Dipole chemisorbed molecules

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. 

Furthermore, as mentioned above the screening of the dipole field by the conduction electrons can be represented by an image dipole inside the metal. This complex of the chemisorbed molecule and its image has a vibration frequency different from that of the free molecule. The electrodynamic interaction between a dipole and its image has been discussed in many works. The theoretical problem is that the calculated frequency shift is extremely sensitive to the position of the image plane (Fig. 3a). One can with reasonable parameter values obtain a downward frequency shift of the order of 5-50 cm S but the latest work indicates that the shift due to this interaction is rather small. 

In order to deduce the orientation of the chemisorbed molecules, the metal-surface dipole selection mle needs to be invoked this states that only vibrations with components perpendicular to the metal surface are HREELS-active. That is, if benzene... 

Fig. 3. Schematic picture of a chemisorbed CO molecule, (a) As a point dipole with its image, representing the screening by the conduction electrons, (b) The spatial extension of the two molecular orbitals involved in the chemisorption bond, the highest occupied 5a and the lowest unoccupied 2n orbital, (c) The density of states of the conduction electrons and the 2n orbital, which by interaction with the metal electrons has broadened into a resonance and shifted down in energy.

In section 3.1 we discussed the present picture of the electronic arrangement of CO chemisorbed on a metal surface, which was schematically shown in Fig. 3. When the molecule is vibrationally excited charge is oscillating between the 2n resonance and the metal. This gives rise to the large increase in the dynamical dipole moment, as was discussed in that section. These local charge... 

Fig. 18. Infrared spectra of C -H vibrations of different coverages of CH30 on Cu(lOO) at 100K, showing the symmetric (at 2800cm ) and two asymmetric stretch modes. Inset shows a tilted chemisorbed methoxide molecule and the orientations of the dynamical dipole moments ji. (Reproduced by permission from...

In summary, the point dipole model with images can be made to account for the experimentally determined effects of intermole-cular dipole coupling. The magnitudes of the effects cannot be predicted from the properties of the free CO molecule but they can be used to estimate the changed values in the chemisorbed state. 

Attempts were also made to observe emission spectra of CO chemisorbed on platinum by wrapping one rod with platinum foil and enclosing it in a gas-tight cell. These efforts were not successful. The significance of these failures has not been determined. It was possible that the foil was not covered by a layer of CO or that the intensity of the emission bands of chemisorbed CO are not as great as those of the corresponding absorption bands. The failure may also have been due to the orientation of the CO molecules. If they were oriented perpendicular to the surface, the radiation would be emitted perpendicular to the direction of the change of dipole moment and would be parallel with the surface of the rod, so that little would enter the slits of the spectrometer. 

A number of locations and orientations of Sarin on the regular nanosurface and on the small fragment of MgO were found. In this study it was revealed that Sarin is physisorbed (the nanosurface and hydroxylated small fragment this is undestructive adsorption) or chemisorbed (destructive adsorption) on MgO (see Figure 16-1). The physisorption of GB on the surface of MgO occurs due to the formation of hydrogen bonds and ion-dipole and dipole-dipole interactions between adsorbed GB and the surface. The chemisorption occurs due to the formation of covalent bonds between the molecule and the surface. The adsorption results in the polarization and the electron density redistribution of GB. The adsorption energy obtained at the MP2/6-31G(d) level of theory for the most stable chemisorbed system is... 

The various ethene adsorbate species can be identified by vibrational spectroscopy (cf. Fig. 43) (46,138,448,470 75). Calibration SFG spectra recorded under UHV include three vibrational features, at 2880, 2910, and 3000 cm (138), which are similar to those characterizing the adsorbates on Pd(l 11). The peak at 2880 cm is attributed to the Vs(CH3) stretch vibration of ethylidyne (MSC-CH3), the feature at 2910 cm results from the Vs(CH2) of chemisorbed di-a-bonded ethene, and the very weak peak at 3000 cm represents the Vs(CH2) of physisorbed 7i-bonded ethene. As has been stated, the Vs(CH2) signal characterizing 7i-bonded molecules on single-crystal surfaces is very weak and explained by the surface-dipole selection rule for metal surfaces (17). 

Interactions at room temperature When CO is first introduced (Fig.l), a increases irtfantaneously and then remains independent of P 0. The fact that a does not decrease means that CO does not dissociate on titania nor at the interface, otherwise the filling of anionic vacancies by atomic oxygen (Eq,-6) would have decreased substantially a by consuming free electrons. The sharp initial increase, on the contrary, shows that CO chemisorb on Pt with a donor effect probably due to the creation of dipoles as proposed for H. chemisorption which renders ohmic the electrical contact between the metal and its semiconductor support (26, 17, 28)Accordlng to these authors, the creation of a dipole layer decreases the work function of the metal which approaches the electron affinity of the semiconductor, thus suppressing the Schottky barrier. Presently CO adsorbs as a donor molecule on Pt decreasing 0, which allows elec-... 

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