Die Winkelabhängigkeit der Resonanzstreuung niederenergetischer Elektronen an N2

Results of scattering experiments with monochromatic electrons in the energy range from a few tenth of an eV to about 16 eV are reported. Below 1.8 eV collision energy no resonance structures have been found either in the total scattering (transmitted current) or in the elastic and inelastic differential cross sections. The resonances above 1.8 eV have been measured in the elastic and inelastic channels (up to vibrational quantum numberν=8 of the N₂-molecule) in the angular range from 10° to 110°. In the inelastic channelsν≧2 especially the first resonance peak appears asymmetric, as predicted by calculations of the associated Franck-Condon-factors. In all inelastic channels the angular dependence (due to the pure resonance scattering) shows maxima at 0° and 90° and a minimum around 55°, probably indicating gerade-symmetry of the associated N ₂ ^? -state. Excitation functions of three states of N₂ were measured between their thresholds (between 11 and 12 eV) and 15 eV, one of them showing several new resonance structures.     

XPS and UPS study of oxygen adsorption on Re(0001) at low temperatures

Oxygen adsorption was studied in the temperature range 50–300 K. Three adsorbed states were found: (i) an atomic state existing in the whole temperature range, (ii) a molecular state (physisorbed oxygen molecule) between 50 and 80 K, (iii) a peroxide state whose bond order between the two oxygen atoms is around one and which exists between 80 and 300 K.     

Microscopic model of NaNO2 in the paraelectric phase

Starting from a sterical hindrance potential for the motion of the NO₂-molecular group in the deformable cage of neighbouring Na-ions, we derive a microscopic model for the NaNO₂ crystal in the paraelectric phase. The dynamical variables are the translational displacements of both the NO₂-groups and the Na-ions, and the reorientations of the NO₂-groups. Reorientations are described by means of symmetry adapted functions. From a numerical study of the model, we conclude that reorientations of the NO₂-groups take place essentially through rotations about the crystallographicc-axis. The model explains why optical experiments have led to the incorrect conclusion of reorientations about thea-axis. By studying the symmetry properties of the bilinear coupling of translations and rotations, we separate optical and acoustical displacements. Only the former couple to the order parameter in the long wavelength limit. Therefore there is no acoustical soft mode at the ferroelectric phase transitions. The bilinear coupling leads to an effective lattice mediated interaction among reorienting NO₂-groups.     

Spektren und Winkelverteilungen der Photoelektronen von Atomen und Molekülen

The photoelectron energy spectra produced by the impact of 584 Å photons on the gases Kr, H₂, N₂, O₂, CO, NO and several alkanes are reported. The angular distribution of photoelectrons corresponding to the simultaneous formation of specific electronic states of the ion have been measured in the range 30°—130°. A preference for electron ejection in the direction of light propagation is shown in the formation of the electronic ground states of NO⁺ and O ₂ ⁺ . Both involve the ejection of an electron from aπ _g orbital. The onset energies for the various electronic states have been obtained with a resolution of ca. 40 meV. The relative transition probabilities for formation of various electronic states in a given molecular ion, as well as the Franck-Condon factors within specific states have been obtained. Similar experiments with the alkanes (methane, ethane, propane, and n-butane) reveal directly the existence of widely separated electron states (or grouping of states) in the ion, as well as the probability of forming these states. This energy distribution function, of vital importance to the study of the unimolecular decay of ions, could only be inferred previously by use of a questionable assumption.     

Die differentiellen Anregungsfunktionen dern+2-Zustände von Helium

Energy and angular dependences for the electron impact excitation of the Helium 2³ S, 2¹ S, 2³ P, and 2¹ P states have been measured using monochromatic (0.05 eV) electrons in the energy range from 19 to 23 eV and for scattering angles ranging from 0 to 110°. The structures near the 2³ S-threshold have been analyzed in terms of resonant and non resonant i.e. direct scattering phase shifts. The first peak in this excitation function at 19.95 eV is mostly due to the rapid change with collision energy of the directs-wave phase of the inelastic scattering, which in turn is a consequence of the existence of the 2² S-resonance at 19.3 eV. The energy position and behaviour with collision energy of the first peak at 20.7 eV in the 2¹ S-channel is in agreement with the predictions for the existence of a virtual resonance state ofBurke et al. A term sceme of the He^? resonances with assignments of configurations is presented.     

Adsorption de l'éthyléne et de l'acétylène sur des films de rhénium evaporés sous ultra-vide; Hydrogénation de l'éthylène: II. Discussion des résultats expérimentaux et interprétation

A qualitative model is proposed in order to explain our experimental results on ethylene chemisorption on evaporated rhenium films and hydrogenation of ethylene (part I). The surface must present at least two kinds of surface sites (A and B). The second type (B), either preexists on the surface, or is induced by the adsorption phenomenon itself. On the most energetic ones (A), dissociation of ethylene and hydrogen is complete. Adsorption of ethylene is characterized by a sticking coefficient value of 0.1 if they are free and 1 if they are hydrogen covered. On sites B, ethylene is adsorbed without full dissociation (sticking coefficients equal to 0.015). independent on adsorption temperature. Hydrogen desorption is due to full dissociation of ethylene on the surface and a displacement reaction while ethane is produced by reaction between non-dissociated adsorbed ethylene and hydrogen in the gas phase. The same Rideal-Eley mechanism applies for hydrogenation of ethylene in quasi-stationary conditions, along with a self-poisoning mechanism involving dehydrogenation leading to C₂H₂ non-hydrogenable adsorbed species.