Analysis of transition dipole moments of band-progression peaks of fluorocarbons: Fundamentals and application to orientation analysis of fluoropolymers

Perfluoroalkyl chains give infrared spectra involving a dominant absorption peak corresponding to the CF2 symmetric stretching vibration mode. The direction of the transition dipole moment of this mode is approximately perpendicular to the chain axis, that is, the vibrational motion belongs to the E1 symmetry species. Accordingly, the band progressions of this mode have also been considered to have the same property. In the present study, we have investigated the properties of the band progressions in detail using the p-polarized multiple-angle incidence resolution spectrometry on an oriented n-C20F42 monolayer. We found that the surface selection rule of band progressions is the same as that of the CF3 symmetric stretching vibrational mode with the aid of quantum chemical calculations. This indicates that the transition dipole moment of the band progressions does not correspond to the E1 mode but is instead oriented predominantly parallel to the chain axis. We also show that this distinction is important for determining the orientation of perfluoroalkyl chains.