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

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

An atomic and molecular database for analysis of submillimetre line observations

Atomic and molecular data for the transitions of a number of astrophysically interesting species are summarized, including energy levels, statistical weights, Einstein A-coefficients and collisional rate coefficients. Available collisional data from quantum chemical calculations and experiments are extrapolated to higher energies. These data, which are made publically available through the WWW at http://www.strw.leidenuniv.nl/~moldata, are essential input for non-LTE line radiative transfer programs. An online version of a computer program for performing statistical equilibrium calculations is also made available as part of the database. Comparisons of calculated emission lines using different sets of collisional rate coefficients are presented. This database should form an important tool in analyzing observations from current and future (sub)millimetre and infrared telescopes.Comment: Accepted for publication in A&A, 14 pages, 5 figure

Electronic states of the Cr₂O₃(0001) surface from ab initio embedded cluster calculations

The electronic structure of the Cr2O3(0001)(0001) surface is studied by means of ab initio embedded cluster calculations. Charge distributions and local d-d excitations are analysed for different geometrical relaxations of the surface. In the ground state there is considerable delocalization of electrons from the oxygen 2p band into the partly occupied 3d AOs of the Cr cations at the surface such that their ionicity is reduced to Cr2+ as compared to Cr3+ in the bulk. The calculated d-d and charge transfer excitations agree very well with the prominent loss peaks observed experimentally by means of electron energy loss spectroscopy (EELS) at the Cr2O3(0001) surface

Laser induced desorption of NO from NiO(100): Characterization of potential energy surfaces of excited states

In order to interpret experimental results such as velocity flux distributions and rotational/vibrational populations of the state resolved UV-laser induced desorption of NO from NiO(100) ab initio calculations at the configuration interaction (CI) and complete active space self consistent field (CASSCF) levels have been performed for the electronic ground state and those excited states which are important for the desorption process. The NO/NiO(100) system was described by a NiO(8-5) cluster embedded in a Madelung field of point charges with NO adsorbed in the on-top position on the central Ni2+ ion. Two-dimensional potential energy surfaces for several electronic states have been calculated as a function of the N-Ni distance and the tilt angle of NO towards the surface normal. The excited states involved in the desorption process are charge transfer states in which one electron is transferred from the oxygen 2p-shell into the NO 2π-orbitals. The dependence of the potential energy surfaces on the N-Ni distance is dominated by a strong Coulomb attraction between the NO− ion formed as an intermediate and the hole created within the cluster. The angular dependence of the potentials favours an upright adsorption geometry if NO− is approaching the surface. This offers an explanation of the strong coupling between translation and rotation, which has been observed experimentally for the system NO/NiO(100) , as well as the absence of such a coupling in the system NO/NiO(111)

Weak thermal reduction of biphase Fe2O3 0001 films grown on Pt 111 Sub surface Fe2 formation

The initial thermal reduction of biphase Fe₂O₃(0001) films grown on Pt(111) has been studied with HREELS, LEED, TDS, and synchrotron-based valence band photoelectron spectroscopy. Ab initio calculations of the electronic excitation energies of Fe²⁺ and Fe³⁺ ions in different oxidic environments were carried out to support the experimental studies. Annealing the biphase Fe₂O₃(0001) at 1000 K results in the desorption of oxygen and a concomitant significant change of the electronic excitation spectra measured with HREELS. On the other hand, studies employing more surface sensitive methods like LEED, vibrational spectroscopy of adsorbates, and surface-sensitive valence band photoelectron spectroscopy reveal barely any changes induced by the desorption of oxygen. Based on these experimental findings we propose that the thermal reduction of biphase Fe₂O₃(0001) occurs mostly below the surface under the chosen conditions