PM-IRRAS SPECTROSCOPY OF 9CB LIQUID-CRYST AL THIN-FILMS DEPOSITED ON A METALLIC SUBSTRATE .1. VIBRATIONAL ANALYSIS

Polarization Modulated InfraRed Reflection Absorption Spectroscopy (PM -IRRAS) has been used to obtain the vibrational spectra of 4-nonyl-4'- cyanobiphenyl (9CB) thin films deposited on a gold substrate. For the different phases of this liquid crystal, PM-IRRAS spectra of 1000 to 2 0 angstrom thin films exhibit important changes relative to the absorb ance spectrum of a bulk sample. Just after deposition, the liquid crys tal has a particular phase and it is necessary to cycle the sample at liquid nitrogen temperature to get the crystal phase. Phase transition temperatures are lower than those observed with a bulk sample. Concer ning the crystalline phase, relative intensities on PM-IRRAS spectra a re different from those mesured on the bulk absorbance spectrum. These differences are due to a well defined molecular orientation in the li quid crystal film.