HREEL spectra

Fig. XVin-5. HREELS spectra for NO2 on Pt(lll) adsorbed in different bonding geometries. [From M. E. Bartram and B. E. Koel, / Vac. Sci. Tech., A6, 782 (1988).]...

Figure 1. HREELS spectra for Increasing H2S exposures on the Pt(lll) surface at 11 OK. A value of one In the exposure units corresponds to saturation of the first layer.

Figure 2 HRSBLS spectra Illustrating sequential CH SH decomposition on the Pt(lll) surface. The HREELS spectra were taken following treatment at the temperatures Indicated on the reference TPD spectra shown In the right panel.

FIGURE 27.45 HREELS spectra for Pd(lll) after emersion from lOOmM CFjCOOH + 1 mM benzene electrolyte at (a) 0.5 V and (b) 0.2 V RHE. The peak intensities were normalized with respect to the elastic peak. (From Kim et ah, 2003, with permission from Elsevier.)... 

Figure 8. HREELS spectra of the nanocrystalline diamond and diamond-like carbon films with various [CH4]/[CO]. (a) [CH4]/[CO] = 4.5.0/0. (b) [CH4MCO] = 4.5/1.0. (c) [CH4]/[CO] = 4.5/10 seem. The elastic peak for (c), reduced by a factor of 25, is shown for comparison. Reprinted with permission from [66], K. Okada et al.. Diamond Relat. Mater. 10, 1991 (2001). 2001, Elsevier Science.

One of the classic examples of an area in which vibrational spectroscopy has contributed to the understanding of the surface chemistry of an adsorbate is that of the molecular adsorption of CO on metallic surfaces. Adsorbed CO usually gives rise to strong absorptions in both the IR and HREELS spectra at the (C-O) stretching frequency. The metal-carbon stretching mode ( 400 cm-1) is usually also accessible to HREELS. 

The invariance of IETS in an M-A-M junction vs an M-I-A-M device is exceptionally well demonstrated by the work of Reed [30], Figure 7 shows the Au-alkanedithiol-Au structure he used to create a single barrier tunnel diode. The IET spectra obtained from this device were stable and repeatable upon successive bias sweeps. The spectrum at 4.2 K is characterized by three pronounced peaks in the 0-200 mV region at 33,133, and 158 mV. From comparison with previously reported IR, Raman, and high-resolution electron energy-loss (HREEL) spectra of... 

Figure 8.14 High-resolution electron energy loss spectroscopy (HREELS) and low-energy electron diffraction of CO adsorbed on a Rh(l 11) surface, along with structure models. The HREELS spectra show the C-O and metal-CO stretch vibrations of linear and threefold CO on rhodium (from R.Linke etal. [56]).

Figure 3. Rh(111) HREELS spectra of a.) 1 L D 0, b. -f.) varying exposures of CO followed by 1 L D O. (Reproduced with permission from Ref. 5. Copyright 1988 Elsevier.)...

HREELS of the H 0 + HF system. The nature of the interaction stabi-iizing HF on thS surface is made clear by the HREELS spectra of Figure 6. As the concentration of HF in the water layer is increased a new peak around 1150 on (and several smaller peaks) first increases and then, as the HF/H-0 ratio exceeds 1, decreases in intensity. By analogy to vibrationaf spectra of acid hydrates of known structure (13-16), this peak is identified as the symmetric bending mode of the pyramidal H 0 ion. We have observed the same peak upon coadsorption of water ana other, stronger, mineral acids. The reaction... 

Figures HREEL spectra of a 12-MLAr film at three E with = 15 ° and analysis angle 0 = 45°.

By scattering within molecular solids and at their surfaces, LEE can excite with considerable cross sections not only phonon modes of the lattice [35,36,83,84,87,90,98,99], but also individual vibrational levels of the molecular constituents [36,90,98-119] of the solid. These modes can be excited either by nonresonant or by resonant scattering prevailing at specific energies, but as will be seen, resonances can enhance this energy-loss process by orders of magnitude. We provide in the next two subsections specific examples of vibrational excitation induced by LEE in molecular solid films. The HREEL spectra of solid N2 illustrate well the enhancement of vibrational excitation due to a shape resonance. The other example with solid O2 and 02-doped Ar further shows the effect of the density of states on vibrational excitation. 

Figure 6 HREEL spectra of a multilayer disordered N2 film recorded at the indicated Ei. (From Ref. 100.)...

