XPS spectra valence band

Fig. 25. XPS valence band spectra for reactively sputtered Ru Ir, x02 electrodes on a Ti substrate after preparation for different compositions x. Note the shift in t2g band position. After [83].

Figure 9 shows the photoemission valence band spectrum of Th metal. A comparison of the high resolution XPS valence band spectra of Th (resolving two distinct peaks at 1.8 and 0.6 eV below Ep) with a calculated total (s-d) density of states, (convoluted for broadening effects as lifetime and instrumental effects) gives a nearly complete agreement. The two peaks are attributed to 6d states. 

The XPS valence band as shown in Fig. 11, and especially the narrow and intense peak just below Ep (observed in all experiments) have been discussed following mainly hne I. Theoretical partial 5f density of states calculations agree in reproducing this feature, which can therefore be attributed to nearly pure 5 f states. But these density of states curves predict additional structmes which, although differing considerably in their position, are not observed experimentally. A maximum, observed only once at 1.8 eV might be qualitatively described by one calculation however, relatively poor statistics (only 100 c/s) may have artificially introduced this structure since it is difficult to understand why other XPS valence band spectra (of comparable or even higher resolutions) do not show it. 

The XPS valence band spectra for the dioxides of the transuranium elements (from Np to Bk) have been presented in an extensive and pioneering work that also includes core level spectra and has been for a long time the only photoemission study on highly radioactive compounds. High resolution XPS spectra (AE = 0.55 eV) were recorded on oxidized thin metal films (30 A) deposited on platinum substrates with an isotope separator. (The oxide films for Pu and the heavier actinides may contain some oxides with lower stoichiometry, since starting with Pu, the sesquioxides of the heavier actinides begin to form in high vacuum conditions.)... 

Despite this controversial part concerning the final state multiplet description the importance of this pioneering work on highly radioactive actinide oxides must be emphasized XPS valence band spectra recorded afterwards for Np02 and Pu02 confirm these early results. 

Figure 11. Gold XPS Valence Band Spectra of (1) Bulk gold (2) AuSn particles prepared in acetone (3) after Ar ion etching.

Some XPS valence band spectra of thick insulating samples have been obtained using a low energy electron emission source (flood gun) to neutralize the positive surface charge. This... 

Figure 3. XPS valence band spectra of the lightest alkanes (from top to bottom and left to rightl methane, ethane, propane, n-butane, n-pentane, n-nonane, successively. The measurements were perform in the gas phase.

Figure 6. XPS valence band spectra of benzene (solid phase), polystyrene, and...

It is the reason why it was also important to study systematically the XPS valence band spectra of simple model polymers substituted by an heteroatom it is necessary to discover also the fingerprint(s) of these substituents in order to understand in the future more complicated systems. 

Fluoro-substituted Polymers. The fluoropolymers were between the first to be studied by the XPS technique because the substitution of F atom(s) in the -CH.-CH - unit induced very large modifications in the XPS core level spectra (shifts up to 8eV) that were easy to detect and interpret. The XPS valence band spectra of similar compounds, namely poly(vinyl fluoride) (PVF), poly(vinylidene fluoride) (PVF2), poly(trifluoroethylene) (PVF3), and poly(tetrafluoroethylene) (PTFE) (26, 27, 28) are also expected to reflect the induction of such strong electronic effects at the valence molecular level. 

Figure 7. XPS valence band spectra of the fluoropolymers PE, PVF, PVF2, PVF3, and PTFE (11)...

Figure 8. XPS valence band spectra for two ethylene-tetrafluoroethylene copolymers (top) alternating structure (Twttom block structure.

Figure 9. XPS valence band spectra of (A) polyvinyl chloride, (B) polyfepi chloro-hydrine), (C) poly(chlorotrifiuoroethylene), (D) cofethylene-tetrafluoroethylene...

Figure 10. XPS valence band spectra of the linear polyethers (I) polyfmethylene oxide), (2) polyfethylene oxide), (4) poly(tetramethylene oxide) (A).

