Phonon spectral densities of Cu surfaces: Application to Cu(211)

Power phonon spectra of vicinal stepped surfaces of Cu(211) have been calculated using a molecular dynamics method combined with a semiempirical potential. The potential is based on an analytic form of inverse powers proposed by Finnis and Sinclair with the parametrization of Sutton and Chen. One of the four independent parameters of the potential was rescaled to reproduce the bulk phonon spectrum of Cu while retaining other properties of the bulk Cu close to the experimental values. Using this potential, we calculated the power surface phonon spectra, projection of the spectra at the high-symmetry points of surface Brillouin zone (SBZ), and the mean square displacements (MSD's) of atoms of the Cu(211) surface. The calculated projected phonon spectra at Γ̅ and at two new SBZ points (at X̅ and Y̅ ) compare favorably with experiment and theory when available. The MSD of the Cu(211) surface is also well reproduced and its temperature dependence shows that anharmonicity of the atomic motion becomes important above 200 K

Potential energy surfaces and dissociation mechanisms of molecular ions

The results of recent ab initio calculations of reaction paths are analyzed. Dissociation mechanisms are as a rule controlled by crossings or avoided crossings between potential energy surfaces. Conical intersections are found to be especially important, because they sometimes provide reaction paths which are both adiabatic and characterized by a low activation energy. Moreover, in the neighbourhood of conical intersections, non-adiabatic transitions between potential energy surfaces can take place. This corresponds to a competition between two reaction paths according as the system remains in the initial electronic state or leaves it. The outcome of the competition is determined by the consequences of the breakdown of the Born-Oppenheimer approximation. Several examples are discussed

Sections efficaces partielles de photoionisation mesurées et calculées pour CO entre 15 et 40 eV.

Vibrationally resolved partial photoionization cross sections of CO have been measured and theoretical partial cross sections have been calculated using the method of Raseev et al. and the two-step MQDT formulation of Giusti-Suzor and Lefebvre-Brion. We discuss the physical nature of the shape resonances appearing in the epsilon-sigma continua and we analyze their vibrational selectivity. A new assignment of the Rydberg series converging to CO+(B2Sigma+) is suggested and their electronic and vibrational selectivity is discussed