Figure 6. HREELS spectra following the adsorption of 1.0 L oxygen on a K i(lOO) layer at 300 K. The K/Ni(100) surface was prepared by evaporating 2 ML of potassium onto a clean Ni(lOO) surface at 300 K.

Figure 8. HREELS spectra following the reaction of Ni0/Ni(100) with hydrogen. The surface was treated at 800 K at a H2 pressure of 2x10-6 torr. All IKEELS spectra were collected at 300 K after the reaction with H2. The corresponding LEED patterns are (a-c) NiO(lOO) + c(2x2)-0 (d) c(2x2)-0 (e) p(2x2)-0 (f) Ni(lOO) + very weak p(2x2)-0.

Figure 9. HREELS spectra following the reaction of K-doped Ni0/Ni(100) with hydrogen at 800 K. All experimental conditions were identical as those described in Figure 8.

The HREELS, Auger electron spectroscopy (AES) and thermal desorption spectrometry (TDS) experiments were carried out in a UHV chamber described previously.6 Briefly, the chamber was equipped with a HREELS spectrometer for vibrational analysis, a single-pass cylindrical mirror analyzer for AES measurements and a quadrupole mass spectrometer for TDS measurements. The HREELS spectra were collected in the specular direction with an incident energy of 3.5 eV and with a spectroscopic resolution of 50-80 cm-1. The TDS data were obtained by simultaneously monitoring up to 16 masses, with a typical heating rate of about 1.5 K s-1. 

Figure 24.2 Comparison of HREELS spectra recorded after the adsorption of 0.4 L CO on (a) clean V(110) and on (b) carbide-modified V(110) surface at 80 K. The v(CO) vibrational modes are detected at above 1000 cm 1. The low-frequency modes, at 465 and 610 cm-1 in spectrum (a) and at 375 cm 1 in spectrum (b), are related to the v(V-O) and v(V-C)...

High-resolution energy-loss spectroscopy (HREELS) spectra of the oli-gothiophenes 4T through 8T have been reported (94JCP(101)6344 95JST (348)405). 

Fig. 2.1 HREEL spectra of C60 multilayer films shown as a function of increasing hydrogen exposure. The primary electron beam energy is 6 eV and the sample temperature is -150°C. (a) no hydrogen exposure, FWHM = 36.5 cm-1 (b) a 45 L hydrogen exposure, FWHM = 34.8 cm-1 (c) a 180 L hydrogen exposure, FWHM = 40.4 cm-1 and (d) a 1,000 L hydrogen exposure, FWHM = 60.4 cm-1. Spectral features labeled for comparison with Table 2.1 (Reproduced by permission of the AAS from Stoldt et al. 2001).

Fig. 9. HREELS spectra of functionalized silicon surfaces prepared via photochemical reactions with H/Si(lll). In each case R represents a saturated alkyl chain (9 or 10 carbon atoms long) covalently attached to the Si surface. The methyl and acid terminated surfaces were prepared via reactions with decene and undecylenic acid respectively while the thienyl terminated surface was prepared by reaction of thienyl Li with an ester terminated surface. The dashed line at 1500 cm-1 represents the typical low frequency cut-off for ATR-FTIR measurements on silicon.

While the detection of the Si-H and Si-C modes indicates HREELS can probe the buried molecule/silicon interface, in general this method will be most sensitive to the terminal groups at the vacuum/monolayer interface. This is illustrated in Fig. 9 where spectra for several modified surfaces with different terminal functionalities are shown. In each case this terminal group is tethered to the surface via a Cio alkyl linker yet the spectra are significantly different. This is particularly evident in the spectra for the thienyl terminated surface in which the aromatic C-H stretch is clearly observed. In contrast this mode is quite small in the FTIR spectra, which are dominated by the contributions of the alkyl linker chain [51]. The observation of strong terminal group modes in the HREELS spectra indicates that these functional groups are likely present at the surface of the film and not buried back towards the H-terminated surface. This is consistent with their availability for sequential reactions as discussed in the previous section. 

Figure 3. HREEL spectra observed after exposure of 2.2 L propanal, 2.6 L acrolein, 1.3 L allyl alcohol, or 2.3 L 1-propanol to the clean Rh(l 11) surface and annealing to 301 K, 247 K, 304 K, and 258 K, respectively, to complete the decarbonylation reaction.

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