Tangential field spectrum

Figure 4.7 (a) Magnitude of the tangential field spectrum on the particle for different values of the surface conductance parameter X (b) effect of surface conductance on the tangential field argument (°). 

Nonhomogenous electric field around metallic nanostmcmre is responsible for the surface selection mles of SERS (Moskovits 1985). According to them, three types of vibrational modes may be found in SERS spectrum (i) those excited by the normal component of the field and resulting in an induced dipole with a strong component perpendicular to the surface, (ii) those excited by the tangential component of the field with a strongly induced dipole tangential to the surface and (iii) mixed cases. The intensity of particular Raman bands then depends on the orientation of the molecule with respect to the surface. 

Now g (and g as well) in Eqs. (82)-(85) are functions of frequency, and ratios such as Ll/L are also functions of exciting frequency. To the blue of the localized surface plasmon peak, the ratio L /L progressively decreases, but to the red of the peak, L L] gradually increases. In fact, for metals, since Re(e)- -oo as the excitation frequency approaches the infrared Eq. (83) shows that L - 0 in the infrared, while Eq. (82) shows that V is finite in the infrared. These results are the basis of the surface selection rules for IR surface spectroscopy, where the electric field of the light incident on metals must be concentrated in the normal direction. They also emphasize that the tangential component of the field will decrease with respect to the normal component as the excitation source is moved towards the red region of the visible spectrum. 

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