 
Angle-resolved
photoemission spectroscopy (ARPES).
ARPES is a powerful tool for the investigation of
the electronic structure of solid surfaces. UV photons impinge on the surface
and electrons are emitted into the vacuum by virtue of the photoelectric effect.
The energy and angle of emission of the photoelectrons is analyzed in an
electron spectrometer. This allows determination of the electronic band
structure as a function of the in-plane electron momentum k//
and electron energy E.
Time- and angle-resolved photoemission spectroscopy (trARPES)
extends and complements conventional ARPES by adding femtosecond
time-resolution. tr-ARPES has the capability of resolving elementary scattering
processes directly in the electronic band structure as function of energy and
electron momentum due to simultaneous measurement of the spectral and dynamic
information.
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In a
pump-probe scheme a femtosecond infrared laser pulse excites the sample by
electron-hole pair creation and a subsequent UV pulse probes the transient
electronic structure after a time delay  t. In detail,
ultrafast changes of the occupied electronic structure including
metal-to-insulator transitions, transient populations in unoccupied states,
cooling of excited carriers due to electron-phonon coupling, and collective
excitation modes such as coherent phonons are studied with tr-ARPES. The wide
range of microscopic processes accessible with tr-ARPES promises new insights on
the non-equilibrium properties of complex correlated materials.
Research group: Kirchmann, Stähler
Review article: S. Hüfner, “Photoelectron Spectroscopy”, Springer
(2003) |
| Time- and Angle-Resolved
Two-Photon Photoemission (TR&AR 2PPE). |
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In
2PPE, contrary to conventional photoemission, electrons are emitted after
absorption of two photons (h 1 and h2) with energies below the sample work function
 . The first photon (h1) is absorbed by an electron below the Fermi
level EF of the sample and excited to an intermediate state, which can, for
instance, be adsorbate-induced. The photoemission occurs when the same electron
absorbs another phonon (h2) that excites
the electron above the vacuum level Evac. The kinetic energy
of these photoelectrons is detected as a function of their emission angle,
yielding information about the dispersion E(k//) of the
intermediate state. The femtosecond time resolution is achieved by the variation
of the time delay between pump (h1) and
probe pulse (h2). The pump pulse creates a
non-equilibrium distribution of electrons in the sample and the resulting
relaxation dynamics in the intermediate state (e.g. population decay or carrier
localization) are monitored by the time-delayed probe pulse.
Research group: Ernstorfer, Stähler, Kirchmann
Review article: T. Fauster et al., Progr. Surf. Sci. 82,
224 (2007) |
Time-resolved second harmonic generation
spectroscopy.
Time-resolved second harmonic (SH) generation spectroscopy is a powerful
table-top tool for the investigation of electron, lattice, and spin dynamics at
surfaces and buried interfaces of centrosymmetric solids. The SH electric field
E=E0+EM, where E0 is independent
of and EM is proportional to the spin
polarization/magnetization M in the detection volume. Owing to its
surface/interface sensitivity, E monitors transient electron distribution
by E0 and M by EM in surface and interface states and in a couple of
monolayers thick subsurface region after excitation by the pump laser pulse.
E0 is also sensitive to band structure
variations originating from the transient lattice distortion produced by the
electron-phonon relaxation processes.
In the case of thin multilayer structures on optically
transparent substrates, the back pump-font probe configuration can be used when
the pump pulse generates hot carriers (HC) in the layer closest to the
substrate. If this layer is ferromagnetic, HC are spin-polarized and traversing
the structure, bring a certain M in the subsurface layer. Finally, HC trigger
electron, lattice and spin dynamics at the sample surface, which is detected by
the probe pulse. This approach allows one to study the HC transport and separate
different origins of observed dynamics by a comparison of different excitation
conditions realized for back and front pumping.
Research group: Melnikov, Stähler
Review article: A. Melnikov et al., Journal of Physics D 41, 164004
(2008) |
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Time Resolved Vibrational Sum Frequency (VSF)
Spectroscopy.
Understanding the localization and flow of energy
through molecular vibrations at interfaces is of use both as a means of probing
molecular level structure in these environments and in understanding their
chemical reactivity. Gaining such insight requires a spectroscopic method that
is interface specific. One approach (see TERS) is to use near field enhancement.
Another is to investigate the sum frequency response: the emission of an
electric field whose frequency is the sum of the frequencies of two incident
fields. Such emission is interface specific by its symmetry selection rules and,
when the frequency of one incident field is tuned to that of interfacial
molecular vibrations, increases dramatically: sum frequency emission gives the
vibrational spectrum of just molecules at an interface. By using an intense
(< 100 fs) infrared pulse as a pump and probing the VSF response, the
dissipation of this excitation, and therefore structural dynamics and
vibrational coupling, can be tracked over timescales from femtoseconds to
several picoseconds with interfacial specificity.
Research group: Campen
Review article: H. Arnolds et al., Surf. Sci. Rep. 65, 45 (2010)
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Terahertz Spectroscopy.
