Core electron excitation selective

It is clear that the chemical reactions induced by core electron excitation are atom-specific. They generally depend upon the atomic site of the core hole and the electronic configuration of the core hole excited state. In this sense, the chemistry is selective. The chemistry can be selected by tuning the photon energy to produce particular core hole excited states. In another... 

Selection rule for core electron excitation into an empty state MO consider also the electric field vector E orientation, respect to the axis direction of the p orbital component in the MO final state the transition is allowed in the case of these two are parallel and forbidden when are at normal angle. Then, electron transition into antibonding MOs are strongly influenced by the a or it character of the antibonding orbitals. 

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... 

Now the question arises, as before, of the degree to which the rare earth s valence electrons (and in particular 5d) are populated by hybridisation with the electron-rich 7T-MO system of the C72 host molecule. Figure 9 shows resonant photoemission spectra of Ce2 C72, with photon energies selected to span the N4>5 core level excitation spectrum, as indicated in Fig. 8. 

For simplicity, the G-n separation is imposed in the VB calculations for the ground and excited states since low-lying excitations only involve the 7t electrons in selected species O3, C3 and C3H5. After the G-n separation, the symmetry of both the ground electronic state and the 7t excited states only depends on 7t valence electronic structures because a doubly occupied a core within the MO formalism is always total symmetric, and does not affect the symmetry of electronic states. If the three p orbitals forming n bonds in O3 are numbered as... 

This yields sensitivity to the surface within a photo-electron mean free path length of the photoexcited atom, which can be chosen with chemical selectivity. Compared to ARUPS, ARXPS is a more convenient approach for the extraction of structural information, since core-level excitations are more easily described than valence-level excitations. Angle-resolved detection is normally used to obtain the most complete structural information, which is interpreted in a procedure very similar to that of LEED with full simulation of the electron emission intensity as a function of energy or angle. 

The electronic transitions probed by x-ray absorption spectroscopy involve the excitation of a core electron into either unoccupied bound electron states near the Fermi level of the material or at higher energies into the continuum of states producing a photoelectron. These electronic excitations must obey spectroscopic selection rules and thus can provide information about the symmetry of an atom s environment, its oxidation state, and sometimes, with the assistance of comprehensive theoretical calculations, details about the geometry of ligands and other nearby atoms. This information is derived from excitations into bound states and low lying resonances above the Fermi level and is referred to as the x-ray absorption near edge structure (XANES). 

---