Jens Güdde

Electron dynamics in time and space: Following molecular excitation and electron transfer processes in time and space within a single experiment is a long-standing goal of spectroscopy in the field of chemistry. Wallauer et al. combined tomographic photoemission imaging with a femtosecond pump-probe scheme to trace the excited state molecular orbitals of surface-adsorbed molecules with both spatial and temporal resolution. The present demonstration opens a new window for investigating the ultrafast electron transfer dynamics in such processes as chemical reactions on surfaces and intermolecular charge transfers. Science , this issue p. 1056

Herein, experimental work on the ultrafast electron dynamics in the topological surface state (TSS) of three‐dimensional (3D) topological insulators (TIs) observed with time‐ and angle‐resolved two‐photon photoemission (2PPE) is reviewed. The focus is laid on the generation of ultrafast photocurrents and the time‐resolved observation of their decay. 2PPE not only allows to unambiguously relate the photocurrents to the spin‐polarized electronic surface states. Probing of the asymmetric momentum distribution of the electrons carrying the current makes it possible to study the microscopic scattering processes that govern the unusual electron transport in the time domain. Ultrashort mid‐infrared pump pulses permit not only a direct optical excitation of the TSS in Sb₂Te₃ but also lead to a strong asymmetry of the TSS population in momentum space. Two‐dimensional band mapping of the TSS shows that this asymmetry is in fact representative of a macroscopic photocurrent, while the helicity‐dependence of the photocurrent is found to be small. The time‐resolved observation of the photocurrent decay reveals a huge mean free path of the electrons in the TSS.

We study the role of electronic interface states on the electron transfer dynamics between layers of the organic semiconductor 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) and the (111) and (100) surfaces of silver. For this purpose, we investigate the change of the decay dynamics of the first (n = 1) image-potential state on these surfaces upon adsorption of an ordered monolayer of PTCDA by means of time-resolved two-photon photoemission (2PPE). We find that the already short lifetime of the (n = 1)-state on Ag(111) is only slightly further reduced by PTCDA adsorption, whereas a much stronger reduction by a factor of three is observed for adsorption on Ag(100) resulting in similar lifetimes for both orientations. We show by model calculations on the basis of an analytical one-dimensional pseudo-potential that the enhanced decay for PTCDA/Ag(100) can be attributed to the opening of an additional channel for electron-electron scattering by the formation of an interface state which is derived from the Shockley-type surface resonance of Ag(100).

The formation of image-potential states at the interface between a graphene layer and a metal surface is studied by means of model calculations. An analytical one-dimensional model-potential for the combined system is constructed and used to calculate energies and wave functions of the image-potential states at the Gamma-point as a function of the graphene-metal distance. It is demonstrated how the double series of image-potential states of free-standing graphene evolves into interfacial states that interact with both surfaces at intermediate distances and finally into a single series of states resembling those of a clean metal surface covered by a monoatomic spacer layer. The model quantitatively reproduces experimental data available for graphene/Ir(111) and graphene/Ru(0001), systems which strongly differ in interaction strength and therefore adsorption distance. Moreover, it provides a clear physical explanation for the different binding energy and lifetime of the first (n=1) image-potential state in the valley and hill areas of the strongly corrugated moire superlattice of graphene/Ru(0001).