Laser‐induced desorption of NO from NiO(100): Ab initio calculations of potential surfaces for intermediate excited states

In order to interpret the experimental results of the state resolved UV‐laser‐induced desorption of NO from NiO(100) (rotational and vibrational populations, velocity distributions of the desorbing NO molecules, etc.), we have performed ab initio complete active space self‐consistent field (CASSCF) and configuration interaction (CI) calculations for the interaction potential between NO and the NiO(100) surface in the electronic ground state and for those excited states which are involved in the desorption process. The NiO(100)–NO distance and the tilt angle between the NO axis and the surface normal have been varied. A cluster model containing a NiO8−5‐cluster embedded in a Madelung potential has been used for representing the NiO(100) surface. The excited states which are important for the desorption process, are charge transfer states of the substrate–adsorbate system, in which one electron is transferred from the surface into the NO‐2π‐orbital. The potential curves of these excited charge transfer states show deep minima (4 eV–5 eV) at surface/NO distances which are smaller than that in the ground state. The angular dependence of these potentials behaves similar as in the ground state. A semiempirical correction to the calculated excitation energies has been added which makes use of the bulk polarization of NiO. With this correction the charge transfer states are considerably stabilized. The lowest excitation energy amounts to about 4 eV which is in reasonable agreement with the onset of the laser desorption observed experimentally at about 3.5 eV. The density of the NO−‐like states is rather high, so that probably several excited states are involved in the desorption process. The potential energy curves for all of these states are quite similar, but the transitions from the ground state into different excited charge transfer states show strongly differing oscillator strengths, which are also strongly dependent on the surface/NO distance. This fact is important for the dynamics of the deexcitation process in the sense of a selection criterion for the states involved. The magnitude of the oscillator strengths is large in comparison with the excitation of NO in the gas phase, which might be an indication for the possibility of optical excitation processes. One dimensional wave packet calculations on two potential energy curves using fixed lifetimes for the excited state in each calculation have been performed and enable us to estimate the mean lifetime of the excited state to be 15 fs≤τ≤25 fs. This implies that the dynamics of the system is dominated by the attractive part of the excited state potential

Theoretical Investigation of Laser Induced Desorption of Small Molecules from Oxide Surfaces: A First Principles Study

State resolved laser induced desorption of NO molecules from a NiO(100) surface is studied theoretically. A full potential energy surface for the excited state was constructed by means of ab initio cluster calculations in addition to the potential energy surface for the ground state. Multidimensional wave packet calculations on these two surfaces allow a detailed simulation of experimental observables, such as velocity distributions and desorption probabilities, on a full ab initio basis

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

SUBSTITUENT EFFECTS ON BONDING PROPERTIES IN DIPHOSPHENES, DISILENES, AND DIIMINES

Schoeller W, STAEMMLER V. SUBSTITUENT EFFECTS ON BONDING PROPERTIES IN DIPHOSPHENES, DISILENES, AND DIIMINES. INORGANIC CHEMISTRY. 1984;23(21):3369-3373

SUBSTITUENT EFFECTS ON THE BONDING PROPERTIES IN DIPHOSPHENES AND DISILENES

Schoeller W, STAEMMLER V. SUBSTITUENT EFFECTS ON THE BONDING PROPERTIES IN DIPHOSPHENES AND DISILENES. PHOSPHORUS SULFUR AND SILICON AND THE RELATED ELEMENTS. 1983;18(1-3):450-450

CI CALCULATIONS OF THE ELECTRONIC SPECTRUM OF THE C2HC_{2}H RADICAL

$^{1}$ T. G. Heil and H. F. Schaefer III, J. Chem. Phys. 56, 958 (1972).Author Institution: IBM San Jose ResearchAb initio SCF and CI calculations have been performed for the ground and lowest excited states of the $C_{2}H$ radical. A medium-sized Gaussian basis set was used (double zeta and polarization functions and diffuse s-functions) and an iterative natural orbital technique adopted for the determination of weakly occupied valence shell orbita1s. $C_{2}H$ has a low-lying $^{2}\Pi$ state, about 0.5 eV $(T_{00})$ above the $^{2}\Sigma^{+}$ ground state, whereas all $\pi\pi^{*}$ excited states are much higher $(T_{00} > 6 \ eV)$. In addition to vertical and adiabatic excitation energies, equilibrium geometries and vibrationa1 frequencies have been determined for a number of excited states