Selection rules infrared

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. 

Infrared selection rule operators acting in the u ( ) H-bond bridge subspaces of centrosymmetric cyclic dimers taking into account some degree r of forbidden transition. 

A molecule which is a symmetric top on account of its symmetry (accidentally symmetric tops are not considered), exhibits two types of normal vibrations, because the oscillating dipole moment may either be oriented parallel to the top axis or perpendicularly to it. The infrared selection rules for a so-called parallel band are... 

The second type of fundamental vibrations involves an oscillating dipole moment, oriented perpendicularly to the unique molecular axis. The corresponding infrared selection rules for the rotational transitions are given by... 

The band at 1338 cm was identified in Ref [16] to be a Bjg band based on its frequency and intensity in the crystal spectra, while a band at 1292 cm is likely to be a shifted variant of the C-H deformation Ag mode at 1303 cm in the single crystal [17]. The other bands correspond to modes which normally show infrared activity (see Figure 13.4). Considering that all the modes occurring upon In and Ag deposition are normal modes of the PTCDA molecule, the observed break-down of the Raman-infrared selection rules was proposed to originate from a weak charge transfer between the molecules and the metal surface mediated by molecular vibrations [9]. 

Column matrix, 297 Combination frequencies, 36, 247 infrared selection rules, 160 Raman selection rules, IGl Combination levels, 36 degeneracy, degree of, 151 species of, 148/., 247, 331 Commutation of matrices, 294, 295, 302, 315... 

Infrared Selection Rules and Intensities for the Harmonic Oscil... 

By convention, these are often called infrared selection rules, to distinguish between the selection rules for vibrational spectra obtained by infrared absorption and Raman spectroscopy (described later in this section). We will stick to the more cumbersome electric dipole to avoid the suggestion that the rules are wavelength-dependent. 

With a little group theory, we can determine whether or not the vibration has a dipole derivative. The same symmetry selection rules apply to vibrations as to electronic transitions for a transition to be allowed, the direct product of the representations for the initial and final states must be one of the representations for the transition moment. The transition moments for electric dipole or infrared selection rules correspond to the functions x, y, and z. For Raman transitions, the transition moments correspond to any of the second-order functions of x, y, and z, such as xz or -I- y. The representation of the ground vibrational state is always the totally symmetric representation, so F, F is equal to Fy for fundamental transitions. Therefore, the selection rule for fundamental transitions is F, (x) Fy = F = F. For example, the group theory predicts that for CO2 the transitions V2 = 0 1 and V3 = 0 1 are infrared-allowed, because those vibrational modes have TTu (x,y) and (z) symmetry, respectively. On the other hand, the symmetric stretch transition Vj = 0 1 is forbidden by infrared selection rules but allowed by Raman selection rules, because that vibrational mode has (x + y, z ) symmetry. Here are the relevant rows from the character table in Table 6.4 ... 

The MathematiceJ Background to Infrared Selection Rules 335 A6.4 Vibrational Modes for Polyatomic Molecules... 

The surface-induced infrared selection rule states that only vibrating dipoles with a nonzero component perpendicular to the substrate surface will be excited by p-polar-ized infrared radiation. This provides a means for determining the average orientation of surface-confined molecules [34]. 

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