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Core electrons formation

When multi-electron atoms are combined to form a chemical bond they do not utilize all of their electrons. In general, one can separate the electrons of a given atom into inner-shell core electrons and the valence electrons which are available for chemical bonding. For example, the carbon atom has six electrons, two occupy the inner Is orbital, while the remaining four occupy the 2s and three 2p orbitals. These four can participate in the formation of chemical bonds. It is common practice in semi-empirical quantum mechanics to consider only the outer valence electrons and orbitals in the calculations and to replace the inner electrons + nuclear core with a screened nuclear charge. Thus, for carbon, we would only consider the 2s and 2p orbitals and the four electrons that occupy them and the +6 nuclear charge would be replaced with a +4 screened nuclear charge. [Pg.4]

Although the occupied orbitals are of main importance, since they are directly involved in the formation of the chemical bond, the unoccupied states also provide complementary information. In X-ray absorption spectroscopy (XAS), often denoted Near Edge X-ray Absorption Fine Structure (NEXAFS), we excite a core electron to the empty states above the Fermi level [3,4,13]. There is a close connection between XES and XAS where the former gives information on the occupied orbitals while the latter relates to the character and symmetry of the unoccupied levels. Both are governed by the dipole selection rule and the localized character of the core orbitals allows a simple atom-specific projection of the electronic structure the major difference is in the final states. In XAS the empty electronic states are probed... [Pg.60]

The formation of a multiply charged ion following the ejection of a core electron often leads to decay of the molecule into several positive fragments, and, in the end, gives different final products than in photochemistry. An ion with a core hole can also make transitions into the highly excited states that cannot be occupied spectroscopically (for instance, into the C2S + state of the ion5 ). [Pg.271]

One can imagine three possible ways for the SES to be formed (1) the excitation of a core electron into a vacant orbital (2) the formation of a Rydberg state converging to / > Ix and (3) the excitation of two valence electrons. [Pg.272]

Clearly any attempt to base FeK on such molecularly defined cores defeats the aims of pseudopotential theory. However, the approximate invariance of atomic cores to molecule formation implies that, of the total of Na electrons which could be associated with the centre A in an atomic calculation, nx are core electrons and n K will contribute to the molecular valence set. Thus we can define a one-centred Fock operator ... [Pg.105]

Obviously the electrons most affected in the process of formation of the condensed state are the valence electrons and in the lighter elements the primary physical effects of the remaining (core) electrons is often included through the concept of a pseudo potential, generally non-local. With this understanding Hamiltonian (1) is then modified to reproduce simply the valence electron spectrum. To within density dependent constants (1) is therefore replaced by... [Pg.7]

For example, in metals, because of their large electrical conductivity, it seems that at least some of the electrons can move freely through the bulk of the metal, while the core electrons remain in their atomic orbital, similar to the isolated atoms forming the metal. For example, let us take into account the formation of a linear array of lithium atoms from individual lithium atoms Li-Li Li-Li-Li Li-Li-Li-Li. Then, the first stage is the formation of a lithium molecule, Li2. This molecule is analogous to the hydrogen molecule, H2 [15,19], In the formation of the H2 molecule, two MOs are formed, that is, the bonding MO... [Pg.26]

To summarize, the formation of a 2s-energy band from the 2s orbitals when N Li atoms are gathered together to form the Li crystal is shown in Figure 1.15. There are, N 2s-electrons but there are 2N states in the band, therefore the 2s band is only half full. Besides, the atomic Is orbital, which is close to the Li nucleus, that is, is the two Is electrons which are the core electrons, remains undisturbed in the solid, that is, each Li atom has a closed K-shell, specifically a full Is orbital. Consequently, in general, when a solid metal is formed, the external orbitals overlap. As a consequence of this process, the outer electrons move without restraint through the metal, while the core electrons remains in their atomic orbital. [Pg.28]

If the Wigner-Seitz cell appropriate to a metal is superimposed on the spatial charge distribution of a free atom, one finds characteristically that a quantity of charge, typically between 2/3 e and 1 e, lies outside the cell boundaries. (13) Since in the metal the cell is of course neutral, this implies that formation of the metal requires compression of the valence charge, and associated with the compression is an increase of the Coulomb interactions of the valence electrons with each other and with the ion core. A lowest order estimate of the shift associated with this effect may be based on truncation of the free atom valence orbitals at the cell radius, rws, and renormalization of the charge within the cell. For a core electron lying entirely inside the valence density the core-valence Coulomb interaction is... [Pg.91]

After core hole formation, relaxation in the valence orbitals can give rise to promotion of valence electrons into unoccupied levels. If this reorganization is fast, and the energy required for this transition is not available to the primary photoelectron, shake-up satellites can show up on the low kinetic energy (high p) side of the main peak. Further loss lines can be created if the photoelectron passing the solid excites group oscillation of the conduction electrons (plasmon loss). [Pg.249]


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See also in sourсe #XX -- [ Pg.51 , Pg.278 , Pg.284 ]




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Core, formation

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