Selection rule dipole-normal

Since the intensity of IR light for s-polarization is very small at the metal surface, only vibrations with a transition dipole component along the surface normal are detected. This necessitates using p-polarized light for the IR beam. The transition dipole vector of the molecule and electric field vector of the IR light must have a non-zero projection on each other, the consequence is that molecules with their transition dipole parallel to the metal are not seen in the SFG spectrum. This effect is referred to as the IR dipole surface selection rule [45,46]. 

Qualitatively, the selection rule for IR absorption for a given mode is that the symmetry of qT ) " must he the same as qT ). Qiianii-talivcly, the transition dipole moment is proportion al to tlie dipole derivative with respect to a given normal mode dp/di. ... 

There is also the normal dipole selection rule in operation, as illustrated in Figure 5.48, due to Liith (1981). Any dipole at a surface induces an image charge within the surface. If the dipole orientation is normal to the surface, the effect is enhanced by the image dipole. If, however, the orientation is parallel to the surface, the effect is annihilated by the image dipole. This orientation selection rule thus strongly favours normally oriented dipoles. 

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

While s-polarized radiation approaches a phase change near 180° on reflection, the change in phase of the p-polarized light depends strongly on the angle of incidence [20]. Therefore, near the metal surface (in the order of the wavelength of IR) the s-polarized radiation is greatly diminished in intensity and the p-polarized is not [9]. This surface selection rule of metal surfaces results in an IR activity of adsorbed species only if Sfi/Sq 0 (/i = dipole moment, q = normal coordinate) for the vibrational mode perpendicular to the surface. 

For a fundamental vibrational mode to be IR-active, a change in the molecular dipole must take place during the molecular vibration. This is described as the IR selection rule. Atoms that possess different electronegativity and are chemically bonded change the net dipole of a molecule during normal molecular vibrations. Typically, antisymmetric vibrational modes and vibrations due to polar groups are more likely to exhibit prominent IR absorption bands. 

Polymer films were produced by surface catalysis on clean Ni(100) and Ni(lll) single crystals in a standard UHV vacuum system H2.131. The surfaces were atomically clean as determined from low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Monomer was adsorbed on the nickel surfaces circa 150 K and reaction was induced by raising the temperature. Surface species were characterized by temperature programmed reaction (TPR), reflection infrared spectroscopy, and AES. Molecular orientations were inferred from the surface dipole selection rule of reflection infrared spectroscopy. The selection rule indicates that only molecular vibrations with a dynamic dipole normal to the surface will be infrared active [14.], thus for aromatic molecules the absence of a C=C stretch or a ring vibration mode indicates the ring must be parallel the surface. 

An important consequence of the presence of the metal surface is the so-called infrared selection rule. If the metal is a good conductor the electric field parallel to the surface is screened out and hence it is only the p-component (normal to the surface) of the external field that is able to excite vibrational modes. In other words, it is only possible to excite a vibrational mode that has a nonvanishing component of its dynamical dipole moment normal to the surface. This has the important implication that one can obtain information by infrared spectroscopy about the orientation of a molecule and definitely decide if a mode has its dynamical dipole moment parallel with the surface (and hence is undetectable in the infrared spectra) or not. This strong polarization dependence must also be considered if one wishes to use Eq. (1) as an independent way of determining ft. It is necessary to put a polarizer in the incident beam and use optically passive components (which means polycrystalline windows and mirror optics) to avoid serious errors. With these precautions we have obtained pretty good agreement for the value of n determined from Eq. (1) and by independent means as will be discussed in section 3.2. 

All this work on the dipole-dipole interaction has been made for modes oriented normal to the surface or for the normal component of n and they predict an upward frequency shift for increasing coverage. Hayden et al. suggested that a downward shift could occur for modes oriented parallel to the surface and this idea has also been used to assign modes of H/W(100). However, it should be clear that the interaction must be much weaker for modes parallel to the surface, as the dipole field in accordance with the infrared selection rule mentioned in section 2 is screened by the metal surface. At least, in a theoretical model this has to be taken into account. 

An alternative but not so general selection rule (it is restricted to the harmonic oscillator approximation) is that jVoI> / v5i dr is zero if dfiJdQ (evaluated for the equilibrium nuclear configuration) is zero, i.e. if there is no linear dependence of the dipole moment on the normal coordinate Q . 

A more detailed discussion of these mode assignments including reference to the IR spectra of model organometallic compounds is presented in reference 24. We assume the adorbate is oriented with its carbon-carbon axis approximately parallel to the surface since only small, broad peaks (1300 - 1400 cm l) are seen in the C-C stretching region. Observation of such a mode in the specular direction is prohibited by the normal dipole selection rule (3,... 

Step 4. For a vibrational mode to be infrared (IR) active, it must bring about a change in the molecule s dipole moment. Since the symmetry species of the dipole moment s components are the same as rx, ry, and 1, a normal mode having the same symmetry as Ix, Ey, or 1 will be infrared active. The argument employed here is very similar to that used in the derivation of the selection rules for electric dipole transitions (Section 7.1.3). So, of the six vibrations of NH3, all are infrared active, and they comprise four normal modes with distinct fundamental frequencies. 

The selection rules are restrictions imposed on the quantum transitions, because of the laws of conservation of angular momentum and parity [59], In the case of IR spectroscopy, within the frame of the harmonic approximation, the applicable rules are the electric dipole selection rules. That is, when the expression in Equation 4.19 has a finite value, the transition is allowed, and when this expression is zero the transition is forbidden. In the Raman case, when one of the integrals given by Equation 4.23 is different from zero, the normal vibration associated is Raman-active. 

To determine whether the vibration is active in the IR and Raman spectra, the selection rules must be applied to each normal vibration. Since the origins of IR and Raman spectra are markedly different (Section 1.4), their selection rules are also distinctively different. According to quantum mechanics (18,19) a vibration is IR-active if the dipole moment is changed during the vibration and is Raman-active if the polarizability is changed during the vibration. 

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