Dipole scattering

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.

L of CO was adsorbed at a pressure of 1 x 10 mbar and T= 200 K. At zero energy loss one observes the highly intense elastic peak. The other peaks in the spectrum are loss peaks. At high energy, loss peaks due to dipole scattering are visible. In this case they are caused by CO vibration perpendicular to the surface. The... 

A type of molecular resonance scattering can also occur from the formation of short-lived negative ions due to electron capture by molecules on surfrices. While this is frequently observed for molecules in the gas phase, it is not so important for chemisorbed molecules on metal surfaces because of extremely rapid quenching (electron transfer to the substrate) of the negative ion. Observations have been made for this scattering mechanism in several chemisorbed systems and in phys-isorbed layers, with the effects usually observed as smaU deviations of the cross section for inelastic scattering from that predicted from dipole scattering theory. 

Fig. 19.9 Plot of scaled total decay rates n3r of Ba 6pmn( J = l + 1 autoionizing states in atomic units vs (. For ( = 0-4 the measured rates (O) shown are the average rates from many n values. The data for the rates for > 4 are for n = 12. The solid line is a simple theoretical calculation based on the dipole scattering of a hydrogenic Rydberg electron from the 6p core electron. Note that the core penetration of the lower states reduces the actual rate from the one calculated using the dipole scattering model. The constant total decay rate for > 8 is the spontaneous decay rate of the Ba+ 6p state (from ref. 39).

The RAIRS spectra for NO on Mo2C display peaks at 1800 and 1761 cm-1. These peaks are characteristic of on-top bonded NO.39 Terminally bonded NO on Ru(001) displays a stretching frequency of about 1790-1820 cm-1. By analogy with the discussion of the CO TPD and RAIRS data, we attribute the peaks at 1800 and 1761 cm-1 to on-top adsorption on two distinct Mo sites. Again, it must be noted that the RAIRS results cannot be used to rule out the presence of tilted or flat-lying states on the Mo2C surface. Indeed, Hayden et al,36 only detected the on-top NO species in a RAIRS study of NO on Ru(001) despite the fact that NO adsorbed in the hollow site exhibits a dipole scattering mode in EELS measurements. 

In the limit of dipole scattering, the zero-angle scattering Jones matrix for a system of anisotropic particles is simply found by extending equation (4.66) to the case were the polarizability is a tensor. In this case,... 

Where infrared and Raman spectroscopy are limited to vibrations in which a dipole moment or the molecular polarizability changes, EELS detects all vibrations. Two excitation mechanisms play a role in EELS, namely dipole scattering and impact scattering. 

The EELS spectrum of DMBM [Fig. 39(a)] and the IR spectrum of solid DMBM in KBR [Fig. 39(b)] are very similar except for the absence of the sulfhydryl SH stretch in the EELS spectrum (2500 cm-1). [The difference in appearance of the EELS and IR spectra results from several causes the resolution of EELS (80 cm-1) was poorer than that of IR (4 cm-1) EELS emphasizes electron impact scattering while IR emphasizes dipole scattering, an experimentally valuable distinction and the intrinsic line widths may be broader for the adsorbed layer than for the solid compound.] Assignments of the EELS bands are proposed in ref. 81. Absence of the SH stretch from EELS spectra of DMBM is evidence for removal of the sulfhydryl hydrogen during adsorption... 

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