Relaxation Dynamics of Photoexcited Charge Carriers at the Bi(111) Surface

Bi possesses intriguing properties due to its large spin-orbit coupling, e.g. as a constituent of topological insulators. While its electronic structure and the dynamics of electron-phonon coupling have been studied in the past, photo-induced charge carriers have not been observed in the early phases of their respective relaxation pathways. Using two-photon photoemission (2PPE) we follow the de-excitation pathway of electrons along the unoccupied band structure and into a bulk hole pocket. Two decay channels are found, one of which involves an Auger process. In the hole pocket, the electrons undergo an energetic stabilization and recombine with the corresponding holes with an inverse rate of 2.5~ps. Our results contribute to the understanding of the charge carrier relaxation processes immediately upon photo-excitation, particularly along the $\Gamma T$-line where the electron dynamics have not been probed with time-resolved 2PPE so far.Comment: 8 pages, 5 figure

Unoccupied electronic band structure of the semi-metallic Bi(111) surface probed with two-photon photoemission

While many photoemission studies have dealt with both the bulk band structure and various surface states and resonances, the unoccupied electronic structure above the Fermi level of the Bi(111) surface has not yet been measured directly although understanding of this model semi-metal is of great interest for topological insulators, spintronics and related fields. We use angle-resolved two-photon photoemission to directly investigate the occupied and unoccupied p bands of Bi, including the bulk hole pocket at the T point, as well as the image potential states and surface states of Bi(111).Comment: 9 pages, 7 figure

Optically and thermally induced molecular switching processes at metal surfaces

Using light to control the switching of functional properties of surface-bound species is an attractive strategy for the development of new technologies with possible applications in molecular electronics and functional surfaces and interfaces. Molecular switches are promising systems for such a route, since they possess the ability to undergo reversible changes between different molecular states and accordingly molecular properties by excitation with light or other external stimuli. In this review, recent experiments on photo- and thermally induced molecular switching processes at noble metal surfaces utilizing two-photon photoemission and surface vibrational spectroscopies are reported. The investigated molecular switches can either undergo a trans–cis isomerization or a ring opening–closure reaction. Two approaches concerning the connection of the switches to the surface are applied: physisorbed switches, i.e. molecules in direct contact with the substrate, and surface- decoupled switches incorporated in self-assembled monolayers. Elementary processes in molecular switches at surfaces, such as excitation mechanisms in photoisomerization and kinetic parameters for thermally driven reactions, which are essential for a microscopic understanding of molecular switching at surfaces, are presented. This in turn is needed for designing an appropriate adsorbate–substrate system with the desired switchable functionality controlled by external stimuli

Photo-induced and thermal reactions in thin films of an azobenzene derivative on Bi(111)

Azobenzene is a prototypical molecular switch which can be interconverted with UV and visible light between a trans and a cis isomer in solution. While the ability to control their conformation with light is lost for many molecular photoswitches in the adsorbed state, there are some examples for successful photoisomerization in direct contact with a surface. However, there the process is often driven by a different mechanism than in solution. For instance, photoisomerization of a cyano-substituted azobenzene directly adsorbed on Bi(111) occurs via electronic excitations in the substrate and subsequent charge transfer. In the present study we observe two substrate- mediated trans–cis photoisomerization reactions of the same azobenzene derivative in two different environments within a multilayer thin film on Bi(111). Both processes are associated with photoisomerization and one is around two orders of magnitude more efficient than the other. Furthermore, the cis isomers perform a thermally induced reaction which may be ascribed to a back-isomerization in the electronic ground state or to a phenyl ring rotation of the cis isomer

Polaron dynamics in thin polythiophene films studied with time-resolved photoemission

Femtosecond time-resolved two-photon photoemission spectroscopy is employed to study the dynamics of an excited state in a thin regioregular poly(3-hexylthiophene) (RR-P3HT) film deposited on a conducting polymer poly(3,4-ethylene-dioxythiophene): poly-(styrenesulfonate) (PEDT:PSS) electrode following optical excitation at 2.1 eV. We found that the biexponential decay of this excited state has a fast component (2.6 ps) assigned to bound polaron pairs which recombine quickly or separate to be added to the slow component (7.6 ps). The latter is attributed to polarons generated via charge transfer between adjacent polymer chains

