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

Laser induced desorption of CO from Cr₂O₃(0001): Ab initio calculation of the four-dimensional potential energy surface for an intermediate excited state

Ab initio cluster calculations for an excited state of the system CO/Cr2O3 (0001) are presented. A CO(a (3)Pi)-like state generated by an internal electronic excitation of the CO molecule (5sigma --> 2pi*) is considered to be a possible intermediate for the laser-induced desorption of CO from Cr2O3 (0001). A four-dimensional potential energy surface (PES) for this state is calculated at the CASSCF level of theory. A few possible alternatives for the desorption intermediate are also discussed. The interaction mechanism between the excited CO molecule and the-surface is analyzed, and it is shown that the change of symmetry of the quadrupole moment of the CO molecule is responsible for the topology of the calculated PES

Adsorption of CO on Cr₂O₃(0 0 0 1)

The adsorption of CO molecules on the Cr-terminated (0001) surface plane of Cr2O3 is studied theoretically by means of quantum chemical cluster calculations and experimentally by thermal desorption spectroscopy and infrared reflection absorption spectroscopy. The combination of these approaches yields a detailed picture of the CO/Cr2O3(0001) adsorption. In the low coverage limit, CO is strongly bound (chemisorbed) and occupies a O3-hollow position, i.e. it is adsorbed above a threefold hollow site in the first full oxygen layer, with the CO axis oriented along a line connecting two Cr ions at the surface and tilted strongly against the surface normal. CO molecules in this position desorb at 175 K, which corresponds to an adsorption energy of 45 kJ/mol, and exhibit a blue shift of the CO stretching frequency of about 35 cm−1 with respect to CO in the gas phase (from 2143 to 2178 cm−1). At higher coverages, a second desorption peak at 105 K is found which is accompanied by a small red shift of −10 cm−1 of the CO stretching frequency. This adsorption state corresponds to a physisorption of CO with an adsorption energy of 28 kJ/mol and is tentatively assigned to CO molecules adsorbed parallel to the surface above one O2− anion in the oxygen layer

Photo-induced desorption of NO from NiO(100): calculation of the four-dimensional potential energy surfaces and systematic wave packet studies

The velocity distributions of the laser-induced desorption of NO molecules from an epitaxially grown film of NiO(100) on Ni(100) have been studied [Mull et al., J. Chem. Phys., 1992, 96, 7108]. A pronounced bimodality of velocity distributions has been found, where the NO molecules desorbing with higher velocities exhibit a coupling to the rotational quantum states J. In this article we present simulations of state resolved velocity distributions on a full ab initio level. As a basis for this quantum mechanical treatment a 4D potential energy surface (PES) was constructed for the electronic ground and a representative excited state, using a NiO5Mg18+13 cluster. The PESs of the electronic ground and an excited state were calculated at the CASPT2 and the configuration interaction (CI) level of theory, respectively. Multi-dimensional quantum wave packet simulations on these two surfaces were performed for different sets of degrees of freedom. Our key finding is that at least a 3D wave packet simulation, in which the desorption coordinate Z, polar angle and lateral coordinate X are included, is necessary to allow the simulation of experimental velocity distributions. Analysis of the wave packet dynamics demonstrates that essentially the lateral coordinate, which was neglected in previous studies [Klüner et al., Phys. Rev. Lett. 1998, 80, 5208], is responsible for the experimentally observed bimodality. An extensive analysis shows that the bimodality is due to a bifurcation of the wave packet on the excited state PES, where the motion of the molecule parallel to the surface plays a decisive role

Single crystalline silicon dioxide films on Mo(1 1 2)

A preparation is reported which, for the first time, results in a thin, crystalline SiO2 film on a Mo(1 1 2) single crystal. The procedure consists of repeated cycles of Si deposition and subsequent oxidation, followed by a final annealing procedure. LEED pictures of high contrast show a crystalline SiO2 overlayer with a commensurate relationship to the Mo(1 1 2) substrate. Surface imperfections have been studied by SPA-LEED and a structure model, consistent with the appearance of antiphase domain boundaries as preferential disorder, is proposed. AES and XPS have been used to control film stoichiometry. A spatial dependence of the Si4+ core level shift with distance from the interface plane is observed and well explained by image charge interaction across the interface. Furthermore, the theoretically predicted insensitivity of the Si4+ core level shift with respect to the degree of crystallinity is experimentally verified for the first time. The wetting of the substrate by the film has been investigated by XPS and TDS. The results prove that the film covers the substrate completely