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.

Combined orbital tomography study of multi-configurational molecular adsorbate systems.

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 [Formula: see text] 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.

Polarization-sensitive pulse reconstruction by momentum-resolved photoelectron streaking.

The precise knowledge of the electric field in close proximity to metallic and dielectric surfaces is a prerequisite for pump-probe experiments aiming at the control of dynamic surface processes. We describe a model to reconstruct this electric field in immediate surface proximity from data taken in photoelectron THz-streaking experiments with an angle-resolved electron analyzer. Using Monte-Carlo simulations we are able to simulate streaking experiments on arbitrary surfaces with a variety of initial electron momentum distributions and to reconstruct the effective electric field at the surface. Our results validate the approach and suggest energy regimes for optimal pulse reconstruction.

Erratum: "Following the molecular motion of near-resonant excited CO on Pt(111): A simulated x-ray photoelectron diffraction study based on molecular dynamics calculations" [Struct. Dyn. 2, 035102 (2015)].

[This corrects the article DOI: 10.1063/1.4922611.].

Following the molecular motion of near-resonant excited CO on Pt(111): A simulated x-ray photoelectron diffraction study based on molecular dynamics calculations.

A THz-pump and x-ray-probe experiment is simulated where x-ray photoelectron diffraction (XPD) patterns record the coherent vibrational motion of carbon monoxide molecules adsorbed on a Pt(111) surface. Using molecular dynamics simulations, the excitation of frustrated wagging-type motion of the CO molecules by a few-cycle pulse of 2 THz radiation is calculated. From the atomic coordinates, the time-resolved XPD patterns of the C 1s core level photoelectrons are generated. Due to the direct structural information in these data provided by the forward scattering maximum along the carbon-oxygen direction, the sequence of these patterns represents the equivalent of a molecular movie.

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.

Intramolecular competition in the photodissociation of C(3)D(3) radicals at 248 and 193 nm.

Motivated by recent experimental work, a theoretical study of the photodissociation of perdeuterated propargyl (D(2)CCD) and propynyl (D(3)CCC) radicals has been carried out, focusing on the C-C bond cleavage and D(2) loss channels. High-level ab initio calculations were carried out, and RRKM rate constants were calculated for isomerization and dissociation pathways. The resulting reaction barriers, microcanonical rate constants and product branching ratios are consistent with the experimental findings, supporting the overall mechanism of internal conversion followed by statistical dissociation on the ground state surface. We found loose transition states and very low exit barriers for two of the C-C bond cleavage channels and an additional CD(2) + CCD channel, which had not been reported previously. Our results probe the extent of propargyl and propynyl isomerization prior to dissociation at 248 and 193 nm and deliver a comprehensive picture of all ongoing molecular dynamics.

Photochemical deactivation pathways of the A-state allyl radical.

Ab initio direct molecular dynamics with trajectory surface hopping methods simulates the photochemical deactivation pathways of the allyl radical, C(3)H(5), following electronic excitation to the A-state. The electronically nonadiabatic dynamics mediated by two conical intersections produces predominantly hot ground state allyl radicals along both the disrotatory and conrotatory photochemical deactivation pathways with a near synchronous rotation of the terminal methylene groups. The electrocyclic transformation of the allyl radical to the cyclopropyl radical is a minor channel accounting for 8% of all trajectories with 98% of them following the disrotatory pathway.

Photodissociation of the propargyl and propynyl (C(3)D(3)) radicals at 248 and 193 nm.

The photodissociation of perdeuterated propargyl (D(2)CCCD) and propynyl (D(3)CCC) radicals was investigated using fast beam photofragment translational spectroscopy. Radicals were produced from their respective anions by photodetachment at 540 and 450 nm (below and above the electron affinity of propynyl). The radicals were then photodissociated at 248 or 193 nm. The recoiling photofragments were detected in coincidence with a time- and position-sensitive detector. Three channels were observed: D(2) loss, CD+C(2)D(2), and CD(3)+C(2). Observation of the D loss channel was incompatible with this experiment and was not attempted. Our translational energy distributions for D(2) loss peaked at nonzero translational energy, consistent with ground state dissociation over small (<1 eV) exit barriers with respect to separated products. Translational energy distributions for the two heavy channels peaked near zero kinetic energy, indicating dissociation on the ground state in the absence of exit barriers.

Spectroscopy and dynamics of A [(2)B1] allyl radical.

The A [(2)B1] <--X [(2)A2] band system between 380 and 420 nm was observed in a supersonic jet expansion. The allyl radical was found to dissociate following electronic excitation, releasing a hydrogen atom. Monitoring the appearance of the hydrogen atom photoproduct as a function of the excitation laser wavelength, similar spectral features are observed as in earlier absorption experiments. Time- and frequency-resolved photoionization of the hydrogen atom product provides information on the unimolecular dissociation dynamics. The measured dissociation rates and kinetic energy releases of both allyl radical, C3H5, and partially deuterated allyl radical, C3DH4, suggest direct loss of the central hydrogen atom, leading to allene as the major product.

Probing centrifugal barriers in unimolecular dissociation of the allyl radical.

Time-resolved photoionization of the hydrogen atom product from the allyl radical, C3H5, dissociation with 115 kcal/mol total energy provides information on the unimolecular dissociation dynamics. Vibrationally hot ground-state allyl radicals in both low and high J-states are prepared by electronic excitation to selected rovibrational states of C-state allyl followed by internal conversion. The measured dissociation rates and kinetic energy release are independent of the allyl parent rotational energy and suggest that centrifugal effects are unimportant in allyl radical dissociation at 115 kcal/mol.