Electron energy loss spectroscopy monolayers

Although the question of orientation order does not arise, these results for argon monolayers illustrate several features of neutron vibrational spectroscopy. Well-defined excitations can be observed at somewhat lower energy transfers than accessible with optical and electron energy-loss spectroscopies. Typical energy resolution in the scans of Fig. 2 is 0.3 meV (2.4 cm"1). The capability of obtaining inelastic spectra at constant non-zero Q is also not present in these other spectroscopies. However, scattered intensities are relatively low. Counting times in Fig. 2 were -30 minutes per point. 

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). 

The product of the collision-induced dissociative chemisorption event is identified by high resolution electron energy loss spectroscopy. Fig. 9a shows the vibrational spectrum of a monolayer of methane at 46 K before bombardment with Ar. The vibrational frequencies are unperturbed from the gas phase values within the resolution of this technique ( 20 cm-1). The loss observed at 1305 cm" is assigned to the V4 mode, the loss at 1550 cm- to the >2 mode and the losses at 2895 cm 1 and 3015 cm- to the vi and V3 modes, respectively. Fig. 9b shows the vibrational spectrum after exposure of the methane monolayer at 46 K to a beam of Ar atoms with a translational energy of 36 kcal/mole. This spectrum has been assigned previously to an adsorbed methyl radical. 

Finally we only mention that, for example, calorimetric investigations [72, 395], LEED [91], high-energy electron diffraction [313], neutron diffraction [45, 371], high-resolution electron energy loss spectroscopy [304], and ellipsometric studies [111, 147, 366] were extended much beyond the fully compressed N2 monolayer into multilayer and film regimes. 

Fig. 2. Experimental number of scattered electrons, N E), of energy, E, versus electron energy for a Rh(lll) surface covered with a monolayer of ethylidyne species (CCH3) -the stable, room temperature structure of chemisorbed ethylene. Boxes and inset figures show how particular scattered electrons are used in (a) Auger electron spectroscopy, (b) high resolution electron energy loss spectroscopy and (c) low energy electron diffraction...

Generally, it is very difficult to identify the chemical enhancement due to its small contribution as compared with the contribution from the electromagnetic enhancement. Figure 5.14 shows as an exception the Raman spectra of pyromellitic dianhydride (PMDA) adsorbed on Cu(lll) excifed at 647 and 725 nm. The lower spectrum is clearly enhanced as compared with the upper one. Figure 5.15 represents the electronic absorption spectrum of fhe same system obtained with electron energy loss spectroscopy (EELS). The intense narrow peak at 1.9 eV (the transition wavelength 653 nm) appears only for copper covered with a monolayer of PMDA, whereas the intrinsic intramolecular ex-... 

Compared to electron energy loss spectroscopy (EELS), the technique offers a significantly higher resolution (refined apparatuses can reach values of 0.005 cm and better). Experimental halfwidths as low as 0.025 cm" have been reported for physisorbed monolayers, e.g. CO-adlayers on NaCl(OOl) [96HEI]. Since, however, the experimental setup for recording IR-spectra of adsorbed monolayers on substrates mounted in an UHV-apparatus is somewhat more complicated, EELS is the more versatile technique. On the other hand, IR-spectroscopy can be applied to insulators in a straightforward way and can be used outside vacuum if the problems related to the absorption by the ambient gas can be overcome. 

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