The XPS valence band spectra picture directly the bonds between the atoms of the molecule, and are more characteristic of the compounds ( ) especially for polymers containing only carbon atoms could the technique (with the use of complementary reference spectra, and/or theoretical density of states calculations) be sensitive enough to allow an identification of isomers ... 

Examples of real isomerism of constitution are very numerous we list here a few series of compounds vdiich were more or less easily identified by their XPS valence band spectra. Generally, they are relatively simple polymers in the sense that their monomeric unit contains few atoms for larger systems, it is expected that the distinction between two isomers will be less evident. We distinguish here the two following groups of polymers, and only list the compounds already studied (the list contains all the compounds already studied in the laboratory or elsewhere it is not restrictive all the isomers studied up to now were found different by their valence band spectra) ... 

Figure 11. XPS valence band spectra of poly( 1-butene) and poly(butadiene l,4)cis...

Stereoisomerism. The first trials to use the XPS valence band spectra to distinguish between two stereoisomers were unsuccessful. Cis- and trans- poly(isoprene) - with short branched chain and small substitution effects-, as well as cis- and trans-poly(l,4dichloro-2,3epoxybutene) - with longer branched chain and more intense substituent effects- did not show in our first measurements significant differences in their valence band spectra that could be attributed to the searched effect. Before... 

That success led us to tackle another more complicated question is it possible with the XPS valence band spectra to distinguish between crystalline and amorphous polymers It has already been shown that core level analysis was not successful (36). From other measurements, mainly on semiconductor materials, it is known that, if the core level peaks of the amorphous materials were slightly broader, the main modifications appear in the valence band where the fine details are smeared out compared to the crystalline material (47). 

Figure 16. XPS valence band spectra of syndio-polypropylene in the helical and zig-zag planar conformations...

For sake of completeness and only briefly (as other reviews are specially dedicated to these subjects), we would like to mention that XPS valence band spectra of polymers can also be used -going beyond a sole interpretation of the molecular orbitals in the valence band- to provide informations on some solid state parameters and on surface structure effects of the polymers. 

Being essentially a surface technique, XPS valence band spectra also allows to monitor modifications occurring at the polymer surface during adsorption, reactions, of degradation... Very few contributions are, up to now, dealing with such studies ( ). The most direct use of the technique is actually a comparison of the core and valence photoelectron line intensities to deduce informations about the surface and the in-depth composition of the polymer, as well as about the orientation of the macromolecular chain at the surface boundary. 

During the past decade the developments in the experimental technique made possible high resolution measurements of the XPS valence band spectra with a good statistics. The use of these measurements for a more rigorous comparison between the theoretical and experimental data are illustrated by recent results obtained for phosphorus [27, 28] and sulphur [29] oxyanions In these studies the theoretical interpretation was obtained from DV-Xa cluster MO calculations. Experimental data on crystal structure information from X-ray diffraction measurements were used to set up realistic model clusters. In the case of the S04 cluster, the results of several model calculations (ab initio, DV-Xa, hybrid models) are also presented. From the comparison of these results a better understanding of the role of the contribution from different effects to the MO one-electron energies can be obtained. 

Fig 5. XPS valence band spectra of Na3P04, Na4P207, (NaPOslnand P2O5 (from [26]). 

Fig 6. Theoretical XPS valence band spectra of PO/ obtained by using (a) Slater parameters ... 

The P207 dimer pyrophosphate anion consists of two P04 units sharing one oxygen atom that is called the bridging oxygen Ob. The other oxygen atoms are called non-bridging or terminal (Ot). Theoretical XPS valence band spectra based on two different cluster models are shown and compared to the respective... 

Fig 9. The P207 XPS valence band spectra of anions corresponding to the hydrated (A) and anhydrous (B) forms of Na4P207, calculated by using the DV-Xa cluster MO method, in comparison with the experimental valence band spectrum (C) of the Na4P207 sample (Ref 28). 

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