Core electron, photoionization

As illustrated in Fig. 7.15, the electromagnetic radiation measured in an XRF experiment is the result of one or more valence electrons filling the vacancy created by an initial photoionization where a core electron was ejected upon absorption of x-ray photons. The quantity of radiation from a certain level will be dependent on the relative efficiency of the radiationless and radiative deactivation processes, with this relative efficiency being denoted at the fluorescent yield. The fluorescent yield is defined as the number of x-ray photons emitted within a given series divided by the number of vacancies formed in the associated level within the same time period. 

For the purposes of this review it is convenient to focus attention on that class of molecules in which the valence electrons are easily distinguished from the core electrons (e.g., -n electron systems) and which have a large number of vibrational degrees of freedom. There have been several studies of the photoionization of aromatic molecules.206-209 In the earliest calculations either a free electron model, or a molecule-centered expansion in plane waves, or coulomb functions, has been used. Only the recent calculation by Johnson and Rice210 explicitly considered the interference effects which must accompany any process in a system with interatomic spacings and electron wavelength of comparable magnitude. The importance of atomic interference effects in the representation of molecular continuum states has been emphasized by Cohen and Fano,211 but, as far as we know, only the Johnson-Rice calculation incorporates this phenomenon in a detailed analysis. 

The lines of primary interest in an xps spectrum are those reflecting photoelectrons from core electron energy levels of the surface atoms. These are labeled in Figure 8 for the Ag 3s, 3p, and 3d electrons. The sensitivity of xps toward certain elements, and hence the surface sensitivity attainable for these elements, is dependent upon intrinsic properties of the photoelectron lines observed. The parameter governing the relative intensities of these core level peaks is the photoionization cross-section, q. This parameter describes the relative efficiency of the photoionization process for each core electron as a function of element atomic number. Obviously, the photoionization efficiency is not the same for electrons from the same core level of all elements. This difference results in variable surface sensitivity for elements even though the same core level electrons may be monitored. 

There are photoionization cross section effects to be considered. XPS ( 1 keV soft X-ray photon excitation) is used to study the core-electron energy levels, while UPS ( 20-40 eV photons) is used to study the valence electron energy distribution. UPS has the... 

The majority of recent PES studies of the halomethanes has concerned photoionization dynamics, and has employed synchrotron radiation sources. Halomethanes are aptly suited for such studies because they exhibit a relatively small number of well-resolved PES bands and contain atoms of very different atomic number, Z. This variation of Z permits fine tuning of the molecular ion potential and opens a window for the study of photoelectron-ion interactions. In addition, the Br3d and I4d shells have large photoionization cross-sections in the SXR region, thus extending the scope of PES studies from the valence to the outer-core electrons. 

It is clear, however, from the discussion involving Eqs. (7)—(9) and from the sudden approximation sum rule that the spectrum associated with the photoionization of a core electron should not, in fact, necessarily consist only of a single line some data observed for RbCl and RbF (40) are shown in Fig. 16. The narrow peaks are the Rb 4s24 6(1S) - -4s14 >6(2S) excitation and the broad peaks, approximately equal in intensity, arise from multiple electron excitation , that is, the production of final states such as 4s24 4 s(2S), where n > 5. Even though the photoemission event is just a one-electron dipole process, multiple excitation can occur because the wavefunction of the instantaneous intermediate state of the (TV—1)-electron ion [Eq. (7)] has overlap with wavefunctions of such multiply excited states that is, i has components which are eigenfunctions, n(N—1), of multiply... 

The main peaks in X-ray Photoelectron Spectroscopy (XPS) for molecules appear because of the photoionization of core electrons. In addition, satellite peaks on the high binding energy side of the main peak have often been observed. These peaks are generally referred to as shakeup satellite peaks. In the sudden approximation, the shakeup process which accompanies photoionization can be considered as a two-step process. First, a core electron is emitted as a photoelectron, creating an inner shell vacancy. In the next step, electron(s) in the same molecule transfer from valence orbital(s) to unoccupied orbital(s) with relaxation of orbital energies. It is important to study these satellites in order to understand the valence and excited states of molecules (1). 

Figure 9 Relative partial photoionization cross sections (RPPICS) of the d-bands of M(CO)6 (M = Cr (a), Mo (b), W (c)). The arrows indicate the energies at which the (n- )p core electrons ionize two energies are a consequence of the spin-orbit splitting of the core hole. " Adapted with permission of The American Chemical Society from Cooper, G. Green, J. C. Payne, M. P. Dobson, B. R. Hillier, I. H. J. Am. Chem. Soc. 1987, 109, 3836.

To understand the origin of low-energy satellites in the 3d spectra of light lanthanides, their implication for a dynamical picture of core level photoionization in the presence of semilocalized screening orbitals to establish their relation to the ground state electronic structure, see fig. 1 (Wertheim and Campagna 1978). 

In extended systems, however, the many-body states are characterized by a constant electrochemical potential (and an indefinite number of electrons). There are two independent reasons for this (i) in an XPS experiment the sample is grounded so that electrons can flow into it and compensate the ejected ones, and (ii) the Coulomb forces in metallic systems move any excess charge to the surface. The second reason (which follows from Poisson s equation) couples with the first to insure charge neutrality of the sample. Since our final states are by construction translationally invariant (the 4f number in every cell is the same), this implies in turn that every WS cell in the final state must be electrically neutral. In particular, a photoionized cell having one fewer core electron must contain an additional valence, or conduction, electron. 

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