NEXAFS spectra

Rgure 5 NEXAFS spectra above the C K-edge for a saturation coverage of pyridine C5H5N on Pt(111), measured at two different polarisation angles with the X-ray beam at normal incidence and at 20° to the sample surface. 

Figure 6 NEXAFS spectra above the C K-edge for the polymers PMPO poly (dimethyl...

The NEXAFS spectra for the structureless background and the 288.OeV. A new feature, however, energy of 284.5 eV and Is assigned Similar excitations have been seen near 284 eV with a number of unsaturated hydrocarbons chemisorbed on Pt(lll) (10,11). The presence of this new resonance at 284.5 eV supports the HREELS and the TPD 300K. 

Recent NEXAFS (11,2A) have confirmed -the ethylldyne structure proposed by LEED analyses (1A,21) and further determined the structure of adsorbed molecular ethylene. Figure 4 shows the NEXAFS spectra for ethylldyne (a) and ethylene (b) on the Pt(lll) surface taken for two Incidence angles of the X-ray beam. The transitions observed In these NEXAFS spectra have been assigned using SCF-Xo calculations (24). For the ethylldyne spectrum taken at 20 Incidence angle peak A Is caused by a C(ls)+o j, transition peak B Is caused by a C(ls)+o (, (, transition. Peak A In the... 

Figure 4. Carbon K-edge NEXAFS spectra of ethylldyne (a) and ethylene (b) adsorbed on the Pt(lll) surface. Spectra for two incidence angles (20 and 90 ) of the X-ray beam are shown.

Fig. 1. NEXAFS spectra of ethylene (T-90K) and ethylidyne (T-300K) chemisorbed on Pt(lll) for normal incidence. The difference between the spectra is also shown to indicate the maximum for ethylidyne at 285.8 eV photon energy.

Figure 37. NEXAFS spectra at the K-edge of chloride for (a) LiCl crystal, (b) 0.2 M LiCl/THF, (c) 0.2 M LiCFTHF + 0.1 M aza-ether that does not have electron-withdrawing substituents on N, and (d) 0.2 M LiCl/THF + 0.1 M linear aza-ether with n = 3 in Table 8. Note the white line peak split when the electron-withdrawing substituents perfluo-romethylsulfonyl are present. (Reproduced with permission from ref 384 (Figure 2). Copyright 1996 The Electrochemical Society.)...

Fig. 13 Carbon K-edge partial electron yield (PEY) NEXAFS spectra collected from the CMPE-SAM (top) and OTS-SAM (bottom). The arrow marks the position of the Is tt transition for phenyl C = C, present only in the CMPE-SAM sample. (Reproduced with permission from [76])...

Figure 6.5. (a) Band structure and (b) total DOS calculated for neutral TTF and PDOS for the S and C atoms (black lines). The S2p and CD NEXAFS spectra of TTF (grey lines) are superposed to the PDOS of S and C, respectively. Energies are referred to the HOMO maximum, (c) Band structure and (d) total DOS calculated for neutral TCNQ and PDOS for the N and C atoms (black lines). The ND and CD NEXAFS spectra of TCNQ (grey lines) are superposed to the PDOS of N and C, respectively. Energies are referred to the LUMO minimum. The F-, X-, Y-and Z-points are defined as in Fig. 6.4. Reprinted with permission from Fraxedas et al., 2003. Copyright (2003) by the American Physical Society.

The calculated total DOS and PDOS for the relevant atoms of TTF-TCNQ, TTF and TCNQ were introduced in Figs. 6.4(b), 6.5(b) and 6.5(d), respectively, and compared to experimental NEXAFS data (grey lines in the figures). The S2p, CI5 and NI5 NEXAFS spectra obtained from thin films of TTF-TCNQ, TTF and TCNQ are also shown in Fig. 6.14 where the relevant MOs of TTF and TCNQ associated with the different features of the spectra are indicated. The differences found in the energies of the peaks in the NEXAFS spectra compared with the peaks in the calculated PDOS are in part due to the fact that the calculated PDOS corresponds to the electronic structure of the ground state while the NEXAFS spectra correspond to the excited state with a core hole. 

Let us start with neutral and charged TTF. The experimental spectrum for S2p (Fig. 6.14(a)) corresponding to neutral TTF is disappointingly unstructured, whereas that for charged TTF is clearly rich in structure. As shown in Fig. 1.31, the S2/ core levels are composed of two spin-orbit split components, 2p3/2 and 2pi/2, separated by 1.3 eV, so that the S2p NEXAFS spectra consists of the superposition of both... 

