Verlag des Forschungszentrums Jülich
JUEL-3512
Bartolucci, Fabrizio
Negative Ionen-Resonanz von adsorbierten Molekülen
150 S., 1998
The adsorption of N2 on Ag(110) at 15 K leads to a physisorption of the
molecules. The properties of the negative ion resonance (NIR) have been investigated
by elctron energy loss spectroscopy (EELS) in the energy range of
the ²[Pi]g- and ²[Sigma]u-resonance. In the case of the ²[Pi]g-resonance the sample were
covered with N2 multilayer. For the ²[Sigma]u-resonance additional measurements
have been done in the mono- and bilayer regime. The excitation function, the
overtone excitation and the emission profile were determined for three different
coverages. The excitation functions yield the resonance energies for different
coverages and exhibits the influence of the image potential. The overtone excitation
ratio exhibits the short lifetime of the ²[Sigma]u-resonance compared to the
²[Pi]g-resonance. Since the overtone excitation ratio is constant in the whole
range of coverages, the image potential has no influence on the lifetime of the
NIR. The emission profile shows distinct changes with increasing coverages.
A simple model without the consideration of multiple scattering effects
determines the emission profiles for different structures of orientational ordering of
the molecules. These results and the LEED experiments confirm that the
molecule orientation changes with increasing coverage in the order herringbone
-+ pinwheel -+ [alpha]- N2. The N2 coverage of the Ag(110) surface were determined
by thermal desorption spectroscopy (TDS). The LEED pictures exhibit
an ordered structure of the N2 layers and strong similarities to the well known
N2 physisorption on graphite (0001). After the O2 exposure at 15 K the chemi-
sorbed and the physisorbed species ([alpha]-02) coexist on the surface. The strong
overtone excitation of the physisorbed layers were used to determine the
dissociation energies of the mono- and multilayer. The excitation function of the
chemisorbed species ([alpha]-02) exhibits a resonce energy of 2 eV. The TD spectra
show that the [alpha]-02 partly exists at 15 K and were partly built from physisorbed
O2 after annealing. Therefore the physisorbed O2 is a precursor state for
[alpha]-02. After further annealing the chemisorbed species approximately desorbs
completely without dissociation. Besides this [alpha]-02 species with an energy loss
at 85.0 meV a second chemisorbed species have been observed with an energy
loss at 79.5 meV ([beta]-02). This species appears after the O2 adsorption at 75 K.
After annealing [beta]-02 dissociates at temperatures below 200 K. The chapter
N0/NiAl(001) deals with the preparation of well-ordered oxynitrid films on
NiAl(001) after the No adsorption at 75 K. Subsequent annealing at 1200 K
leads to a aluminumoxynitride film (AlON). Even the exposure with NH3 and
O2 leads to the formation of AlON after annealing. The EEL spectrum
exhibits 5 Fuchs-Kliewer-Phonons, which are shifted in the energy compared to the
spectrum of [Theta]-A1>SUB>2O3. The band gap were determined to be 6.6 eV. The LEED
pictures show the same 2 dimensionallattice structure as [Theta]-A1>SUB>2O3. From auger
electron spectroscopy (AES) the atomic mass ratio were estimated to be about
2 nitrogen to 1 oxygen.
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