EELS Dipole Vibration

Relaxation of the Chemical Bond, Springer Series in Chemical Physics 108, DOI 10.1007/978-981-4585-21-7 6, Springer Science+Business Media Singapore 2014 

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Wliile infrared and Raman speetroseopy are limited to vibrations in whieh a dipole moment or the moleeular polarizability ehanges, EELS deteets all vibrations. Two exeitation meehanisms play a role in EELS dipole... 

Figure Bl.25.12 illustrates the two scattering modes for a hypothetical adsorption system consisting of an atom on a metal [3]. The stretch vibration of the atom perpendicular to the surface is accompanied by a change m dipole moment the bending mode parallel to the surface is not. As explained above, the EELS spectrum of electrons scattered in the specular direction detects only the dipole-active vibration. The more isotropically scattered electrons, however, undergo impact scattering and excite both vibrational modes. Note that the comparison of EELS spectra recorded in specular and off-specular direction yields infomiation about the orientation of an adsorbed molecule.

Figure Bl.25.12. Excitation mechanisms in electron energy loss spectroscopy for a simple adsorbate system Dipole scattering excites only the vibration perpendicular to the surface (v ) in which a dipole moment nonnal to the surface changes the electron wave is reflected by the surface into the specular direction. Impact scattering excites also the bending mode v- in which the atom moves parallel to the surface electrons are scattered over a wide range of angles. The EELS spectra show the higlily intense elastic peak and the relatively weak loss peaks. Off-specular loss peaks are in general one to two orders of magnitude weaker than specular loss peaks.

Carbon monoxide on metals forms the best-studied adsorption system in vibrational spectroscopy. The strong dipole associated with the C-O bond makes this molecule a particularly easy one to study. Moreover, the C-0 stretch frequency is very informative about the direct environment of the molecule. The metal-carbon bond, however, falling at frequencies between 300 and 500 cm1, is more difficult to measure with infrared spectroscopy. First, its detection requires special optical parts made of Csl, but even with suitable equipment the peak may be invisible because of absorption by the catalyst support. In reflection experiments on single crystal surfaces the metal-carbon peak is difficult to obtain because of the low sensitivity of RAIRS at low frequencies [12,13], EELS, on the other hand, has no difficulty in detecting the metal-carbon bond, as we shall see later on. 

Mechanism EELS applied in the off-specular mode detects all vibrations, whereas RAIRS only detects vibrations in which a dipole moment changes perpendicular to the surface. 

An ordered monolayer of molecules having a large dynamical dipole moment must not be regarded as an ensemble of individual oscillators but a strongly coupled system, the vibrational excitations being collective modes (phonons) for which the wavevector q is a good quantum number. The dispersion of the mode for CO/Cu(100) in the c(2 x 2) structure has been measured by off-specular EELS, while the infrared radiation of course only excites the q = 0 mode. 

The use of a surface selection rule in infrared spectroscopy and in EELS has been widespread and has yielded much valuable information on surface orientation of adsorbed species. The rule is a very simple one for adsorbates at metal surfaces, only vibrational modes which modulate a component of the molecular dipole perpendicular to the surface are active in these spectroscopies. This rule arises from the fact that metals have high electrical conductivities at vibrational mode frequencies and this results in the parallel (but not the perpendicular) component of the radiation field going to zero at the surface. 

A distinct advantage of EELS is that electrons can excite the vibrational modes of the surface by three different mechanisms dipole scattering, impact scattering, and resonance scattering. By analyzing the angular dependence of the inelastically scattered electrons a complete symmetry assignment of the surface-adsorbate complex can be made. 

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