HREELS selection rules

While the underlying mechanisms of HREELS are pretty well understood, many important details relating to selection rules and scattering cross sections remain unknown. 

Figure 5.48. Schematic illustration of the operation of the normal dipole selection rule in HREELS.

High Resolution Electron Energy Loss Spectroscopy has been "discovered" at about the same time as the previous cited techniques - die first reported experiment is related to a study of small molecules adsorbed on a (100)W surface and is dated from 1967 (1). During the last 15 years, the characterization of adsorption states of molecules on metal and semiconductor surfaces was the principal attribute of HREELS information on the elemental composition, on the chemistry, and the kinetics of surface reactions (versus temperature and/or time) were studied. One significant "plus" of HREELS is its ability to identify adsorption sites on a metal, by using the "dipole-selection rule" it is therefore possible to gain information on the short-scale structure or morphology of a surface with HREELS. 

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. 

The effect of alkyl chain length on the structure of alkanethiols on Au(lll) was studied with CH3(CH2) iSH, where n = 2,4, 6, 8, 10, 11, 12, 14, 15, 16, and 18).i The results, in terms of HREEL spectra, are displayed in Figure 11. It is most interesting to note that the intensity of CH3 a-deformation mode at 1380 cm (171 meV) is profoundly dependent on the number of carbons in the alkyl chain It is present only when the number of carbon atoms is even (cf, the spectra labeled Cio, C12 and C le) it is absent when the number is odd (cf, the spectra labeled Cu andCis). This odd-even trend is caused by the fact that the orientation of the CH3 head is parallel to the surface for odd number of carbon atoms but perpendicular when the number is even (cf, the inset in Figure 11). As dictated by the dipole selection rules, only the oscillator that has a component perpendicular to the surface (as in the even number chain) would show HREELS activity. It can also be seen in the frequency region below 220 cm (27.3 meV) that more than one peak, separated by about 30 cm (3.7meV) are present this indicates the existence of multiple adsorption sites for the subject alkanethiols on Au(lll). 

Figure 5 shows the SFG vibrational spectra of carbon monoxide obtained at 10 -700 Torr of CO and at 295 K. When the clean Pt(lll) surface was exposed to 10 L (1 L=10 Torr sec) of CO in UHV, two peaks at 1845 cm and 2095 cm were observed which are characteristic of CO adsorbed at bridge and atop sites. LEED revealed that a c(4 X 2) structure was formed in which an equal number of carbon monoxide molecules occupied atop and bridge sites [15]. Such results are in agreement with previous HREELS [16] and reflection-absoiption infrared spectroscopy (RAIRS) [17] studies. ITie much higher relative intensity of atop bonded CO to bridge bonded CO in the SFG spectra is due to the specific selection rule for the SFG process [18]. As mentioned earlier, SFG is a second order, nonlinear optical technique and requires the vibrational mode under investigation to be both IR and Raman active, so that the SFG intensity includes contributions from the Raman polarizability as well as the IR selection mle for the normal mode. 

On metal surfaces, two additional selection rules apply. The first is that only vibrations perpendicular to the surface are HREELS active. This rule follows from two phenomena unique at metal surfaces (i) Electromagnetic waves polarized perpendicularly to the plane of incidence (parallel to the plane of the surface) undergo a 180° phase shift upon reflection. That is, at the metal surface, the out-of-phase electric-field vectors of the incident and reflected waves cancel each other as a result, no field exists that can couple with dipoles that oscillate parallel to the surface, (ii) The dynamic dipole moment generated by an oscillator that vibrates in the surface-parallel direction is cancelled by that of its image dipole (Figure 1) hence, there the net dynamic dipole moment is zero. On the other hand, if the real dipole is oriented perpendicularly to the surface, its dynamic dipole moment is reinforced by that of its image dipole. This selection rule is the same as that for infrared reflection-absorption spectroscopy (IRAS). =... 

HREELS (Table 4.1) has the advantage of detecting all types of vibration this is because there are two excitation mechanisms, viz. dipole scattering and impact scattering. The former is subject to the same selection rules as RAIRS and gives strong features on-specular, but the latter excites all vibrational modes. There are however supplementary selection rules that apply to impact scattering in the on-specular direction. As noted earlier, this technique is not applicable to supported metal catalysts. 

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. 

Since both HREELS and RAIRS are vibrational spectroscopies, and the same selection rules apply, their information contents must overlap. This is demonstrated in Fig. 8 [2], in which the HREELS and RAIRS spectra from a Cu(l 11) surface covered with about 10 molecular layers of cyclohexane at low temperature are shown. The vibrational spectra appear at the same energetic positions in both techniques, but it should be noted that whereas RAIRS has the advantage of better energy resolution HREELS is able to record spectra down to losses close to 0 cm. For reasons of IR transmission of window materials, the cutoff in RAIRS is in the region 400-800 cm". ... 

Figure 11 A comparison of the IR spectrum from ethene adsorbed on Pt(111) at room temperature with that of the model compound (CH3C)Co3(CO)9. Asterisks indicate absorptions of the model compound allowed in the spectrum of the adsorbed species by the metal-surface selection rule arrows indicate other bands observed by HREELS. Courtesy Chesters MA.

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