Bt Thole

MOLECULAR POLARIZABILITIES CALCULATED WITH A MODIFIED DIPOLE INTERACTION

Abstract The point dipole interaction model for molecular polarizability recently proposed by Applequist, Carl, and Fung is modified by replacing the point dipole interaction by an interaction between smeared out dipoles. Rules are developed to indicate plausible forms for this modified interaction. The polarizabilities of a wide range of chemically different molecules can be calculated, using for each atom one polarizability independent of its chemical enviromnent. The errors are comparable to experimental uncertainty. Special care is taken to produce a model that tends to avoid infinite polarizabilities without use of cutoffs at short distances.

SPIN-MIXED GROUND-STATE OF FE PHTHALOCYANINE AND THE TEMPERATURE-DEPENDENT BRANCHING RATIO IN X-RAY ABSORPTION-SPECTROSCOPY

Analysis of the Fe L2,3 absorption spectrum together with the temperature-dependent susceptibility of FePC indicates a ground state which is a mixture of quintet and triplet. Temperature-dependent measurements of the X-ray absorption spectrum, particularly the variation in the branching ratio, are proposed to determine the precise nature of the ground and low-lying excited states.

Charge transfer satellites and multiplet splitting in X-ray photoemission spectra of late transition metal halides

Abstract Core-level X-ray photoemission spectra (XPS) are calculated for Ni and Co dihalides with an MX6 cluster model (M = Ni, Co; X = F, Cl, Br), where intra-atomic multiplet coupling as well as covalency mixing is taken into account. The effects of the intra-atomic configuration interaction between (3s)1(3p)6(3d)n and (3s)2(3p)4(3d)n+1, which is known to be important in the 3s XPS, are treated phenomenologically. The overall spectral shape of the Ni (Co) 2p XPS can be explained by a charge transfer mechanism, and the difference in the line shape between the Ni (Co) 2p 3 2 and 2p 1 2 XPS spectra is shown to originate from the multiplet splitting of their charge transfer satellites. However, the Ni (Co) 3s XPS are so complex that it is difficult to discriminate between exchange split pairs and their charge transfer satellites. As for Co dihalides, the spectral splitting of the 3s XPS mostly represents the 3s-3d exchange splitting of the (3s)1(3d)8 configuration. However, in Ni dihalides the first satellite peak is mainly due to charge transfer.

In situ RHEED and XPS studies of epitaxial thin α-Fe2O3(0001) films on sapphire

In situ RHEED and XPS measurements of epitaxial alpha-Fe2O3(0001) films are reported as a function of the number of deposited monolayers. The films were prepared on alpha-Al2O3(0001) substrates by MBE. The RHEED patterns suggest that layer-by-layer growth of alpha-Fe2O3(0001) occurs for the first few monolayers. Subsequently, the growth mode changes to three-dimensional growth. The in-plane lattice constant of the first monolayer of alpha-Fe2O3(0001) is expanded relative to that of the bulk, although in the case of lattice matching between alpha-Al2O3 and alpha-Fe2O3 a contraction would be expected. This can be explained by assuming a basic hexagonal structure for the first monolayer with a random distribution of ferric ions over the octahedral sites between the close-packed oxygen layers. Beyond the first monolayer, the ordered corundum structure is formed. The lineshapes of the XPS Fe 2p core level spectra are also found to be thickness-dependent. The deviation of the Madelung potential at the surface shifts the positions of the Fe 2p peaks to lower binding energies. For the first few monolayers, the satellite intensity is reduced because the interplanar contraction leads to a shorter Fe-O distance.

The direct reaction field hamiltonian: Analysis of the dispersion term and application to the water dimer

Abstract The induction and dispersion terms obtained from quantum-mechanical calculations with a direct reaction field hamiltonian are compared to second order perturbation theory expressions. The dispersion term is shown to give an upper bound which is a generalization of Alexanders upper bound. The model is illustrated by a calculation on the interactions in the water dimer. The long range Coulomb, induction and dispersion interactions are reasonably reproduced.

