EFFECT OF SUBSTRATES ON THE INFRARED EXTERNAL REFLECTION SPECTRA OF LANGMUIR-BLODGETT-FILMS

The Fourier transform infrared (FTIR) polarized external reflection (E R) spectra of 9-monolayer cadmium stearate Langmuir-Blodgett (LB) film s were measured on various kinds of materials (Ge, ZnSe, and GaAs) in order to investigate whether their spectra could be used for a quantit ative analysis of the molecular orientation. The FTIR ER spectra on do uble-side-polished substrates with LB films on both sides would not fi t in, even qualitatively, with the 5-layer system (IR//air/LB/substrat e/LB/air) theoretical prediction. On the other hand, the ER spectra of a single-side-deposited LB film (IR//air/substrate/LB/air) were quali tatively explained by a simple 3-layer system (IR//substrate/LB/air) i n which the reflection in the substrate was treated as a single reflec tion. This indicated that the output rays from the substrate in the co nventional 5-layer system did not interfere sufficiently with each oth er. A 5-layer system calculation without any interference effect quali tatively explained the ER spectra of LB films on a double-side-polishe d substrate. It was eventually concluded that double-side-polished mat erials are not suitable for a precise analysis of ER spectra, since it is almost impossible to estimate the area ratio of interferential and non-interferential rays.