Initial state effect

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. 

Although the dynamic final state effect can also explain the broadening of the core levels, the expected shift for the Fermi-edge and the core levels should still be quite similar. In fact, in numerous experiments [83,84,86,96,100,101] significantly different shifts were observed for the different spectral elements, which indicates that beyond the final state effects - which probably still give important contributions to shifts - initial state effects also have to be considered. 

Series, from 335.5 eV for Ca/Pd = 0 to 335.1 eV when Ca/Pd = 2. The latter result is consistent with TEM findings, as well as HChS data, which indicate that the Pd crystallites are larger the higher the Ca to Pd ratio is (Table 1). The Ca 2p3/2 signal was always typical of Ca " (347.5 eV), with consistently shifted values (+ 0.7 eV) from those of bulk CaCOs or CaSiOs, which can be attributed to final or initial state effects quite similar to those observable whenever an oxide is dispersed onto an inert support [4]. The presence of a C Is signal at 289.3 eV only in Ca-containing samples, and in both series, indicates the possible association of the carbonate anion with Ca cations. 

A priori, it is not clear what causes the core level shifts seen in Figures 4-7 and, in particular, if these shifts come from initial state effects [60,61]. In principle. 

Early theoretical studies based on a semi-empirical self-consistent tight-binding scheme indicate that the core-level shifts in the Pd/W(l 10) and PtAV(l 10) systems come from initial state effects (d-s,p rehybridisation, for example) [37]. The calculated shift for the Pd core level was 0.7 eV versus the value of 0.8 eV measured experimentally [53]. More sophisticated calculations (fiill-potential linear muffin-tin orbital method with LDF) for the Pd/Mo(110) system also indicate that the Pd 3d core-level shifts reflect initial state effects (substantial polarization of electrons around Pd) [40]. In this case, the calculated Pd 3ds/2 core level (0.9 eV) is identical to the experimental value and most of it (0.77 eV) comes from initial state effects while the rest (0.13 eV) originates in changes in the screening of the core hole [40]. 

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