XPS peaks

Photoelectron peaks are labelled according to the quantum numbers of the level from which the electron originates. An electron coming from an orbital with main quantum number n, orbital momentum / (0, 1, 2, 3,. .. indicated as s, p, d, f,. ..) and spin momentum s (+1/2 or -1/2) is indicated as For every orbital momentum / > 0 there are two values of the total momentum j = l+Ml and j = l-Ml, each state filled with 2j + 1 electrons. Flence, most XPS peaks come in doublets and the intensity ratio of the components is (/ + 1)//. When the doublet splitting is too small to be observed, tire subscript / + s is omitted. 

The surface atomic ratios (Cd/F, S/F and C(naflon)/F) calculated from the XPS peak areas are listed In Table 1. The results Indicate that the cublc-CdS film has a higher surface concentration of CdS than the hexagonal film. In both, the C(ls)/F(ls) ratio Is similar to Naflon Itself. 

XPS also yields chemical information directly. Eor instance, if an element in a sample exists in different valence states, the XPS peak may broaden and show a shoulder. It is possible to deconvolute the peaks and determine valence states and the relative amount of each state in the sample. It is important to do this type of work by comparison of values of standard reference compounds. 

The effects of adsorbate coverage (film thickness) on the Pd 3d5/2 XPS peak positions of the Pd/W(l 1 0), Pd/ Re(0001), and Pd/Mo(l 10) systems were systematically investigated [63]. The peak positions reported for Pd coverage in excess of 1 ML represent a product of electrons emitted from surface and subsurface atoms. For the case of Pd(lOO), theoretical calculation suggest that the Pd 3d5/2 XPS BE of the surface atoms is 0.4 eV lower than that of bulk Pd. A similar difference has been observed experimentally for Ni and Pt surfaces. These shifts in BE are a consequence of variations in the coordination number of the surface atoms compared to bulk atom. If we reference the combined peak of bulk and surface atoms in 40 ML of Pd on W(1 1 0) to that of Pd(l 00) a difference of —0.8 eV is obtained between the Pd 3ds/2 BE of a pseudomorphic monolayer of Pd on W(110) and that of the surface atoms of Pd(l 00). The corresponding shifts... 

The intensity of a peak depends on the value of the photoelectron cross-section, a, which is a measure of the efficiency of the photon interaction with the electron. Each orbital has its own cross-section, so the intensities of XPS peaks will not be identical even when all else is ideal. 

Figure 2.5 illustrates two spectra recorded from a sample of iron using (a) Al Ka radiation, and (b) Mg Ko, radiation. The binding energy of the peaks are characteristic of each element. There is a difference in hv between these sources of 233 eV, so, as expected from equation (2.1), the XPS peaks on spectrum (a) are displaced 233 eV relative to those in spectrum (b). The spectrum was taken over a wide energy range to detect all possible peaks of elements present in the surface. The 2p and 3p peaks from iron are identified, as well as the Is peak from carbon which was present as a contaminant. 

Van t Blik et al. [3] exposed a highly dispersed 0.57 wt% RI1/AI2O3 catalyst (H/M=1.7) to CO at room temperature and measured a CO uptake of 1.9 molecules of CO per Rh atom. Binding energies for the Rh 3ds/2 XPS peak increased from 307.5 eV for the reduced catalyst under H2 to 308.7 eV for the catalyst under CO. The latter value equals that of the [Rh+(CO)2Cl]2 complex, in which rhodium occurs as a Rh+ ion. The infrared spectrum of the Rh/Al203 catalyst under CO showed exclusively the gem-dicarbonyl peaks at 2095 and 2023 cm-1. All results point to the presence of rhodium in Rh+(CO)2 entities. However, how can a rhodium particle accommodate so much CO ... 

FIGURE 6.8 Comparison of typical (a) In3d and (b) Sn3d XPS peaks measured for ITO films prepared at different hydrogen partial pressures of 0, 7.9x 10 6, and 1,6x 10-5 torr. 

TPSR experiments were carried out. Based on the products detected, propose a mechanism for the reaction(s) that take place on the surface. What is the main purpose of the XPS data reported in Figure 2 of the same article within the context of these studies Justify the assignments provided for the I 3d5/2 XPS peaks at 620.0 and 619.5 eV to molecular iodopropane and atomic adsorbed iodine, respectively. 

Fig. 7. Plots of halogen XPS peak area vs carbon 1 s peak area for vinyl halides adsorbed on a platinum 100 surface.

X-Ray photoelectric ionization is believed to take place in a time interval of about 10-18 s. Therefore separate XPS peaks are possible for atoms if the lifetime of the asymmetric electronic state is greater than about 10 18 s, whether or not the atoms are structurally equivalent. We may represent the ground state of a localized mixed valence compound (involving two metal atoms differing in oxidation state by one unit) by the following formula, where the dot represents the extra valence electron M—M. The two possible XPS transitions can then be represented as follows, where the asterisk indicates core ionization,... 

The first transition would be expected to be of higher energy than the second from simple atomic charge considerations. Because the two atoms are of equal abundance, the two peaks have essentially equal intensities. Unfortunately, the observation of two XPS peaks does not rule out the possibility of delocalized valence electrons in the ground state. Two transitions are expected even in that case because of polarization of the excited state by the core ionization 123 The ground state of a delocalized mixed valence compound can be crudely represented by the formula M-M, where the intermediate position of the dot indicates that the odd valence electron is equally shared by the two metal atoms. The two XPS transitions can then be represented as follows,... 

Notice that the excited states are similar to those formed from the localized ground states. Again the first transition is of higher energy than the second. Hush has made a theoretical study of these transitions123. He concluded that, in the case of symmetrical delocalized mixed valence complexes, the two XPS peaks will occur at energies... 

The appearance of only one XPS peak for a mixed valence compound is consistent with a delocalized ground state (and excited state). Bifeirocenylene (II, III) picrate, whose structure is shown in Fig. 8, probably fits in this category. The Mossbauer spectrum of the complex indicates only one kind of iron atom, and the Fe 2p3,2 spectrum consists of only one peak with a weak shoulder at higher binding energy 29). It should be recognized, however, that even in the case of a localized system in which two XPS peaks are expected, if the chemical shift between the two peaks is less than the resolution of the spectrometer, only one peak will be observed. 

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