Metal-surface selection rule

Rh(CO)2 [16]. Such a dicarbonyl should possess two vibration modes. However, only the symmetric mode is observable in the IR spectrum. The asymmetric mode is inaccessible to an IR experiment on a metal surface due to the so-called metal surface selection rule, which prohibits the observation of dipole excitation if the transition dipole moment is oriented parallel to the surface. It should be noted that the observed frequencies fit well to values observed for Rh(CO)2 on technical Rh/Al203 catalysts [35-40] ( 2100 cm ) and Rh(CO)2 on planar TiO2(110) surfaces [41] (2112 cm ). 

A variant of IRRAS is polarization modulation IR reflection absorption spectroscopy (PM-IRRAS). In this method, the polarization of the IR beam incident on the sample is modulated between parallel and perpendicular polarization. When the sample is metallic, only the parallel-polarized light yields signals from adsorbed molecules, because the electric field amplitude of perpendicular-polarized light vanishes at the metal surface. This statement is the basis for the metal surface selection rule 100,109). When the medium above the sample (gas or liquid phase) is isotropic, both polarizations are equivalent. The PM-IRRAS method thus enables the measurement of signals from adsorbates on a metal surface in the presence of an absorbing gas or liquid phase. 

There is no mention in Table 4.2 of structures, either on surfaces or in complexes, wherein two or more of the same type of hydrocarbon molecule are attached to the same metal atom or ion. Because of the wide occurrence of carbonyl complexes, where the M—CO bond is similar to that in the alkene complexes, this is somewhat surprising, but in view of their greater size it is likely that steric repulsion would inhibit their formation except perhaps at atoms of low CN. Somewhat unstable complexes of the form " E2M (M=Pt, Pd) are known, that with platinum being the more stable, and a number of complexes denoted as " E2M and " E3M (M=Co, Rh, Ni, Pd, Pt and Cu) have been formed by matrix isolation in a solid Group 0 element at very low temperature, as well as " EM species. The infrared spectrum of " EPd closely resembles that of the adsorbed structure 2, but there are additional bands that, because of the metal-surface selection rule, are not visible for the adsorbed species. Moreover the measurement of UV-visible spectra is possible. This fascinating area of -complex chemistry, which has produced several prophetic insights into catalytic mechanisms, has been sadly neglected for many years it merits renewed attention. 

Figure 1 Charges and their images near metal surfaces the origin of the metal surface selection rule (MSSR).

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.

Greenler [32] has shown that this selection rule is, in fact, operational down to metal nanoparticles with diameter 20 A or more. Therefore, IR experiments conducted on supported metal particles can also be analyzed within this context [22, 23]. Note that this selection rule, often known as the metal-surface selection rule, applies only to metal surfaces. Its application to semiconductor surfaces depends largely on the dielectric response of those surfaces, which depends on doping levels, while it does not apply at all to insulator surfaces. For the detailed behavior of IR radiation with metal, semiconductor, and insulator surfaces, the reader is pointed to Chabal s [4] excellent review. 

In terms of symmetry arguments, the metal-surface selection rule can be more usefully applied by the following considerations. Given that only a field normal to the surface can be generated in a RAIRS experiment at a metal surface, only the electric dipole moment operator, normal to the surface is now relevant to the transition moment matrix, which now becomes... 

Since i, the ground vibrational state, and fj,z both transform as the totally symmetric representation, the transition moment matrix would only be nonzero if f also transforms as z, that is, effectively transforms as the totally symmetric representation for a surface system. Therefore, the metal-surface selection rule can perhaps more powerfiiUy be expressed as follows only the totally symmetric vibrational modes will be formally allowed in a RAIRS experiment at a metal surface. Thus, when applying character tables to determine IR activity, it can be seen that the general IR activity predicted for the hnear functions x, y, and z discussed earlier for a molecule in a free state now becomes restricted for a molecule adsorbed on a metal surface and only the totally symmetric vibrations are now allowed (for an adsorbed system setup with the z vector normal to the surface, only those vibrations that transform as the totally symmetric z hnear vector are allowed, while those that transform as x or y are not). 

S-ProUne on Cu(110) From the section above, it can be seen that a general description of the amino acid species present in an adsorbed overlayer can be teased out from the RAIRS data, aided by a qualitative application of the metal surface selection rule. However, a major challenge exists for the field, namely, can one map the conformation and bonding interaction of each molecule to the surface. 

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