What is the spatiotemporal limit of a macroscopic model that describes the optoelectronic interaction at the interface between different media? This fundamental question has become relevant for time-dependent photoemission from solid surfaces using probes that resolve attosecond electron dynamics on an atomic length scale. We address this fundamental question by investigating how ultrafast electron screening affects the infrared field distribution for a noble metal such as Cu (111) at the solid-vacuum interface. Attosecond photoemission delay measurements performed at different angles of incidence of the light allow us to study the detailed spatiotemporal dependence of the electromagnetic field distribution. Surprisingly, comparison with Monte Carlo semiclassical calculations reveals that the macroscopic Fresnel equations still properly describe the observed phase of the IR field on the Cu(111) surface on an atomic length and an attosecond time scale.

The electronic band structure governs the electron dynamics in solids. It defines a group velocity and an effective mass of the electronic wave packet. Recent experimental and theoretical studies suggest that an electron acquires the effective mass of its excited state over distances much larger than the lattice period of the solid. Therefore, electron propagation on atomic length scales was typically considered to be free-electron-like. Here, we test this hypothesis by probing attosecond photoemission from a Cu(111) surface. We use attosecond pulse trains in the extreme-ultraviolet (21-33 eV) to excite electrons from two initial bands within the 3d-valence band of copper. We timed their arrival at the crystal surface with a probing femtosecond infrared pulse, and found an upper limit of 350±40 as (1 as=10−18 s) for the propagation time an electron requires to assume the effective mass of its excited state. This observation implies that a final-state Bloch wave packet forms within a travel distance of 5-7 Å, which is at most two atomic layers. Using well-established theory, our measurements demonstrate the importance of the band structure even for atomic-scale electron transport. The electronic band structure governs the electron dynamics in solids. It defines a group velocity and an effective mass of the electronic wave packet. Recent experimental and theoretical studies suggest that an electron acquires the effective mass of its excited state over distances much larger than the lattice period of the solid. Therefore, electron propagation on atomic length scales was typically considered to be free-electron-like. Here, we test this hypothesis by probing attosecond photoemission from a Cu(111) surface. We use attosecond pulse trains in the extremeultraviolet (21-33 eV) to excite electrons from two initial bands within the 3d-valence band of copper. We timed their arrival at the crystal surface with a probing femtosecond infrared pulse, and found an upper limit of 350 40 as (1 as 10 −18 s) for the propagation time an electron requires to assume the effective mass of its excited state. This observation implies that a final-state Bloch wave packet forms within a travel distance of 5-7 Å, which is at most two atomic layers. Using well-established theory, our measurements demonstrate the importance of the band structure even for atomic-scale electron transport.

Molecular reactivity is determined by the energy levels and spatial extent of the frontier orbitals. Orbital tomography based on angle-resolved photoelectron spectroscopy is an elegant method to study the electronic structure of organic adsorbates, however, it is conventionally restricted to systems with one single rotational domain. In this work, we extend orbital tomography to systems with multiple rotational domains. We characterise the hydrogen evolution catalyst Co-pyrphyrin on an Ag(110) substrate and compare it with the empty pyrphyrin ligand. In combination with low-energy electron diffraction and DFT simulations, we fully determine adsorption geometry and both energetics and spatial distributions of the valence electronic states. We find two states close to the Fermi level in Co-pyrphyrin with Co character that are not present in the empty ligand. In addition, we identify several energetically nearly equivalent adsorption geometries that are important for the understanding of the electronic structure. The ability to disentangle and fully elucidate multi-configurational systems renders orbital tomography much more useful to study realistic catalytic systems.

Electronic wave functions of planar molecules can be reconstructed via inverse Fourier transform of angle-resolved photoelectron spectroscopy (ARPES) data, provided the phase of the electron wave in the detector plane is known. Since the recorded intensity is proportional to the absolute square of the Fourier transform of the initial state wave function, information about the phase distribution is lost in the measurement. It was shown that the phase can be retrieved in some cases by iterative algorithms using a priori information about the object such as its size and symmetry. We suggest a more generalized and robust approach for the reconstruction of molecular orbitals based on state-of-the-art phase-retrieval algorithms currently used in coherent diffraction imaging (CDI). We draw an analogy between the phase problem in molecular orbital imaging by ARPES and of that in optical CDI by performing an optical analogue experiment on micrometer-sized structures. We successfully reconstruct amplitude and phase of both the micrometer-sized objects and a molecular orbital from the optical and photoelectron far-field intensity distributions, respectively, without any prior information about the shape of the objects.