Selection Rules for a Polarized IR Beam

If the IR beam is s-polarized, the electric field at the interface is parallel to the interface plane hence, the beam undergoes absorption only by species with a dynamic dipole parallel to the interface [see e in Eq. (1)]. On the other hand, if the beam is p-polarized, the % and z components of the electric field are nonvanishing hence, it undergoes absorption by species with dynamic dipoles either parallel (fix ) or perpendicular to the interface [s z in Eq. (2)]. Since the three coefficients A A as given by Eqs. (3-5) exhibit comparable values for typical values of the refractive indexes and an incidence angle of 45°, practical consequences are as follows  

An interface species with dynamic dipole perpendicular to the interface leads to absorption only for a p-polarized beam. 

An interface species with dynamic dipole parallel to the interface, and randomly oriented in the surface plane, leads to about the same absorption for s-and p-polarized beams (Ay compared to Af). 

An interface species with fuUy random orientation leads to twice as much absorption for a p-polarized beam as for an s-polarized beam (A +A, compared 

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