Abstract
This thesis is concerned with the photodesorption process of
diatomic molecules from oxide surfaces. As different systems
are considered, it is divided with respect to the desorbate motional
degree of freedom.
Stereodynamical effects of CO-molecules desorbing from a chromiumoxide
surface are treated within the category of rotational excitation.
A DIET (Desorption Induced by Electronic Transitions) process is
simulated, taking into account a two state model which includes
the electronic ground state and an electronically excited
state of the adsorbate-substrate system.
The potential energy surfaces for the two states involved were obtained
by ab initio calculations, the excited state is based on an internal CO excitation.
A stochastic wavepacket approach is applied in three dimensions
(one distance coordinate and two angular degrees of freedom) and yields
mechanistic insight into the desorption process. The rotational alignment
of the desorbing CO-molecules is understood by considering both
the electronic ground and excited state topology. The wave packet
calculations demonstrate the importance of a high-dimensional
treatment of photodesorption from surfaces.
Within the category of vibrational excitation ab initio and wave
packet calculations with the intention of simulating the vibrational
excitation of NO desorbing from a NiO(100) surface after laser irradiation
are reported.
Quantitative agreement between experimental and theoretical vibrational
state populations is obtained if the electrostatic field of the positively
charged surface is taken into account. This field leads to a considerable
shortening of the equilibrium bond lenght of the anionic intermediate,
whereas the equilibrium distance of the neutral NO-molecule is nearly
unaffected.
The role of the thermal population of the molecule-surface vibration is
studied using a model potential adopted to the system NO on chromiumoxide.
Calculations are performed concerning the temperature dependence of the
desorption yield and of the velocity distributions after laser induced desorption.
Furthermore, the thesis includes a discussion of the numerical treatment
of angular coordinates and a detailed description of the highdimensional
wavepacket code.
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