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Struktur und Dynamik Poster: Mi., 14:00–16:30 M-P160
Adsorption geometry of large organic molecules studied by NIXSW: Snphthalocyanine
and NTCDA on Ag(111)
Christoph Stadler 1 , Sören Hansen 1 , Florian Pollinger 1 , Jörg Stanzel 1 ,
Achim Schöll 1 , Christian Kumpf 1 , Eberhard Umbach 1
1 Experimentelle Physik II, Universität Würzburg, Am Hubland, 97074 Würzburg
The properties of organic thin films largely depend on the balance between moleculemolecule
and molecule-substrate interactions and can often be tuned by changing the
preparation conditions like temperature or coverage. This is also the case for 1,4,5,8-
Naphthalenetetracarboxylic dianhydride (NTCDA) and the family of phthalocyanines
(Pc) which are frequently studied as adsorbates on various surfaces.
Here we present results obtained for SnPc, a non-planar representative of the phathalocyanines,
and its adsorption on the Ag(111) surface in a coverage up to one monolayer.
We used a combination of the synchrotron-based NIXSW (normal incident x-ray standing
wave) method and SPA-LEED (spot profile analysing low energy electron diffraction)
to study the vertical and lateral adsorption geometry. Various different adsorption
phases where found, depending on coverage and substrate temperature, and tow phases
were investigated in detail. In the incommensurate monolayer structure at room temperature
(RT) with one molecule per unit cell it adsorbs in a tin-down geometry, where
the Sn is closer to the uppermost silver atoms than the N and C atoms. In the commensurate
submonolayer structure at 150K with two molecules per unit cell a mixed
structure with both, tin-up and tin-down geometries is found. The latter is only stable
at low temperatures and the corresponding RT structure is disordered. Furthermore,
the phenyl rings lie always closer to the surface than the porphyrine N atoms, an effect,
which is even stronger for the LT phase than for the RT phase. This bending is an
important information which clearly shows a strong interaction between the phenyl
rings of the molecule and the substrate.
In the case of NTCDA/Ag(111) the so-called relaxed monolayer was studied by NIXSW
and a significant vertical distortion of the molecule is found upon adsorption. The core
level shift of the carboxylic oxygen and the anhydride oxygen was used to determine the
individual atom heights above the silver surface. The carboxylic oxygen at the corners
of the molecule is significantly bended downwards, whereas the anhydride oxygen in
the bridge position is at the same height as the naphthalene core.
For all the analysis non dipolar effects of the photo electrons are taken into account.
In the case of SnPc/Ag(111) a multilayer is investigated to determine the asymmetry
parameter of the non dipolar contributions to the photoemission yield. In the case of
NTCDA/Ag(111) a comparison of O KLL and O1s data allows the determination of
this asymmetry parameter and the portion of electron induced Auger processes.