Dr. habil. Michael Martins
|University of Hamburg
Faculty of Mathematics, Informatics and Natural Sciences
Department of Physics
Luruper Chaussee 149
The main topic of my research is core level spectroscopy of small quantum systems
as atoms, molecules and clusters in the gas phase to study the electronic
structure and dynamic of these systems. Core level spectroscopy is an ideal tool
to study these properties of complex materials, due to its element and site
These experiments, as photoelectron and x-ray absorption spectroscopy are
performed at modern third generation synchrotron radiation sources as Bessy II
or Petra III and the free electron laser (FEL) FLASH in Hamburg. A core part
of my work is the development of new instrumentation for these facilities as
e.g. beamlines for photons and ions.
In particular the work at FLASH is done different collaborations with the PTB in
Berlin and Hasylab. Hence, I am also a member of the
Electronic structure of free mass selected cluster ions
Clusters are a new class of materials with properties between those of the free atoms and the corresponding solids. The properties of small clusters in the size range from several atoms up to a few hundred atoms per cluster are dominated by quantum size effects, e.g. a special physical or chemical property of the cluster can vary strongly by adding or removing just one atom from the cluster. This behavior is well known as "Every Atom Counts".
Thus, for experiments on clusters it is essential to mass selected them, before analyzing their properties, which can be done quite easily with cluster ions. However, such mass selected cluster beams can only be created with low intensities with only a few hundred clusters per mm3. Therefore up to now only a few studies using soft x-ray radiation have been performed on mass selected clusters, because the synchrotron radiation sources are just not intense enough.
To perform such experiments, either a very long interaction region of the cluster beam with the soft x-ray radiation has to be realized or the number of photons has to be increased dramaticly. An increase of the photon number has become possible with the FEL FLASH. FLASH is a pulsed source and can deliver in 100 fs about 1013 photon, which is the same number a 3. generation synchrotron radiation delivers in 1 s. First core level photoelectron spectra of mass selected lead clusters have been taken during two beamtimes within the BMBF cluster collaboration.
FLASH is perfectly suited to perform photoelectron spectroscopy on clusters, however x-ray absorption spectroscopy is not possible, because the photon energy of FLASH can not be scanned.
To perform x-ray absorption spectroscopy on mass selected clusters within a BMBF collaboration we will setup the experiment PIPE (Photon Ion Spectroscopy at Petra III). In PIPE the cluster will interact with the soft x-ray radiation collinear for about 0.5-1 m, thus increasing the interaction region by a factor of 1000. PIPE will be permanently installed at the new XUV beamline at Petra III.
Nonlinear processes in the soft x-ray regime
The novel free electron laser FLASH delivers preeminent quality and quantity
photons in the soft x-ray regime, opening up new fields of physics and quantum
chemistry. For the first time nonlinear processes in the soft x-ray regime are
accessible. However due to the statistical nature of the SASE FEL a detailed
knowledge of every single FEL pulse in terms of intensity and spectral
distribution is mandatory. Therefore we have developed a new method to measure
the SASE spectra of every single FLASH pulse using photoelectron spectroscopy
of rare gases.
Radiation Chemistry and Properties of transient species
The chemistry of transient molecular species, as free radicals or molecular ions
is very important for many chemical processes. Such species can be created by UV
light and play a dominant role e.g. in the chemistry of the upper atmosphere.
One example is the ozone cycle in which stratospheric ozone is created and
destroyed by UV radiation.
Due to their huge chemical reactivity these transient species can only be produced
in very small amounts, similar as mass selected cluster. Here FLASH opens up a new
way for expriments on such reactive molecules, because due to the high photon flux
in the VUV radicals or molecular ions can be created and probed within one FEL
Inner-shell photoion and photoelectron spectroscopy of open shell atoms
During the last years a main part of my work concentrated on the
2p and 3p core level excitation of the 3d transition metal atoms and some 3d metal containing molecules.
Aim of these studies is to clarify the role of atomic effects in
solid state core level spectra of these important materials, in particular for magnetic dichroism
spectroscopy. These studies are of high relevance for condensed
matter physics, due the size reduction of structures for
technological applications. For such small structures in the nm
size region atomic and molecular effects might have an important
influence and models including these effects have to be applied.
Electronic and magnetic properties of deposited transition metal clusters
In particular for applications clusters as nanostructures have to be deposited on surfaces. Due to this cluster - substrate interaction the properties of the clusters might be strongly modified. Within this project core level spectroscopy is applied to small, mass selected transition metal clusters, deposited onto well defined surfaces. In particular we are interested in the magnetic structure of these nano structures, which is analyzed using the x-ray magnetic circular dichroism (XMCD) method.