|Time- and Angle-Resolved Two-Photon Photoemission (TR&AR 2PPE).
In 2PPE, contrary to conventional photoemission, electrons are emitted after absorption of
two photons (h1 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
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, Rettig
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)
||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)
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)
||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)
The reflectivity of a material is determined by the response of the
materials 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 materials 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 materials structure. When this force is sudden,
the material can be made to ring in terms of its characteristic frequencies, or phonons.
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
||K. Ishioka and O.V. Misochko. In: Progress in Ultrafast Intense Laser Science V, Springer Series in Chemical Physics 98, 23 (2010)
||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: Kumagai
Review article: Y. Kuk et al., Rev. Sci. Instrum. 60, 165 (1989)
|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.
|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: Kumagai
Review article: B. Pettinger, Mol. Phys. 108, 2039 (2010)
|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.
|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