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For two external spherical tips with equal positive charges, the ground state of a hydrogen molecule is variationally determined within the quantum Monte Carlo scheme. For finiteness, the system is enclosed in a spherical container with randomly reflecting unstructured walls. The 12-dimensional system of two protons and two electrons is investigated ab initio without any adiabatic restrictions. We focus on the hydrogen center of mass (COM) distribution in continuation of previous work on ground state and vibration modes at a COM that was fixed in space. Our general purpose is to control the molecule's position and orientation by external means, such as by two charged tips. To this end, knowledge is needed in order to find specific COM regions with a desired molecular orientation in the container at significant probability density of the molecule. [ABSTRACT FROM AUTHOR]

The role of the variational quantum Monte-Carlo method for homogeneous systems and bulk material, on one hand, and finite aggregates like molecules, on the other, is now well established, mainly for the determination of ground state properties. It has been recently shown that it may be also used in surface investigations. Furthermore, dynamic behavior seems to be accessible, too, in view of the numerical resolution currently achieved. This applies also to surface dynamics. Considering the still smaller energy scales as represented by the electron–phonon interaction represents the actual borderline of accessible resolution. Nevertheless, localized electronic excitations could be investigated in bulk solids in order to yield the changes in excitation energy induced by a deformation, which shows thus a possible access to the deformation potentials. [Copyright &y& Elsevier]

Profound gradual changes of surface state energies were predicted for varying surface terminations of the periodic crystal potential in one-dimensional models.[sup 1] This situation can be realized in superlattices with different thicknesses of topmost layers. For the ideally terminated (100) surface of a very thin superlattice (GaAs)[sub 2](AlAs)[sub 2], the shift of the energy of the surface state over the whole minigap in the lower part of the valence band has been found for different terminations of the topmost layer. In the center of the surface Brillouin zone the surface state shift follows model trends. The changes of the energy distribution of photoemitted electrons as determined from the one-step photoemission calculation[sup 2] indicate that experimental observation by the surface-sensitive technique of angle-resolved photoemission should be feasible, and preliminary data indicate this. The results show a straigthforward tuning of surface electron structure by geometrical means. [ABSTRACT FROM AUTHOR]

The current status of photoelectron-diffraction studies of surface structures is briefly reviewed, and several recent developments and proposals for future areas of application are then discussed. The application of full-solid-angle diffraction data, together with simultaneous characterization by low-energy electron-diffraction and scanning-tunneling microscopy, to epitaxial growth is considered. Several new avenues that are being opened up by third-generation synchrotron-radiation sources are also considered. These include photoelectron diffraction from surface and interface atoms, the possibility of time-resolved measurements, and circular dichroism in photoelectron angular distributions. The addition of spin to the photoelectron-diffraction measurement is also considered, and can be achieved either through core-level multiplet splittings or by circular-polarized excitation of spin-orbit-split levels. This last development should make it possible to study short-range magnetic order, perhaps even in a holographic fashion. [ABSTRACT FROM AUTHOR]

The surface sensitivity of electron diffraction and of electron spectroscopies is determined by the imaginary component of the electron self-energy. In crystals, the energy and direction dependence of the electron attenuation and of the escape depth should be taken into account at very low energies. Strong anisotropy of the electron attenuation has been obtained around 20 eV from peak shapes in VLEED intensity profiles from (111) transition metal surfaces. Extension of the local density approximation in the density functional formalism provides quantitative description of the electron self-energy. The one-step model of angular resolved photoemission incorporating the self-energy predicts a strong energy and angle dependence of the escape depth of low energy photoelectrons emitted from GaAs(110). [ABSTRACT FROM AUTHOR]