No abstract

No abstract

We present a comparative study of the electronic properties of the clean Si͑100͒ and the hydrogenated Si͑100͒:H surfaces performed with a low-temperature ͑5 K͒ scanning tunneling microscope. Various surface structures such as single silicon dangling bonds and bare silicon dimers created by local desorption of hydrogen atoms from the Si͑100͒:H surface are also investigated. The experimental scanning tunneling spectroscopy ͑STS͒ curves acquired locally on each of these structures are compared with STS measurements performed on the Si͑100͒ and Si͑100͒:H surfaces. First principle density-functional theory calculations of the projected local density of states, taking into account the influence of the dopant atoms ͑As͒, enable to assign the observed STS spectra.

We present both differential and total cross sections for the direct ionization of water vapor by protons in the incident energy range 0.1-100 MeV. Different theoretical models are investigated within the framework of the Born approximation in order to evaluate the influence of each pairwise Coulomb interaction term among the ejected electron, the scattered proton, and the residual ionized target in the final state. In all these models, the ground state of the water molecule is described by means of an accurate molecular wave function proposed by Moccia ͓J. Chem. Phys. 40, 2186 ͑1964͔͒. The results of these full ab initio quantum-mechanical treatments are compared to experimental data. Good agreement is generally observed, showing that sophisticated Born models are sufficient to explain all the experimental data, including doubly differential, singly differential, and total cross sections.

Controlling the intrinsic optical and electronic properties of a single molecule adsorbed on a surface requires electronic decoupling of some molecular orbitals from the surface states. Scanning tunneling microscopy experiments and density functional theory calculations are used to study a perylene molecule derivative (DHH-PTCDI), adsorbed on the clean 3 × 3 reconstructed wide band gap silicon carbide surface (SiC(0001)-3 × 3). We find that the LUMO of the adsorbed molecule is invisible in I(V) spectra due to the absence of any surface or bulk states and that the HOMO has a very low saturation current in I(z) spectra. These results present a paradox that the molecular orbitals are electronically isolated from the surface of the wide band gap semiconductor even though strong chemical bonds are formed.