Equivalent Core Research Articles

Adsorption and decomposition of ammonia on a W(110) surface: Photoemission fingerprinting and interpretation of the core level binding energies using the equivalent core approximation

We report the first XPS data for ammonia adsorption, condensation and decomposition on a W(110) surface. Monolayer, “second layer” and multilayer NH 3 as well as NH 2, NH and N species can be characterized by a specific N(1s) electron binding energy. We discuss the observed binding energies within a thermodynamic framework, using the “equivalent core approximation”. This model has been previously successfully applied to core level binding energies of gaseous molecules and solids. The agreement between calculated and experimental N(1s) binding energies for some species is excellent, and we conclude that for the considered adsorbates the variation of the N(1s) binding energies is primarily determined by the ground state properties rather than by different relaxation energies in the final state. We also briefly discuss the activity of the W(110) face towards NH 3 decomposition and also present some data for NH 3 dissociation on an oxygen predosed surface.

Symmetry breaking and ionization from symmetry equivalent inner shells and lone pairs in Xα theory

The superiority of the localized, broken symmetry description for ionization from symmetry equivalent core holes and lone pairs in Xα theory is demonstrated by calculations on N 2 (1s) and by analysis of the literature results for O 3, OF 2, Ne, CF 4 and Cr(CO) 6. The differences between the Xα, HF, and experimental values, for localized ionization, can be traced directly to the approximate treatment of shell self-repulsion in Xα. When shell self-repulsion is properly taken into account via a simple perturbation treatment, the corrected Xα ionization energies are in excellent agreement with experiment. The strange result that the Xα delocalized solution has a lower total energy than the localized solution is similarly accounted for by correcting for the exact shell self-repulsion. The error in the Xα delocalized solution is much larger than the error for the localized solution with some important implications. The concepts of “Xα Koopmans energy” and Xα relaxation energy are shown to have practical importance is assessing whether the localized or delocalized solution is physically correct.

Semi-empirical calculation on core ionizations and simultaneous excitations by use of the π-electron and equivalent core approximations. The theoretical foundations of the method and the application to some π-conjugated molecules

The theoretical foundations of the semi-empirical calculation with the equivalent core approximation are discussed. The equivalency of the core- hole-localized model with the core-hole-delocalized model and the importance of including doubly-excited configurations in describing the core ionization and the simultaneous excitation (the shake-up transition) are shown from the discussion on a simplified diatomic model system. Numerical calculations with the equivalent core approximation are performed on ethylene, benzene and butadience by use of the full CI semi-empirical method within the framework of the π - electron approximation.

Single-mode optical fibre microbending loss in a loose tube coating

45 km of single-mode fibre of differing detailed design has been loose-tube packaged under identical conditions. The spectral loss between 1.1 and 1.75 μm was measured before and after packaging. The incremental loss thus deduced, and related to equivalent core size and Δn, provided information about fibre design for safe packaging.

Weathering-Induced Chemical Changes in Surface Coals from Paleocene Fort Union Formation, Red Rim Area, Sweetwater and Carbon Counties, Wyoming: ABSTRACT

Statistically significant differences in chemical composition between 20 outcrop and 22 stratigraphically equivalent core coal samples were demonstrated by proximate and ultimate analyses, heat-of-combustion and forms-of-sulfur determinations, and chemical analyses for 37 elements. The outcrop samples were collected from depths of 30 to 72 in. (75 to 180 cm) on very steep (> 45°) slopes. Core samples were from depths of 31 to 190 ft (9 to 58 m) at distances between 300 and 1,500 ft (91 and 457 m) from the outcrop. Mean annual precipitation in the Red Rim area is ~ 11 in. (28 cm); mean monthly temperatures range from 22°F (-5.5°C) in January to 68°F (20°C) in July. Apparent rank of unweathered coal is subbituminous C coal. Compared with the core samples (moisture and Btu/lb, as-received basis, all others moisture-free basis) the outcrop samples have significantly higher mean moisture (37.4 vs 23.8%), volatile matter (43.8 vs 35.1%), oxygen (24.9 vs 15.6%), and nitrogen (1.0 vs 0.8%); lower ash (13.3 vs 18.2%), hydrogen (3.0 vs 4.7%), sulfur (0.6 vs 1.1%) and heat of combustion (5,330 vs 7,690 Btu/lb); similar fixed carbon (42.5 vs 46.6%) and carbon contents (56.7 vs 60.3%). The sulfate part of the total sulfur increased from 5% in core to 18% in outcrop samples; pyritic sulfur decreased from 14 to 10%, and organic sulfur decreased from 84 to 71%. As indicated by differences in element/Al ratios between core and outcrop samples, the relative rates of removal of Si, K, B, Cr, Ga, La, Li, and V from coal during weathering are signi icantly greater than Al; Cd, Co, F, Ni, Se, and Zn are significantly greater than Al; Cd, Co, F, Ni, Se, and Zn are significantly less than Al; and Ca, Mg, Na, Fe, Ti, As, Ba, Be, Cu, Mg, Mn, Mo, Nb, Pb, Sb, Se, Sr, Th, U, Y, Yb, and Zr are similar to Al. Additional significant chemical changes in outcrop samples should be expected in areas of warmer, more humid climates, where coal is of lower rank, or in samples collected on less-steep slopes or at shallower depths. End_of_Article - Last_Page 936------------

