Up spectra

In the case of atoms UPS is unlikely to produce information which is not available from other sources. In addition many materials have such low vapour pressures that their UPS spectra may be recorded only at high temperatures. The noble gases, mercury and, to some extent, the alkali metals are exceptions but we will consider here only the specttum of argon. 

Quantum chemical predictions of tautomeric equilibria Ab initio multireference Cl studies of tautomers UPS spectra Combined approach to the tautomerism in azaaromatic heterocycles by N, C, and H NMR Fluoro-azoles Experimental data and MNDO studies Tautomerism A review... 

Figure 5-13. Low binding energy Purl of Ihe Me I UPS spectra ol DHPPV recorded during successive" deposition of calcium. The inset shows the fully doped (one Ca atom per monomer) DltPPV with a simple estimate of the inelastic electron background to emphasize the calcium-induced structures (lixnn Ret. lfifip.

Two Hell UPS spectra of poly(3-hexylthiophene), or P3HT, compared with the DOVS derived from VEH band structure calculations 83], arc shown in Figure 5-14. The general chemical structure of poIy(3-a ky thiophcne) is sketched in Figure 5-4. The two UPS spectra, were recorded at two different temperatures, +190°C and -60 "C, respectively, and the DOVS was derived from VEH calculations on a planar conformation of P3HT. Compared to unsubslitutcd polythio-phene, the main influence in the UPS spectra due to the presence of the hexyl... 

Valence band spectra provide information about the electronic and chemical structure of the system, since many of the valence electrons participate directly in chemical bonding. One way to evaluate experimental UPS spectra is by using a fingerprint method, i.e., a comparison with known standards. Another important approach is to utilize comparison with the results of appropriate model quantum-chemical calculations 4. The combination with quantum-chcmica) calculations allow for an assignment of the different features in the electronic structure in terms of atomic or molecular orbitals or in terms of band structure. The experimental valence band spectra in some of the examples included in this chapter arc inteqneted with the help of quantum-chemical calculations. A brief outline and some basic considerations on theoretical approaches are outlined in the next section. 

The UPS spectra (not shown here) recorded upon Al deposition onto the conjugated thiophene systems shows only small visible changes in the positions of the peaks in the spectra [84]. The main effect is a rapid decrease in intensity, which indicates that a metallic overlayer is formed since the cross-sections for the Al(3p) or Al(3s) are much lower than for the C(2p) or S(3p) orbitals. This is consistent with the Al(2p) XPS spectra discussed above. 

The experimental UPS spectra of the emeraldine base form of polyaniline is compared with VEH-derived DOVS in Figure 5-18 97. The DOVS were derived from the VEH band structure calculations shown at the bottom of Figure 5-18. 

Figure 2.16. Work function changes versus CO exposure for clean and K-covered Pt(l 11) at 300 K measured from the onset of the electron emission of He I UPS spectra.42 Reprinted with permission from Elsevier Science.

Figure 5.43. UP-spectra of Ag YSZ electrodes for (a) cathodic and (b) anodic polarization of the galvanic cell Ag YSZ Pd,PdO at 547°C. In (b), the shift of the Fermi edge of the small silver particles on YSZ under anodic polarization is shown enlarged (5x).24 Reprinted with permission from Wiley-VCH.

Figure 3. Valence band spectra of Co/Si(100). Upper curve UPS spectra for 100 nm thick Co/Si(l 1 1) film middle curve thinned 4-5 nm Co/Si(l 1 1) film after ion etching (Co nanoparticles) lower curve clean silicon substrate after removing the Co layer by in situ sputtering. The photoemission data were obtained by He(I) excitation. (Reprinted from Ref [78], 1994, with permission from Springer.)...

In Figure 8 [146] we present the valence band XPS and UPS spectra of the silver nanoparticles at different stages of the size reduction process. The contribution of the substrate was subtracted. The parameter at each spectrum is the measured Ag/Si ratio. 

