19 valence electron rule conversion

In many respects, the successes of this model are remarkable. Iron(O) possesses a total of eight electrons in its valence shell. To satisfy the eighteen-electron rule, five two-electron donors are needed, and compounds such as [Fe(CO)5] are formed. These molecules also obey simple VSEPR precepts, and [Fe(CO)s] adopts a trigonal bipyramidal geometry. Conversely, the use of two five-electron donor ligands such as the strong r-acceptor cyclopentadienyl, Cp, gives the well-known compound ferrocene (9.3). 

For each photoelectron that leaves the surface, an atom with a core hole is left behind in a highly excited state, which relaxes both by radiative and nonradiative processes. In a radiative recombination process, the core hole is filled in an electronic transition from a core level of lower binding energy or a valence level. The surplus energy is released by the emission of an X-ray photon, in a so-caUed X-ray fluorescence process. In this process, the emitted photon has a lower energy than the exciting photon and dipole selection rules apply for both, excitation and de-excitation. Conversely, Auger processes are nonradiative de-excitation channels... 

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