Vibrational Absorption of Species outside the Double-Layer

Vibrational Absorption of Species outside the Double-layer 

Whether the geometry is that of external or internal reflection, the IR beam goes through one medium and penetrates into the other at least over a depth on the order of a fraction of the wavelength ( 5 in the ATR case). The IR absorption is then sensitive to possible changes in the two media, far from the double layer region. 

The latter mode, associated with the breathing mode of the SOj ion, is normally IR-non-active, but it becomes active in the complex because the tetrahedral symmetry of the anion is broken. It is interesting to note that Ga ions can be detected in this way even though IR is blind to the naked Ga ions [11, 

The method can be made quantitative by calibration of the IR signals in solution. 

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