Very useful information concerning the surface of emersed electrodes, however, can be deduced from UPS spectra directly, like the electronic density of states at the Fermi level, the position of the valence band with respect to the Fermi level or possible band gap states. The valence band of UPD metals might help to explain the respective optical data (see Sections 3.2.1 and 3.2.5). 

Fig. 17. UP spectra for (top) a clean Au electrode, (middle) after anodization with a current density of 0.1 mA/cm2, and (bottom) after rinsing the oxidized sample with ultra pure water.

The increase in intensity ratio indicates the accumulation of oxygen species on the surface of the electrode. After formation of the thick oxide layer the ratio O/Ru becomes constant. The fact that the adsorbed species do form bonds with the Ru of the electrode surface is clearly shown in the respective UPS spectra for different electrode potentials [74]. 

Fig. 21. UPS spectra of a clean (top) polycrystalline Ru electrode and after emersion at different potentials from 0.1 mol L 1 HC104. Shaded peak corresponds to ilq band of the oxide.

UPS valence band spectra were also obtained for the UPD system Ag on Pt. These spectra exhibit again a shift of the Ag4d level of the adatom to lower binding energies when compared to the bulk value. For both Cu and Ag adatoms on Pt the UPS spectra clearly show that bulk properties of the adsorbate layer are achieved for coverages of about 3 monolayers. 

The OVGF function method provides a quantitative account of ionisation phenomena when the independent-particle picture of ionisation holds and as such is most applicable in the treatment of outer-valence orbitals. It provides an average absolute error for vertical ionisation energies below 20 eV of 0.25 eV for closed shell molecules. The TDA and ADC(3) methods allow for the breakdown of one particle picture of ionisation and so enable the calculation of the shake up spectra. The ADC(3) is correct up to 3rd order, is size consistent and includes correlation effects in both the initial and final states. 

Biomolecular spectroscopy on frozen samples at cryogenic temperatures has the distinct disadvantage that the biomolecules are in a state that is not particularly physiological. Recall that EPR spectroscopy is done at low temperatures to sharpen-up spectra by slowing down relaxation, to increase amplitude by increasing Boltzmann population differences, and to decrease diamagnetic absorption of microwaves by changing from water to ice. Certain S = 1/2 systems, notably radicals and a few mononuclear metal ions, have sufficiently slow relaxation, and sufficiently limited spectral anisotropy to allow their EPR detection in the liquid phase at ambient temperatures, be it in aqueous samples of reduced size. 

Elastic tunneling spectroscopy is discussed in the context of processes involving molecular ionization and electron affinity states, a technique we call orbital mediated tunneling spectroscopy, or OMTS. OMTS can be applied readily to M-I-A-M and M-I-A-I -M systems, but application to M-A-M junctions is problematic. Spectra can be obtained from single molecules. Ionization state results correlate well with UPS spectra obtained from the same systems in the same environment. Both ionization and affinity levels measured by OMTS can usually be correlated with one electron oxidation and reduction potentials for the molecular species in solution. OMTS can be identified by peaks in dl/dV vs bias voltage plots that do not occur at the same position in either bias polarity. Because of the intrinsic... 

Figure 33 shows the UPS spectra of 2,5-bis(diphenylmethy-lene)-2,5-dihydrothiophene and di-hydroselenophene, as compared with that of 2,5-bis(dithienyl methy-lene)-2,5-dihydrothiophene, together with the spectra of benezene and thiophene for references, irrespective of the central ring with sulphur or selenium the UPS spectra are found to be almost same. However, the spectra were significantly different with changing the substituent rings at the exocyclic double bonds from the phenyl to the thienyl groups. In both the phenyl and the thienyl substituents, the spectra are very similar to those of benzene and thiophene... 

Figure 33. UPS spectra of 2,5-bis(diphenyl-methylene)-2,5-dihydrothiophene, dihydro-selenophene and 2,5-bis(dithienylmethy-lene)-2,5-dihydrothiophene, as compared with those of benzene and thiophene.

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