127 research outputs found
Electron penetration in the nucleus and its effect on the quadrupole interaction
A series expansion of the interaction between a nucleus and its surrounding
electron distribution provides terms that are well-known in the study of
hyperfine interactions: the familiar quadrupole interaction and the less
familiar hexadecapole interaction. If the penetration of electrons into the
nucleus is taken into account, various corrections to these multipole
interactions appear. The best known one is a scalar correction related to the
isotope shift and the isomer shift. This paper discusses a related tensor
correction, which modifies the quadrupole interaction if electrons penetrate
the nucleus: the quadrupole shift. We describe the mathematical formalism and
provide first-principles calculations of the quadrupole shift for a large set
of solids. Fully relativistic calculations that explicitly take a finite
nucleus into account turn out to be mandatory. Our analysis shows that the
quadrupole shift becomes appreciably large for heavy elements. Implications for
experimental high-precision studies of quadrupole interactions and quadrupole
moment ratios are discussed. A literature review of other small quadrupole-like
effects is presented as well
Density-functional study of hydrogen chemisorption on vicinal Si(001) surfaces
Relaxed atomic geometries and chemisorption energies have been calculated for
the dissociative adsorption of molecular hydrogen on vicinal Si(001) surfaces.
We employ density-functional theory, together with a pseudopotential for Si,
and apply the generalized gradient approximation by Perdew and Wang to the
exchange-correlation functional. We find the double-atomic-height rebonded D_B
step, which is known to be stable on the clean surface, to remain stable on
partially hydrogen-covered surfaces. The H atoms preferentially bind to the Si
atoms at the rebonded step edge, with a chemisorption energy difference with
respect to the terrace sites of >sim 0.1 eV. A surface with rebonded single
atomic height S_A and S_B steps gives very similar results. The interaction
between H-Si-Si-H mono-hydride units is shown to be unimportant for the
calculation of the step-edge hydrogen-occupation. Our results confirm the
interpretation and results of the recent H_2 adsorption experiments on vicinal
Si surfaces by Raschke and Hoefer described in the preceding paper.Comment: 13 pages, 8 figures, submitted to Phys. Rev. B. Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
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