71 research outputs found
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 -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
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