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 substrate s were measured for s- and p-polarized beams at various angles of inci dence from 25-degrees to 80-degrees. For the 9-monolayer LB film, the reflection absorbances for both polarized beams were also analyzed the oretically as a function of the angle of incidence. In the case of p-p olarization measurements, the vibration bands with transition moments parallel to the film surface give rise to negative absorbances at inci dent 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 t hroughout the whole angle of incidence, and their absolute values decr ease with the increase in the incident angle. It was concluded from mi nute comparisons between the theoretical and experimental results that the molecules of cadmium stearate were oriented almost perpendicularl y to the film surface. The absorbance values for the symmetric CH2 str etching bands were found to show excellent linearities against the num ber of monolayers. This reveals that the molecular orientation of cadm ium stearate is kept unchanged even when the number of monolayers is c hanged.