehensive introduction to standard ARPES and time-resolved two photon photoemission and then focus on our efforts towards time-resolved versions of photoelectron diffraction and orbital tomography. Both, optimization of experimental parameters and data acquisition procedures, as well as new numerical tools are needed in order to realize such challenging full stop missing after experiments

Mechanism of Laser-induced Field Emission

We have measured electron energy distribution curves (EDCs) of the laser-induced field emission from a tungsten tip. Field emission from photo-excited nonequilibrium electron distributions were clearly observed, while no enhanced field emission due to optical electric fields appeared up to values of 1.3 V/nm. Thus, we experimentally confirm the emission mechanism. Simulated transient EDCs show that electron dynamics plays a significant role in the laser-induced field emission. The results should be useful to find optimal parameters for defining the temporal and spectral characteristics of electron pulses for many applications based on pulsed field emission.Comment: 4 pages 4 figures 1 table, submitted to Physical Review Letter

Laser-induced Field Emission from Tungsten Tip: Optical Control of Emission Sites and Emission Process

Field-emission patterns from a clean tungsten tip apex induced by femtosecond laser pulses have been investigated. Strongly asymmetric field-emission intensity distributions are observed depending on three parameters: (1) the polarization of the light, (2) the azimuthal and (3) the polar orientation of the tip apex relative to the laser incidence direction. In effect, we have realized an ultrafast pulsed field-emission source with site selectivity of a few tens of nanometers. Simulations of local fields on the tip apex and of electron emission patterns based on photo-excited nonequilibrium electron distributions explain our observations quantitatively. Electron emission processes are found to depend on laser power and tip voltage. At relatively low laser power and high tip voltage, field-emission after two-photon photo-excitation is the dominant process. At relatively low laser power and low tip voltage, photoemission processes are dominant. As the laser power increases, photoemission from the tip shank becomes noticeable.Comment: 12 pages, 12 figures, submitted to Physical Review

Importance of surface oxygen vacancies for ultrafast hot carrier relaxation and transport in Cu2_2O

Cu$_2$O has appealing properties as an electrode for photo-electrochemical water splitting, yet its practical performance is severely limited by inefficient charge extraction at the interface. Using hybrid DFT calculations, we investigate carrier capture processes by oxygen vacancies (V$_\mathrm{O}$) in the experimentally observed ($\sqrt{3} \times \sqrt{3}$)R30$^{\circ}$ reconstruction of the dominant (111) surface. Our results show that these V$_\mathrm{O}$ are doubly ionized and that associated defects states strongly suppress electron transport. In particular, the excited electronic state of a singly charged V$_\mathrm{O}$ plays a crucial role in the non-radiative electron capture process with a capture coefficient of about 10$^{-9}$~cm$^3$/s and a lifetime of 0.04~ps, explaining the experimentally observed ultrafast carrier relaxation. These results highlight that engineering the surface V$_\mathrm{O}$ chemistry will be a crucial step in optimizing Cu$_2$O for photoelectrode applications

Time-Resolved Photoelectron Spectroscopy to Probe Ultrafast Charge Transfer and Electron Dynamics in Solid Surface Systems and at Metal- Molecule Interfaces

Photoelectron spectroscopy (PES) is a versatile tool, which provides insight into electronic structure and dynamics in condensed matter, surfaces, interfaces and molecules. The history of PES is briefly outlined and illustrated by current developments in the field of time-resolved PES. Our group's research is mostly aimed at studying ultrafast processes and associated lifetimes related to electronic excitation at solid surfaces

Surface electronic structure of Ni-doped Fe3O4(001)

Magnetite (Fe3O4) doped with earth-abundant metals has emerged as a promising catalyst material, with Ni-doped magnetite (Ni/Fe3O4) being a cost-effective, durable, and highly active material for photocatalytic and electrochemical water oxidation. While previous studies have investigated the incorporation of Ni atoms into Fe3O4 single-crystalline surfaces using surface science characterization methods and density functional theory calculations, an experimental study is still required to understand the impact of Ni incorporation on the electronic structure of Ni/Fe3O4 systems. To address this, we employed angle-resolved photoemission spectroscopy, analyzed within the one-step model of photoemission by a real-space multiple scattering code to investigate the electronic structure of the reconstructed magnetite surface. Moreover, the half-metal to semiconductor phase transition upon Ni incorporation is reflected in an almost complete disappearance of states near the Fermi level. Finally, we report on the systematic changes in the unoccupied states observed with the increasing amount of Ni dopant. These findings offer insights into the influence of Ni incorporation on the electronic structure of Ni/Fe3O4, which can link to an increased catalytic activity

Optical Control of Field-Emission Sites by Femtosecond Laser Pulses

We have investigated field emission patterns from a clean tungsten tip apex induced by femtosecond laser pulses. Strongly asymmetric modulations of the field emission intensity distributions are observed depending on the polarization of the light and the laser incidence direction relative to the azimuthal orientation of tip apex. In effect, we have realized an ultrafast pulsed field-emission source with site selectivity on the 10 nm scale. Simulations of local fields on the tip apex and of electron emission patterns based on photo-excited nonequilibrium electron distributions explain our observations quantitatively.Comment: 4 pages, submitted to Physical Review Letter