High resolution EELS

The other vibrational spectroscopies, laser Raman and magic angle spinning NMR, have also been useful. Despite its low resolution, high resolution EELS has been usefiil in UHV work for assessment of surface cleanliness and for the identification of adsorbed species. 

The other technique is HREELS (high resolution EELS) which utilises the inelastic scattering of low energy electrons in order to measure vibrational spectra of surface species. The use of low energy electrons ensures that it is a surface specific technique, and is often chosen for the study of most adsorbates on single crystal substrates. 

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 ... 

We end the section on EELS with an example from a state-of-the-art instrument enabling routine measurement of spectra with a resolution of about 2 meV to be made. In such cases it is customary to refer to the technique as high resolution EELS or HREELS. 

In order to be effective, EELS spectrometers must satisfy a number of stringent requirements. First, the primary electrons should be monochromatic, with as little spread in energy as possible, preferably around 1 meV or better (1 meV = 8 cm-1). Second, the energy of the scattered electrons should be measured with an accuracy of 1 meV, or better. Third, the low-energy electrons must effectively be shielded from magnetic fields. The resolution of EELS has steadily been improved, from typically 50 to 100 cm-1 around 1975 to better than 20 cm-1 for the currently available spectrometers. When the latter value comes close to the line width of a molecular vibration, the technique is usually referred to as high-resolution EELS (HREELS). 

The existence of hot O adatoms was also believed to be responsible for the desorption of CO2 at 140 K from the Pt(lOO) hex surface partly covered with COads and 02,ads- Fadeev et al (58) used TPR and high-resolution EELS (HREELS) to investigate CO oxidation on Pt(lOO). Some of the results are summarized in Fig. 13. 

In addition, protrusions were observed, as shown in Figure 8,9, which were considered to have a local structure of (1 ll)-( /3 x V3) R30°. Unlike the inference given in Ref. [146], the -CH3 units are chemisorbed on every other carbon atoms at the top surface, and there is no possibility of H H interference in this model. The presence of both C H and -CH3 terminations at homoepitaxial CVD diamond (111) surface was separately confirmed by the vibrational spectra in high-resolution EELS [148, 149],... 

Figure 3. High resolution EELS spectra of Ni MnOx after 500°K reduction for various times, followed by 2x10 L CO exposure. Lower curve - zero minute, upper curve - 10 minutes.

Fig. 15 Ge composition line scans determined by high-resolution EELS on (A) a Sio.sGeo.s island grown at 700°C, and (B) a Sio.8Gco.2 island grown at 500° C. The profiles shown below each XTEM image correspond to the line across the substrate/island interface. Scan direction is from the substrate to the vacuum. (From Ref. °l)...

The spectral range accessible with high-resolution EELS is quite large. Typical experiments examine between 200 and 4000 cm but much larger regions can be analyzed. Vibrational modes as far out as 16,000 cm have been examined. Besides fundamentals, energy losses due to overtones, combination bands, and multiple losses are distinguishable. 

The use of IR spectroscopy of adsorbed CO for the characterisation of supported metal catalysts takes advantage of the many surface science studies performed on metal monocrystal faces. In fact, the technique of low temperature CO adsorption followed by vibrational spectroscopies (such as IRAS, high resolution EELS (HREELS), or, more recently, sum frequency generation spectroscopy (SEG)) represents a largely used technique in metal surface science studies. These techniques give very precise reference data for catalyst characterisation and allow good comparison among the respective results. 

L.H. Dubois, Oxygen-chemisorption and cuprous-oxide formation on Cu(lll)—a high-resolution EELS study. Surf. Sci. 119(2-3), 399-410 (1982)... 

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