Dynamics of optically excited electrons in the conducting polymer PEDT:PSS

Femtosecond time-resolved two-photon photoemission spectroscopy is employed to study the dynamics of the non-equilibrium electron distribution in the conducting polymer poly(3,4-ethylene-dioxythiophene): poly-(styrenesulfonate) (PEDT:PSS) film following optical excitation at 2.1 eV. We found that the electron thermalization occurs on a ultrafast timescale of around 60 fs analogous to the relaxation times of optically excited electrons in Au(111)

Image potential states at chevron-shaped graphene nanoribbons /Au(111) interfaces

Image potential states (IPSs) have been observed for various adsorbed carbon structures, such as graphene or carbon nanotubes. Graphene nanoribbons (GNRs) are intriguing nanostructures with a significant band gap which promise applications in nanotechnology. In the present paper we employ two-photon photoemission (2PPE) to investigate the unoccupied electronic structure and particularly the IPS of chevron-shaped GNR which are synthesized in a thermally activated on-surface synthesis on Au(111). Angle- and time-resolved 2PPE are utilized to gain further insights into the properties of the IPS. Compared to the pristine surface, reduced effective masses between 0.6 and 0.8 electron masses are observed and the lifetimes of the IPS are below the experimental detection limit, which is in the femtosecond regime. Independent of the concentration of N dopant atoms introduced in the GNR we observe a constant binding energy with respect to the vacuum level of the system

Azobenzene versus 3,3',5,5'-tetra-tert-butyl-azobenzene (TBA) at Au(111): Characterizing the role of spacer groups

We present large-scale density-functional theory (DFT) calculations and temperature programmed desorption measurements to characterize the structural, energetic and vibrational properties of the functionalized molecular switch 3,3',5,5'-tetra-tert-butyl-azobenzene (TBA) adsorbed at Au(111). Particular emphasis is placed on exploring the accuracy of the semi-empirical dispersion correction approach to semi-local DFT (DFT-D) in accounting for the substantial van der Waals component in the surface chemical bond. In line with previous findings for benzene and pure azobenzene at coinage metal surfaces, DFT-D significantly overbinds the molecule, but seems to yield an accurate adsorption geometry as far as can be judged from the experimental data. Comparing the trans adsorption geometry of TBA and azobenzene at Au(111) reveals a remarkable insensitivity of the structural and vibrational properties of the -N=N- moiety. This questions the established view of the role of the bulky tert-butyl-spacer groups for the switching of TBA in terms of a mere geometric decoupling of the photochemically active diazo-bridge from the gold substrate.Comment: 9 pages including 6 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

Electronic structure of an iron porphyrin derivative on Au(1 1 1)

Surface-bound porphyrins are promising candidates for molecular switches, electronics and spintronics. Here, we studied the structural and the electronic properties of Fe-tetra-pyridil-porphyrin adsorbed on Au(1 1 1) in the monolayer regime. We combined scanning tunneling microscopy/spectroscopy, ultraviolet photoemission, and two-photon photoemission to determine the energy levels of the frontier molecular orbitals. We also resolved an excitonic state with a binding energy of 420 meV, which allowed us to compare the electronic transport gap with the optical gap

Ultrafast Exciton Population, Relaxation, and Decay Dynamics in Thin Oligothiophene Films

Femtosecond time-resolved two-photon photoemission spectroscopy is utilized to determine the electronically excited states dynamics at the α-sexithiophene (6T)/Au(111) interface and within the 6T film. We found that a photoinduced transition between the highest occupied molecular orbital and lowest unoccupied molecular orbital is essential in order to observe exciton population, which occurs within 100 fs. In thin 6T films, the exciton exhibits a lifetime of 650 fs. On a time scale of 400 fs, an energetic stabilization is observed leading to the formation of a polaron or electron trapping at defect states. The lifetime of this state is 6.3 ps. Coverage-dependent measurements show that apart from the excited state decay within the film, a substrate- mediated relaxation channel is operative. The present study demonstrates that two-photon photoemission spectroscopy is a powerful tool to investigate the whole life cycle from creation to decay of excitons in an organic semiconductor