Orientation of Organic Molecules at the Electrode Surface

Determination of the Orientation of Organic Molecules at the Electrode Surface 

When linearly polarized light is absorbed by a sample, the integrated intensity of the absorption band is proportional to the square of the absolute value of the dot product of the transition dipole vector and the electric field vector of the incident radiation [48, 49]  

At the metal surface, the direction of the electric field vector of the p-polarized radiation is normal to the surface. Therefore, the integrated band intensity of adsorbed molecules is proportional to cos 0, where 0 is the angle between the average direction of the transition dipole moment and the surface normal. If the direction of the transition dipole with respect to the coordinates of the adsorbed molecule is known, Eq. (42) may be used to determine the orientation of that molecule at the metal surface. The direction of the transition dipole moment with respect to coordinates of a molecule can be determined by performing normal coordinate ah initio calculations using commercial software such as Gaussian 98 [50]. 

The numerical value of the ratio y iP/ y ii = 1.5 was determined from the absorbance ratio in an independently measured transmission spectram. The relative method is very useful when either the surface concentration is not available or the MSEFS is difficult to calculate. 

The second method, developed by AUara et al. [42, 48, 49], relies on calculation of the theoretical reflection absorption spectrum for the same angle of incidence and the thin-cavity thickness as the values used during the collection of the experimental data. The optical constants of the window, electrolyte, and metal can be taken from the literature [22, 37-39], while the isotropic optical con- 

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