Neon K-shell excitation studied by electron energy-loss spectroscopy

The electron energy-loss spectrum of gaseous neon in the region of 1s (K-shell) excitation has been investigated using small-angle inelastic scattering of 2.5 keV electrons. The spectrum is compared with previous optical studies of Ne 1s excitation. Term values are in reasonable agreement with those for the equivalent core atom, sodium. In addition to the optically allowed 1s to np series, the optically forbidden 1s to 3s electric quadrupole transition has been observed at 865.1 eV. Values for the Ne 1s ionisation potential (870.09 eV) and an upper bound to the natural linewidth of the 1s to 3p transition (0.31 eV) have been derived.

Nitrogen K-shell excitation in N2, NO and N2O by high-resolution electron energy-loss spectroscopy

The excitation of nitrogen 1s electrons has been observed in N2, NO, and N2O by using the electron energy-loss technique at an incident energy of 1.5 keV, and with an energy resolution of 0.075 eV. The energies, widths and vibrational structures of the inner-shell excited states have been obtained. The spectroscopic constants (internuclear separation Re, vibrational constants omega and omega x and quantum defect delta ) of the inner-shell excited states of N2 have been derived and have been found to be similar to those of the equivalent core states. The energies and vibrational structures of the inner-shell excited states of NO and N2O have also been interpreted in terms of the equivalent core model.

Term-values and valence-rydberg mixing in core-excited states, SiH 4 and PH 3

The controversial interpretation on the XUV spectra of PH 3 and SiH 4 have been examined experimentally and theoretically. An assigned consistent with measured 2p-absroption spectra of gaseous and solid SiH 4 and PH 3, and with CI calculation on core-excited states of SiH 4 is presented. The predictions of the equivalent ionic core virtual orbital model are confirmed. Term-values for core-excited states differ systematically from those for valence-excited states.

Chain length dependence of relaxation energy in K-shell ions of linear alkanes

Core-level binding energies of carbon atoms in several compounds of the linear alkane series of molecules are obtained via ab initio SCF calculations. The general behaviour of the relaxation energy along the series is examined and simple models are suggested for estimating its value. The use of extended basis sets within the Equivalent Core (EC) approximation turns out to provide good agreement with the experimental sign and magnitudes of the binding energy shifts measured for the gas phase compounds of the whole series with respect to the methane molecule.

Inner shell electron energy loss studies of HCN and C 2N 2

Small angle inelastic scattering of electrons with several kilovolts impact energy has been used to study carbon and nitrogen K-shell excitation and ionization of HCN and C 2N 2. The observed energy loss structures are assigned to promotions of K-shell electrons to valence and Rydberg orbitals. High resolution spectra (<100 meV fwhm) of the most intense inner shell transitions in HCN are reported. Excitation and ionization energies of the radicals HN 2 and HCO are derived from the HCN inner shell spectra using the equivalent core analogy.

Studies on the shake-up satellites of auger spectra for potassium and calcium compounds

Abstract Strong satellite peaks were observed in the KL 23 L 23 Auger electron spectra for potassium and calcium in their compounds. Satellite peaks for the potassium KL 23 L 23 Auger electron spectra were located at the lower kinetic energy side of the main peak and their intensities were functions of the nature of the bonding ions. Satellites due to the electrons with larger kinetic energies than the Ca KL 23 L 23 main Auger line were observed for calcium oxide, calcium sulfate and calcium carbonate. All of the satellites of KL 23 L 23 Auger electrons for both elements were interpreted on the basis of the charge-transfer shake-up process in terms of the Mulliken's concept of charge-transfer interaction. The equivalent core approximation was applied for the description of a doubly-ionized core state.

Investigation of the energy and vibrational structure of the inner shell (1s) −1(π2p) 1Π state of the nitrogen molecule by electron impact with high resolution

The vibrational structure of the (1s) −1(π2p) 1Π inner shell excited state of N 2 has been observed, enabling the decay width and the shape of the potential function of the state to be deduced. The validity of the equivalent core model is discussed.

Gas phase ESCA studies of trithiapentalene and its 2,5-dimethyl derivative

1,6,6a-trithiapentalene and its 2,5-dimethyl derivative have been studied in the gas phase by means of ESCA in order to elucidate the unusual type of SS bonding in these molecules. The high resolution spectra show very wide S 2p lines from the terminal sulfur atoms due to vibrational broadening. The effect of core ionization on the SS bond lengths in trithiapentalene has been estimated using the equivalent core approximation and the CNDO/2 method. Ground and final state potential curves have been calculated and explain qualitatively the differences in line widths for the terminal and central sulfur atoms. The observed chemical shifts agree with predictions from CNDO/2 and ab initio calculations.

