Core-level binding energy

This case is particularly interesting since the surface segregation energy can be directly compared to surface core level binding energy shifts (SCLS) measurements. Indeed, if we assume that the excited atom (i. e., with a core hole) is fully screened and can be considered as a (Z + 1) impurity (equivalent core approximation), then the SCLS is equal to the surface segregation energy of a (Z + 1) atom in a Z matrixi. in this approximation the SCLS is the same for all the core states of an atom. 

W. F. Egelhoff Jr., Thermochemical Values for Cu-Ni Surface and Interface Segregation Deduced from Core-Level Binding Energy Shifts, Phys. Rev. Lett. 50 587 (1983)... 

Further studies were carried out on the Pd/Mo(l 1 0), Pd/Ru(0001), and Cu/Mo(l 10) systems. The shifts in core-level binding energies indicate that adatoms in a monolayer of Cu or Pd are electronically perturbed with respect to surface atoms of Cu(lOO) or Pd(lOO). By comparing these results with those previously presented in the literature for adlayers of Pd or Cu, a simple theory is developed that explains the nature of electron donor-electron acceptor interactions in metal overlayer formation of surface metal-metal bonds leads to a gain in electrons by the element initially having the larger fraction of empty states in its valence band. This behavior indicates that the electro-negativities of the surface atoms are substantially different from those of the bulk [65]. 

Figure 2 displays a qualitative correlation between the increase or decrease in CO desorption temperature and relative shifts in surface core-level binding energies (Pd(3d5/2), Ni(2p3/2), or Cu(2p3/2) all measured before adsorbing CO) [66]. In general, a reduction in BE of a core level is accompanied by an enhancement in the strength of the bond between CO and the supported metal monolayer. Likewise, an opposite relationship is observed for an increase in core-level BE. The correlation observed in Figure 2 can be explained in terms of a model based on initial-state effects . The chemisorption bond on metal is dominated by the electron density of the occupied metal orbital to the lowest unoccupied 27t -orbital of CO. A shift towards lower BE decreases the separation of E2 t-Evb thus the back donation increases and vice versa. 

Bjomeholm O, Nilsson A, Tillborg H, Bennich P, Sandell A, Hemnas B, Puglia C, Martensson N. 1994. Overlayer stmcture from adsorbate and substrate core level binding energy shifts CO, CCH3 and O on Pt(lll). Surf Sci 315 L983-L989. 

Of direct interest for photoemission of supported catalysts is that similar increases in the width of d-bands have been observed by Mason in UPS spectra of small metal particles deposited on amorphous carbon and silica substrates [48]. Theoretical calculations by Baetzold et al. [49] indicate that the bulk density of states is reached if Ag particles contain about 150 atoms, which corresponds to a hemispherical particle 2 nm in diameter. Concomitant with the appearance of narrowed d-bands in small particles is the occurrence of an increase in core level binding energies of up to 1 eV. The effect is mainly an initial and only partly a final state effect [48], although many authors have invoked final state - core hole screening effects as the only reason for the increased binding energy. 

Table 5. Core level binding energies, and transition energies and intensities for low energy shake up structures in poly para-substituted styrenes...

The surface core level shift is defined as the shift in the core level binding energy for a surface atom relative to that of a bulk atom. Different theoretical approaches have been used to calculate surface shifts5,9 and for metals it has recently been shown21 that both initial- and final-state effects have to be included in ab initio calculations to obtain consistent agreement between experimental and calculated results. The basic assumption in this theoretical approach is that the final state is completely screened so the... 

During an interface experiment, an overlayer is stepwise deposited onto a substrate. By monitoring the substrate and overlayer core-level binding energies during deposition, the evolution of the valence band maxima of the substrate and of the overlayer can be followed during interface formation [33-36], The procedure is outlined in Fig. 4.3. As will be shown in Sects. 4.2.3.3 and 4.3.3, care has to be taken when applying this standard procedure to study surfaces and interfaces of sputter-deposited ZnO films, as BEvb(CL) depends on the deposition parameters for this material. 

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