Figure 6.15. Schematic representation of the relevant TTF unoccupied MOs for the analysis of the NEXAFS spectra of TTF and TTF-TCNQ. The symmetry labels are those appropriate for the Dih symmetry. The energy values given (in eV) are relative to those of the HOMO of TTF in the isolated TTF molecule. Reprinted with permission from J. Fraxedas, Y. J. Lee, I. Jimenez, R. Gago, R. M. Nieminen, R Ordejon and E. Canadell, Physical Review B, 68, 195115 (2003). Copyright (2003) by the American Physical Society.

Figure 6.16 shows the NEXAFS spectra as functions of the incidence angle 9e for the oriented TTF-TCNQ hlms. For 0e = 0° and Fe close to 90° E lies parallel and perpendicular to the molecular ai>-plane, respectively. In this case the planar geometry of both TTF and TCNQ molecules is a clear advantage strongly simplifying the analysis. 

Figures 6.16(b) and (c) show the angular dependence of the Clx and Nix NEXAFS spectra, respectively. The most salient feature of Fig. 6.16(c) is the intensity increase of the n n b3g, Uu)) + cr (7t(big, bin)) peak for increasing 9 values. Flowever, the benzenic-type n a , biv) and n b2g) peaks remain nearly unchanged. For TCNQ the C2 molecular axis forms an angle of about 36 with the c -direction. Since CN bonds form an angle of 60° with this C2 molecular axis within the molecular plane, the intensity associated to o jt b3g, a )) should increase for larger 0. However, the intensity associated with jt n) orbitals, the benzenic-hke orbitals and n n b3g, a )), should exhibit the opposite behaviour because they are perpendicular to the o jt) orbitals. From Fig. 6.16(c) it is clear that the n n b3g, a )) -I- o n b g, 2u)) peak increases with regard to the benzenic-type orbitals for increasing 9e values because of the increasing a ( r) and...

Let us conclude this section with the intriguing observation of the absence of the a (7t(ag, b f)) contribution in the Nls NEXAFS spectra of both neutral TCNQ and TTF-TCNQ. Let us recall the absence of precisely the Og and b u contributions in the CI5 spectra of TTF discussed above. However, the Cls NEXAFS spectrum of TCNQ shows some intensity in the a (7r(ag, b f)) region [between n a , bif) and TT (jr(b3g, af>) + a n b g, 2u))]- This is due to the signihcant 6 -contribution from carbon for neutral TCNQ while nitrogen contributes negligibly. Thus, it seems that in addition to the intra-atomic selection rules there are additional restrictions apparently symmetry-related in MOMs like those discussed here. This unexplained phenomenon certainly calls for both theoretical and experimental future work. 

While the cross-bridge local adsorption site of acetylene on Cu(lll) and Ni(l 11) is essentially identical, on the structurally similar Pd(lll) surface the molecule adsorbs in a hollow site, as illustrated in Figure 1.7. This different adsorption site, first proposed on the basis of quantitative evaluation of NEXAFS spectra [86] and subsequently confirmed by PhD [87], provides a rationale for the significantly different behaviour seen in vibrational spectroscopy [78] for this system. 

The NEXAFS experiments reported here were carried out at the U1 Beamline of the National Synchrotron Light Source, Brookhaven National Laboratory. Details concerning the optics of the beamline, as well as the UHV chamber with facilities for high pressure reactions, have been described previously.7,8 In our experimental set-up, NEXAFS spectra can be recorded by measuring either the electron yield or fluorescence yield. While the electron yield method is sensitive only to the top few atomic... 

Figure 24.4 Comparison of carbon K-edge features of VC/V(110), VC powder and graphite. The NEXAFS spectra were recorded by measuring the partial electron-yield.

Well-characterized vanadium carbide thin films can be prepared by exposing a clean V(110) surface to ethylene or 1,3-butadiene at 600 K. The formation of vanadium carbide, rather than other forms of carbon-containing species such as graphite or carbonaceous overlayers, was confirmed by the characteristic AES and NEXAFS spectra.4 The stoichiometry and average thickness of the thin carbide films can be estimated by... 

Figure 17.13. Stacked C (Is) NEXAFS spectra of NOM from various sources and environments showing molecular-level structural and compositional heterogeneity. The spectra were recorded in transmission mode (fungi, bacteria, fresh charcoal, black C particle from Liang et al., 2006 black-C rich humic substance from Solomon et al., 2007b litter J. Lehmann, unpubl. data).

Figure 17.17. Principal component analysis map of sample (left) and color-coded spectra (right) from a sample of marine suspended particulate matter. The lower three spectra are characteristic of low organic mineral phases, while the upper three organic phases have distinctively different C-NEXAFS spectra. Background regions are shown in black (J. Brandes, unpublished data 2007). See color insert.

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