Struktur der kinken und einschnürungen in schrauben- und 71°-versetzungen in kubisch-raumzentrierten metallen

Abstract The structure of kinks and constrictions of Screw and 71° Dislocations in B.C.C. Metals The dislocation cores of screw and 71° dislocations are calculated using the interatomic potential of Chang and Graham (1966) for α-Fe. In the case of screw dislocations, two equivalent core configurations exist. Therefore, six types of kinks and two types of constrictions may be defined, whereas in the case of 71 ° dislocations only two types of kinks may exist. It is shown that various reactions between kinks and constrictions may occur.

Ab initio calculations concerning core Auger shifts in some silicon compounds

Ab initio LCAO MO SCF calculations within the spin unrestricted Hartree–Fock formalism have been performed on SiH4, SiH3F, SiH2F2 and CH3SiH3 molecules, their Si 1s*, 2s* and 2p* core hole states, the double 2s** core hole states and the isoelectronic equivalent core P+ and S2+ cations. The results are used to discuss shifts in KL1L1 Auger energies in terms of hole state calculations, orbital energies and the equivalent cores method. There is good agreement between the methods. Shifts in core electron binding energies have also been calculated and indicate that experimental shifts are overestimated.

Theoretical studies of excited states of H 3O and of core-excited NH 3: Comparisons with equivalent core analysis of NH 3 core-excitation spectra and implications for the radiation chemistry of aqueous systems

The H 3O radical has been studied within the ab initio LCAO SCF MO model. A flexible basis set including diffuse basis functions at both O and H has been used in order to represent the excited states adequately. Calculated excitation energies are 1.87, 2.87–3.16, and 3.36–3.47 eV; the calculated ionization energy is 4.75 eV. These represent well the experimental values (good to ±0.3 eV) of 1.6, 2.9, and 3.5 eV for excitation, and 5.0 eV for ionization, deduced by equivalent core analysis of high energy electron impact energy loss studies of NH 3. Similar explicit calculations on the N-1s core-excited states of NH 3 have also been made to examine directly the equivalent core concept. “Excited states” (relative to the lowest bound core-excited state) at 1.72, 2.85–3.09, and 3.32 eV, and the “ionization energy” of 4.68 eV, agree well with experiment and support the equivalent core concept. The possible significance of these H 3O results in the radiation chemistry of aqueous media is discussed in view of the fact that the maximum in the absorption spectrum of the hydrated electron lies near that of H 3O.

Some theoretical aspects of vibrational fine structure accompanying core ionizations in N2 and CO

Ab initio calculations have been carried out on CO and N2 and the relevant core hole states with different basis sets to investigate differences in geometries and force constants. From these calculations vibrational band profiles of the core level ESCA spectra for these molecules have been interpreted, obviating the need to rely on data pertaining to the equivalent core species. The agreement with experimental profiles is excellent. The O1s level of CO which has not been subjected to detailed theoretical analysis previously, is predicted to show substantial vibrational structure in excellent agreement with recently acquired experimental data. The effect of temperature on the band profiles has also been considered. Theoretically derived core binding and relaxation energies of these systems have been investigated both as a function of basis set, and of internuclear distance. Density difference contours have been computed and give a straightforward pictorial representation of the substantial electron reorganizations accompanying core ionizations. Small basis sets with valence exponents appropriate to the equivalent core species when used in hole state calculations describe bond lengths, force constants, core binding energies and relaxation energies with an accuracy comparable to that appropriate to the corresponding extended basis set calculations.

Multiple-excitation processes in transition-metal atoms: II. The inner auger electrons

The KLL Auger spectrum from the nonradiative decay of K-hole configurations of some transition metals is discussed from a theoretical and computational viewpoint, in order to establish the physical validity of the ‘equivalent core’ (EC) model in processes with excitation energies high above threshold (suddenness conditions). The method already employed in paper I of this series appears to be very effective in reproducing both L-shell relaxation and K-shell rearrangement. The obtained binding energies for such electrons are compared with experiments and a complete calculation of the K-LL Auger multiplets in the intermediate coupling is also presented for V, Ti Cr and Mn.

MINDO/3 calculations of ESCA chemical shifts

Calculations of ESCA chemical shifts, using Jolly's equivalent core approximation and the MINDO/3 semi-empirical SCF MO method, have given results in reasonable agreement with experiment.

Calculation of chlorine Kα 1,2 X-ray emission shifts in molecules

Shifts of the chlorine Kα 1,2 X-ray emission line in gaseous molecules and in crystalline NaCl are correlated with effective atomic charges from CNDO/2 molecular orbital calculations made (i) for the neutral atomic ground states. and (ii) within the equivalent core approximation. For the latter case CNDO/2 parameters are derived for argon. The best agreement with experimental chemical shifts is obtained with the equivalent cores approximation. The results for NaCl depend strongly upon the CNDO/2 treatment of sodium.