Many fundamental
elementary excitations in physical systems exhibit transition energies of the
order of 10meV, for example free electrons, excitons, magnons as well as lattice
and molecular vibrations. We investigate such excitations by means of
electromagnetic pulses with frequencies in the terahertz (THz) range. Thanks to
their low photon energy (photon energy 4.16nbsp;meV at 1THz), THz waves can
couple resonantly to these excitations. In part, we use THz pulses as a kind of
“ultrafast Ohm-meter” to measure the instantaneous conductivity of a sample that
was excited by a femtosecond laser pulse. Using appropriate models, the
instantaneous conductivity allows us to infer properties of the current sample
state (such as the temperature of the electron gas). On the other hand, intense
THz pulses can be also used to control matter. As shown in the figure, a THz
pulse drives the sample (here a spin wave in the antiferromagnet NiO) whose
state (here its magnetization) is probed by a subsequently arriving femtosecond
probe pulse. Using a second pump pulse, one can switch the excitation off.
Research group:
Kampfrath
Review article: R. Ulbricht, Rev. Mod. Phys. 83, 543
(2011) |
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Time-resolved electron diffraction.
Time-resolved electron
diffraction is a spectroscopic technique that provides atomic-level structural
information with femtosecond temporal resolution. A pulse of electrons with
femtosecond duration (1 fs = 10-15 s) diffracts
from a sample and gathers the atomic structure of the sample within this
ultrashort time window. By varying the arrival time of the electron pulse
relative to the arrival of a femtosecond laser pulse, we are able to watch
structural changes in the sample in response to the energy deposited by the
laser pulse. We develop a new method of electron diffraction that will provide
time-resolved structural information of surfaces.
Research group: Ernstorfer
Review article: R.J.D. Miller et al., Acta Cryst. A 66,
137-156 (2010) |
Transient reflectivity.
The reflectivity of a material is determined by
the response of the material’s free and bound charges to an incident light
field. When a strong pump laser beam excites a sample, the distribution of these
charges are modified and the material’s reflectivity changed. By probing the
reflectivity as a function of time, information on the relaxation and
propagation dynamics of the free and bound charges can be obtained. In addition,
prompt excitation of charges creates forces on the material’s structure. When
this force is sudden, the material can be made to ring in terms of its
characteristic frequencies, or phonons. |
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This ringing can be observed as a modulation
of the reflectivity, and enables transient reflectivity to measure both
electronic and structural processes in a material. By tuning the probe
wavelength, therefore, detailed information on the electronic and physical
structure can be obtained.
Research group: Ernstorfer, Stähler
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| Review article: |
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K. Ishioka and O.V. Misochko. In: Progress in Ultrafast Intense
Laser Science V, Springer Series in Chemical Physics 98, 23 (2010)
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Scanning tunneling microscopy (STM).
An atomically sharp
metallic tip is approached towards a conducting surface until the tunneling
current sets in at a distance of about 1 nanometer. When laterally scanning the
tip in this regime over the surface, the microscope takes advantage of the
strong dependence of the tunneling current on the tip-surface distance. By
keeping the current constant (see figure), the pathway of the tip reflects the
topography and the electronic structure of the surface and adsorbates with
atomic resolution. We use such instruments under ultrahigh vacuum conditions and
at low temperatures of around 5 K for imaging and spectroscopy, but also for
manipulation of single atoms and molecules by chemical forces, tunneling
electrons or the electric field in the junction. Furthermore, photochemical
processes are induced by light illumination.
Research group: Grill
Review article:
Y. Kuk et al., Rev. Sci. Instrum. 60, 165 (1989)
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Tip-enhanced Raman Spectroscopy
(TERS).
A new approach combines Raman spectroscopy at interfaces with a local
electromagnetic near-field enhancement provided by an illuminated STM tip and is
denoted as tip-enhanced Raman scattering (TERS). The tip is usually made from a
thin Au or Ag wire and has a sharp end of ~20 nm radius. The strong near-field
enhancement near the tip apex arises from the excitation of local surface
plasmon modes when laser light is focused onto the tip. In a sense, the tip acts
as an optical antenna that amplifies both the incident as well as the outgoing
(radiated) electromagnetic fields. The total enhancement may be million-fold or
higher. |
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Only molecules in the enhanced near-field
zone contribute to TERS via enhanced inelastic light scattering, which involves
the annihilation of an incident photon, the excitation of a molecular vibration
and the emission of a photon with a correspondingly altered energy. Thus, two
different types of information can be achieved simultaneously: (i) a TERS
spectrum that shows the vibrational states of the adsorbate, (ii) an STM image
of the same region, which exhibits the local environment of the adsorbate(s). In
other words, TERS simultaneously delivers topographic and chemical information
with very high sensitivity and nanometer resolution.
Research group: Pettinger
Review article: B. Pettinger, Mol. Phys. 108, 2039 (2010)
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| Computational simulations by coarse-grained
descriptions. |
Atomically resolved simulations of complex chemical systems are
computationally demanding. Even for single macromolecules, such as proteins,
all-atom molecular dynamics computations on supercomputers only allow following
their conformational dynamics up to a microsecond, whereas the characteristic
operation timescales of molecular motors and other protein machines lie in the
millisecond range. Therefore, coarse-grained descriptions are needed. In the
elastic-network approach, entire atomic groups (i.e., amino acids) are treated
as individual particles and the particles are viewed as connected by a set of
elastic strings. With these simplifications, complete operation cycles of
molecular machines can be traced. |
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The figure shows a detail of the atomic
structure of myosin and the respective part of its elastic network. Different
reduced descriptions are being used by us in the computational studies of other
complex systems, such as genetic expression and cellular signal transduction
networks.
Research group: Mikhailov
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