Simultaneous evaluation of molecular-orientation and optical parameters in ultrathin films by oscillators-model simulation and infrared external-reflection spectrometry

A new analytical technique is presented for the simultaneous evaluation of molecular orientation and optical parameters in organic ultrathin films on a dielectric substrate by integration of oscillators-model simulation analysis and infrared external-reflection (ER) spectroscopy. The surface-normal and -tangential components of vibrational TO and LO modes in the film were independently represented by two dielectric dispersion functions with the use of Kim's oscillators models. The reflection absorbance of each band in the ER spectra was analyzed by use of the two functions on the assumption of the anisotropic optical system. Two p-polarization infrared ER spectra observed at two angles of incidence were subjected to an optimization calculation, so that the parameters in the two functions were simultaneously converged. The converged results yielded refractive-index dispersions and orientation angles at the same time. As an example study, the molecular structure in a 5-monolayer cadmium stearate Langmuir-Blodgett (LB) film has been analyzed with the new technique. The evaluated optical parameters were consistent with other experimental results. The new technique has also revealed the fine molecular orientation around the alpha -carbon atom, which has not been shown experimentally thus far. The new method proved to enable us to discuss fine properties in thin condensed matter without using library values of optical parameters.