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Ch. Rameshan, A. Bukhtiyarov, G. Rupprechter:
"Synchrotron based AP-XPS for obtaining correlations between surface structure and catalytic selectivity";
Talk: Research at European Neutron and Synchrotron Facilities by Austrian Scientists, Wien (invited); 2013-11-11 - 2013-11-12; in: "Research at European Neutron and Synchrotron Facilities by Austrian Scientists", TU Wien, Wien (2013), 32.

The interface between solids and gas govern many processes in the environment, the heterogeneous
catalysis and in energy generation. As examples the removal of harmful components
from automotive exhaust streams, the reaction of fuels at the electrodes of solid oxide
fuel cells or the cloud droplet nucleation on atmospheric aerosol particles. There are several
surface sensitive spectroscopies and microscopies that can be used to study vapor/solid interfaces,
such as infrared spectroscopy (IR), vibrational sum frequency generation (VSFG), Xray
emission spectroscopy (XES), surface X-ray diffraction (SXRD), scanning tunneling
microscopy (STM), transition electron microscopy (TEM) and scanning electron microscopy
(SEM).
X-ray photoelectron spectroscopy (XPS) is one of the most versatile methods for the investigations
of surfaces on the atomic scale [1]. lt gives quantitative information about the elemental
composition and about the chemical specificity (e.g., oxidation state) of the surface.
Due to the strong interactions of electrons with atoms at typical electron energies used in XPS
( 100 e V - 1500 e V) the mean free path of the electrons is only on the order of several monolayers,
giving XPS exquisite surface sensitivity [2]. Especially by using tunable synchrotron
radiation the surface sensitivity can be enhanced to a maximum.
For measurements under elevated pressures it has to be considered that photoelectrons are
strongly scattered by gas molecules, which complicates the application of XPS at this conditions.
For instance, the inelastic mean free path of electrons with 100 eV kinetic energy in 1
mbar water vapor is about 1 mm [3]. This is much shorter than the typical working distance
between sample and analyzer. The attenuation of photoelectrons by gas molecules can be
overcome by the use of differentially pumped analyzer lens systems and a special geometry
between X-ray source, sample and analyzer.
The proposed talk will focus on the method of synchrotron based XPS for obtaining detailed
geometric and electronic surface structure on different materials. With the use of tunable synchrotron
radiation a 3-dimensianl picture of the near surface region can be obtained via depth
profiling. Additionally the principles of XPS at elevated pressures (AP-XPS) and the needed
advanced experimental equipment will be highlighted. On the example of selected research
topics the possibilities of this spectroscopic method will be demonstrated. In particular how
correlations between the electronic and geometric structure and the chemical reactivity of
model systems can be obtained.

AP-XPS, Synchrotron, Catalysis