Three-Dimensional Ab Initio Quantum Dynamics of the Photodesorption of CO from Cr₂O₃(0001): Stereodynamic Effects

Having performed the first three-dimensional ab initio quantum dynamical study of photodesorption from solid surfaces, we gained mechanistic understanding of the rotational alignment observed in the CO/Cr2O3(0001) system. Our study is based on potential energy surfaces obtained by embedded cluster calculations for both the electronic ground and excited state of the adsorbate substrate complex. Stochastic wave packet calculations demonstrate the importance of the angular degrees of freedom for the microscopic picture of the desorption process in addition to the desorption coordinate

First-principles study of the polar O-terminated ZnO surface in thermodynamic equilibrium with oxygen and hydrogen

Using density-functional theory in combination with a thermodynamic formalism we calculate the relative stability of various structural models of the polar O-terminated (000-1)-O surface of ZnO. Model surfaces with different concentrations of oxygen vacancies and hydrogen adatoms are considered. Assuming that the surfaces are in thermodynamic equilibrium with an O2 and H2 gas phase we determine a phase diagram of the lowest-energy surface structures. For a wide range of temperatures and pressures we find that hydrogen will be adsorbed at the surface, preferentially with a coverage of 1/2 monolayer. At high temperatures and low pressures the hydrogen can be removed and a structure with 1/4 of the surface oxygen atoms missing becomes the most stable one. The clean, defect-free surface can only exist in an oxygen-rich environment with a very low hydrogen partial pressure. However, since we find that the dissociative adsorption of molecular hydrogen and water (if also the Zn-terminated surface is present) is energetically very preferable, it is very unlikely that a clean, defect-free (000-1)-O surface can be observed in experiment.Comment: 10 pages, 4 postscript figures. Uses REVTEX and epsf macro