Surface Selection Rule for Dielectrics

The calculated angle-of-incidence dependences of the absorbance in the s- and -polarized IRRAS of a 1-nm hypothetical isotropic layer on silicon and on glass (Figs. 3.89, 2.09, and 2.10) show that 

The net absorbance in the /j-polarized spectra A shown is further resolved into two components, A and A, which describe the film s interaction with the E and components of the electric field, respectively (Fig. 3.82). The sign of these components differs from that of the sum. For both substrates, 

Relationships (3.64) constitute the surface selection rule (SSR) for dielectrics (see also Fig. 3.18). 

The SSR for dielectrics allows one to obtain a quick, qualitative impression about molecular orientations in ultrathin films. For example (Fig. 3.90), the spectrum of a SAM, obtained by IRRAS at pi = 80 3°, exhibits positive and negative absorbance due to the methylene and methyl stretching modes. 

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