REACTION FIELD EFFECTS ON PROTON-TRANSFER IN THE ACTIVE-SITE OF ACTINIDIN

The feasibility of the inclusion of reaction field effects in accurate ab initio self-consistent field-molecular orbital calculations was studied in the case of proton transfer in the active site of actinidin. The effects of the polarizability of the environment were included, using the direct reaction field model, which treats the environment as a set of interacting polarizable atoms. Up to 1000 of these atoms could be treated but about 300 were sufficient. The full geometry of the active site and the environment was taken into account. The stabilization of the ion pair was calculated to be 3.5 kcal. but this value may be 10 kcal depending on the geometry used. The effect of the static field from the long alpha-helix present in the enzyme was also studied. Dispersion effects are shown to be unimportant. The orientational polarizability of side chains and water molecules was not included.

Magnetic X-Ray Dichroism Study of the Nearest-Neighbor Spin-Spin Correlation Function and Long-Range Magnetic Order Parameter in Antiferromagnetic NiO

A new effect on the X-ray absorption line shape is described which is proportional to the nearest-neighbor spin-spin correlation function. We present theory and demonstrate the use of linear polarized x-ray absorption spectroscopy to study the temperature dependence of the long-range order parameter and the nearest-neighbor spin-spin correlation function in antiferro-magnetic NiO.

Core hole polarization in resonant photoemission

A theory is presented for core hole polarization probed by spin polarization and magnetic dichroism in resonant photoemission. The resonant photoemission is considered as a two-step process, starting with an excitation from a core level to the valence shell, after which the core hole decays into two shallower core holes while an electron is emitted. The two core holes form well defined states which can be selected by the energy of the emitted electron. The non-spherical core hole and the selected final state cause a specific angle and spin distribution of the emitted electron. The experiment is characterized by the magnetic and nonmagnetic moments being measured, the polarization and direction of the light and the spin and angular distribution of the emitted electron. The intensity is a sum over ground state expectation values of tensor operators multiplied by the probability of creating a polarized core hole using polarized light, multiplied by the probability for decay of such a core hole into the final state. Using diagrammatic methods we derive general expressions for the angle and spin dependent intensity in various regimes of Coulomb and spin-orbit interaction, LS, LSJ and jjJ coupling. This core polarization analysis generalizes the use of sum rules in x-ray absorption spectroscopy where the integrated peak intensities give ground state expectation values of operators such as the spin and orbital moments. The photoemission decay makes it possible to measure new linear combinations of operators. The general formula for second-order processes shows that in the presence of core-valence interactions the two-step model may break down due to interference terms between intermediate states separated by more than their lifetime width. We present tables for the resonant p core hole decays in 3d transition metals. The 2p3/23p3p decay in ferromagnetic nickel is calculated using Hartree-Fock values for the radial matrix elements and phase factors. Recent measurements show an effect which is smaller in the 3P final state but stronger in the 1D, 1S peak. Spin polarization is due to odd moments of the core hole. We discuss and plot angular distributions and suitable geometries for spin polarized detection.

The importance of the magnetic dipole term in magneto-circular x-ray absorption dichroism for 3d transition metal compounds

The application of the magneto-circular x-ray absorption dichroism (MCXD) spin sum rule for 3d transition metal compounds faces two problems: the unknown value of the magnetic dipole operator and the division between the and edges. A systematic study of the order of magnitude of the -operator for ions is presented. The variation of the -values with temperature is described and analysed, for all cases from to cations in two different situations. Firstly the perfect octahedral case is considered. It is shown that is non-zero for low temperature; but, as it originates only from d-electron spin - orbit splitting, it is washed out at room temperature. Secondly, a model of the surface situation is considered. In this case originates mainly from the crystal-field splitting. It then exhibits quite large values at any temperature and can by no means be neglected when applying the sum rule. The error introduced in the sum rule due to the mixing of and edges has been estimated.

ANGULAR DEPENDENT PHOTOELECTRIC YIELD - MEASUREMENT OF THE LA3D AUGER AND AUTOIONIZATION LIFE TIME WIDTHS

Abstract The undistorted line shape in X-ray absorption may be obtained by analyzing the total photoelectric yield from solids as a function of the angle of incidence. This method is applied to the La3d absorption spectrum of LaF3. An Auger half-width of Γ = 0.22 eV is found for the La3d 94f1 levels, whereas the autoionization from the 3d9 3 2 4f1 level to the continuum gives an additional broadening of Γ = 0.3 eV.