INFRARED EXTERNAL REFLECTION STUDY OF MOLECULAR-ORIENTATION IN THIN LANGMUIR-BLODGETT-FILMS

FT-IR external reflection spectra of 1-19-monolayer Langmuir-Blodgett (LB) films of cadmium stearate deposited on gallium arsenide substrates were measured for s- and p-polarized beams at various angles of incidence from 25-degrees to 80-degrees. For the 9-monolayer LB film, the reflection absorbances for both polarized beams were also analyzed theoretically as a function of the angle of incidence. In the case of p-polarization measurements, the vibration bands with transition moments parallel to the film surface give rise to negative absorbances at incident angles smaller than the Brewster angle (73-degrees), while those with transition moments perpendicular to the film surface give rise to positive absorbances at the same incident angles. However, the signs of these absorbances are completely reversed at incident angles larger than the Brewster angle. In the case of s-polarization measurements, on the other band, all the vibration bands give negative absorbances throughout the whole angle of incidence, and their absolute values decrease with the increase in the incident angle. It was concluded from minute comparisons between the theoretical and experimental results that the molecules of cadmium stearate were oriented almost perpendicularly to the film surface. The absorbance values for the symmetric CH2 stretching bands were found to show excellent linearities against the number of monolayers. This reveals that the molecular orientation of cadmium stearate is kept unchanged even when the number of monolayers is changed.