High resolution electron energy loss vibrational studies

Figure 2 Schematic of a 127° high-resolution electron energy-loss spectrometer mounted on an 8-in flange for studies of vibrations at surfaces.

In recent years there is a growing interest in the study of vibrational properties of both clean and adsorbate covered surfaces of metals. For several years two complementary experimental methods have been used to measure the dispersion relations of surface phonons on different crystal faces. These are the scattering of thermal helium beams" and the high-resolution electron-energy-loss-spectroscopy. ... 

The success of the isotope dilution experiment for CO on Pt(lll) was accompanied by a serious difficulty in reconciling the magnitude of the shift, which determines Oy/3Q), with the intensity of the band, which also determines Oy/3Q). When due allowance is made for the resultant surface field and geometric factors (36) in RAIR spectroscopy the intensity is almost consistent with the vibrational polarizability av = 0.057 X3 (39), corresponding to the gas phase intensity, as has been concluded for CO adsorbed on copper films (40) from infrared studies and for CO on Pt(lll) (41) and Cu(100) (42) from high resolution electron energy loss spectroscopy. This value of av is an order of magnitude smaller than that deduced from the frequency shift. 

There is a number of vibrational spectroscopic techniques not directly applicable to the study of real catalysts but which are used with model surfaces, such as single crystals. These include reflection-absorption infrared spectroscopy (RAIRS or IRAS) high-resolution electron energy loss spectroscopy (HREELS, EELS) infrared ellipsometric spectroscopy. 

Another class of techniques monitors surface vibration frequencies. High-resolution electron energy loss spectroscopy (HREELS) measures the inelastic scattering of low energy ( 5eV) electrons from surfaces. It is sensitive to the vibrational excitation of adsorbed atoms and molecules as well as surface phonons. This is particularly useful for chemisorption systems, allowing the identification of surface species. Application of normal mode analysis and selection rules can determine the point symmetry of the adsorption sites./24/ Infrarred reflectance-adsorption spectroscopy (IRRAS) is also used to study surface systems, although it is not intrinsically surface sensitive. IRRAS is less sensitive than HREELS but has much higher resolution. 

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. 

The mechanism for the dissociation of CH4 on Ni(lll) is studied by molecular beam techniques coupled with high resolution electron energy loss spectroscopy. The probability of the dissociative chemisorption of CH4 increases exponentially with the normal component of the incident molecule s translational energy and with vibrational excitation. Dissociation can also be induced by the impact of an Ar atom incident on a monolayer of CH4 physisorbed on Ni(lll). 

Electron energy loss spectroscopy (EELS) and high resolution electron energy loss spectroscopy (HREELS) can also provide vibrational information of adsorbates on surfaces [165], Because these methods employ electrons instead of electromagnetic radiation, surface selection rules (see p. 76) are not effective this allows investigation of modes not observed with infrared spectroscopy. Unfortunately the use of electrons both as probe and signal prevents in situ application. Studies of electrode surfaces are feasible with these methods after emersion of the electrode from the solution, but they have been reported only infrequently. 

Electrons scattering off surfaces can lose energy in various ways. One of these ways involves excitation of the vibrational modes of atoms and molecules on the surface. The technique to detect vibrational excitation from surfaces by incident electrons is called high-resolution electron energy loss spectroscopy (EELS). This is the most common type of vibrational spectroscopy used for studying surface-absorbate complexes on single-crystal surfaces. 

Sexton BA (1979) Observation of formate species on a copper(lOO) surface by high resolution electron energy loss spectroscopy. Surface Sci 88 319-330 Sexton BA, Madix RJ (1981) A vibrational study of formic acid interaction with clean and oxygen-covered silver(llO) surfaces. Surface Sci 105 177-195 Sheldon RA, Kochi JK (1968) Photochemical and thermal reduction of cerium(IV) carboxylates. Formation and oxidation of alkyl radicals. J Am Chem Soc 90 6687-6698... 

---