Quantum mechanics: cleaning up metaphysical baggage

The dynamic electronic structure of atoms and molecules can be directly observed by means of the (e, 2e) reaction, which measures the distribution of energies and momenta of two electrons in coincidence after a knockout reaction initiated by an electron beam of known momentum incident on a molecular gas target. The molecular state for each event is identified by the electron separation energy. The recoil momentum for each event is known from the difference of measured initial and final momenta. It has been verified that values of this momentum are equal under suitable conditions to the momentum of the electron in the target immediately before knockout. Thus the spherically-averaged electron momentum distribution for each molecular orbital is measured. This is directly related to molecular orbitals calculated by the methods of quantum chemistry. Properties obtained by this method for different types of molecules are discussed.     

Cyanogen on Ni(110): an experimental and theoretical study

Adsorption of cyanogen, C₂N₂, on a Ni(110) surface has been investigated by combining low energy electron diffraction, thermal desorption and angular resolved ultra-violet photoemission experiments as well as model cluster calculations using the linear combination of Gaussian-type orbitals local density functional method as well as a force field approach. Results of mirror plane photoemission experiments on the ordered c(2 × 2) monolayer could be rationalized by invoking adsorbates bonded to the surface via the π electrons, with their axis oriented along the [001] direction, across the grooves of the (110) surface, leading to an assignment of all six adsorbate-derived valence orbitals. However, unlike in a previous study on the analogous chemisorption system Pd(110)/C₂N₂, only one mirror plane was detected. A possible tilt of the adsorbates in the (1 0) plane was related to crowding on the closer spaced nickel surface by estimating the lateral interaction within the adsorption layer using force field models. Electronic structure calculations on various chemisorption model clusters confirmed the experimental orbital assignment and the orientation of the adsorbate axis essentially along the [001] direction. Best agreement with UPS data was found for the orbital splitting pattern of an adsorption geometry where the C-N groups bind sideways on-top of nickel atoms in the first crystal plane. On the other hand, bonding along in the troughs of the (110) surface leads to a distinctly different ordering of the valence orbitals, at variance with the experimental assignment.