Wolfgang Schattke

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.

The geometric configuration and energy of a hydrogen molecule centered between two point-shaped tips of equal charge are calculated with the variational quantum Monte Carlo (QMC) method without the restriction of the Born-Oppenheimer (BO) approximation. The ground-state nuclear distribution, stability, and low-vibrational excitations are found to deviate significantly from the BO treatment based on a potential energy surface obtained with the same QMC accuracy. The quantum-mechanical distribution of the molecular axis direction and the bond length at a subnanometer level is fundamental for understanding nanomechanical dynamics with embedded hydrogen. The cylindrical symmetry of the tip arrangement yields a uniform azimuthal distribution of the molecular axis vector relative to the tip-tip axis. For fixed tip separation, the QMC sampling shows that the polar angle distribution of the molecular axis is centered around the equatorial plane for positive tip charge (transverse alignment) and around the tip-tip direction for negative tip charge (bridge alignment). These deviations from spherical symmetry are magnified as the tip-tip distance decreases. Our results thus show that the molecular orientation in the junction can be controlled by the tip charge and separation, suggesting an application in the field of molecular machines.

Nonlinear effects in photoemission are shown to open an access to the band structure of unoccupied states in solids, totally different from hitherto used photoemission spectroscopy. Despite its second-order nature, strong resonant transitions occur, obeying exact selection rules of energy, crystal symmetry, and momentum. Ab initio calculations are performed to demonstrate that such structures are present in low-energy laser spectroscopy experimental measurements on Si. Similar resonances are predicted in ultraviolet angle-resolved photoemission spectra, as shown with a theoretical calculation on bulk Al.

Photoelectron spectra from (100) and (111) surfaces of aluminum in the photon energy range 44–100 eV are measured and calculated within an ab initio one-step theory. Dispersion of spectral structures is interpreted in terms of unoccupied electronic structure of a semi-infinite crystal. The energy dependence of complex self-energy is derived from the experiment. The lifetimes of the (100) and (111) surface states and the photon energy dependence of the intensity of photoemission from the surface states are determined. A broad spectral structure is experimentally observed at the (111) surface, which disappears at the room temperature. It is tentatively interpreted as a surface resonance.