Energy-loss spectrum HREELS

In order to show that the strongly bound species was actually an EpB molecule, high-resolution electron energy loss spectroscopy (HREELS) was used to study the species present at the various dosing temperatures. When dosed at lower temperatures, most of the observed peaks in the HREELS matched those of the vibrational spectrum of liquid EpB, suggesting that intact EpB is interacting with the silver surface at lower temperatures. However, the silver surface dosed with EpB at 300 K showed noticeable differences in the HREELS spectrum. In addition, DPT calculated vibrational frequencies of the surface bound oxaraetallacylce matched well with those determined experimentally. 

HREELS High-resolution electron energy-loss spectroscopy [129, 130] Same as EELS Identification of adsorbed species through their vibrational energy spectrum... 

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... 

As noted in the introduction, vibrations in molecules can be excited by interaction with waves and with particles. In electron energy loss spectroscopy (EELS, sometimes HREELS for high resolution EELS) a beam of monochromatic, low energy electrons falls on the surface, where it excites lattice vibrations of the substrate, molecular vibrations of adsorbed species and even electronic transitions. An energy spectrum of the scattered electrons reveals how much energy the electrons have lost to vibrations, according to the formula ... 

In Fig. 14 we show HREEL spectra of ethylene adsorbed at Ag(4 1 0) and at Ag(2 1 0) at T = 105 K and compare them with the spectra recorded for Ag(l 00). On stepped surfaces (upper two spectra) C2H4 was dosed with a pure beam. Non-activated adsorption is witnessed by the loss in the 121-125 meY region. No adsorption takes place, on the other hand, on the extended (100) terraces of Ag(l 00) [90] up to much higher energies (see spectrum recorded at Ex = 0.31 eV in Fig. 14). Chemisorption on Ag(l 0 0) is observed when the ethylene exposure is performed with Ex — 0.35 eV. Adsorption on the flat surface is therefore translationally activated for extended (100) terraces and non activated for stepped surfaces. Physisorbed molecules do not contribute to the HREEL spectra since desorption takes place within a few seconds after the end of the dose at 105 K (as evident from Fig. 2) and recording a spectrum requires many minutes. 

The adsorption of ethylene on the Rh(lll) surface provides a typical example. The high-resolution electron-energy-loss (HREEL) spectrum at 77 K in Figure 2.25 has been attributed to ethylene adsorbed molecularly intact on the Rh(lll) surface [101]. However, vibrational frequencies measured are markedly different from those for gas-phase ethylene, indicating a strong interaction between ethylene and the rhodium surface. 

The basic experiment in HREELS in the backscattering geometry is straightforward [37], A monochromatized electron beam of 1-10 eV is directed toward the surface and the energy distribution of the reflected electrons is measured in an electron analyzer with a resolution of up to 7 meV. The spectrum consists of the elastic peak and peaks due to energy losses to the sample surface by the excitation of molecular vibrations. If plotted as wave numbers, these vibrations are very similar to those observed in IR techniques. The resolution achievable in this technique is, however, considerably less than in IR, which becomes clear if one considers that 1 meV = 8.066 cm , so the spectral resolution in HREELS is of the order of 100 cm (in IR the resolution is typically around 4 cm" or better). Detection of crystallinity or other high-resolution details as is possible in IR is therefore currently not achievable in HREELS. 

Figure 16 High-resolution electron energy loss (HREEL) spectrum of a metallized PET sample (a) clean PET (b)-(g) increasing coverage of the PET by Al atoms. (From Ref. 78.)...

At a concentration of 1.3 Na/a-6T HREELS spectra of Na-doped sexithiophene [318] reveal a new, very broad loss-structure at 1.7 eV (Fig. 46) (for bare sexithiophene compare also Fig. 38). With further Na-deposition the intensity of this loss peak increases very strongly and dominates the spectrum at the highest concentration of 8.3 Na/a-6T. Simultaneously a loss peak at 3.9 eV arises. Both effects lead to a less clear peak structure at energies between 1.0 and 4.5 eV due to the broadness of the peaks. Depending on the Na concentration there may also maxima be detected at 1.3 and 2.1 eV which arise within the 1.7eV peak structure as shoulders. There may also be a new peak at 0.95 eV for 3.2 Na/a-6T accompanied by a slight increase of the peak intensity at 0.75 eV. This intensity increase can only be explained by an additional electronic energy loss at 0.75 eV because the intensity of the overtone of the C—H stretch vibration is constant. The new peaks can either be explained as... 

Figure 12 In-specular (bottom spectrum) and off-specular (middle and top) HREELS measurements for 0/Ag(21 0), recorded for the same electron energy and for the same scattering angle, 0S. The loss at 56meV has a remarkably strong impact scattering component which leads to an inversion of the intensity ration with the 40 meV loss for out-of-specular conditions.

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