Abstract
As reported in the preceding paper, a recording device with high optical contrast can be formed by high-speed irradiation of laser beam on thin crystalline films of long-chain fatty acids grown on glass substrates with vacuum-evaporated Cr thin films. This article describes molecular-level analysis of the recording process, using a model substance of palmitic acid. Polarized FT-IR spectra of the palmitic acid crystals of the recorded and original films were analyzed in a comparative manner. The irradiation of the laser beam induced the change in the orientation of the long-chain axis of the crystalline film through the process of partial melting and recrystallizaion. The long-chain axis orientation of the original film was normal to the film plane, while that of recrystallized portion was parallel. In more detail, the intensity changes of CH2 scissoring and CH2 stretching bands in IR spectra with varying polarization directions showed that the c-axis of unit cells in the recrystallized portions of the recorded film was parallel to the b-axis, and hence normal to the a-axis, of the original film. To confirm this, the crystal growth process occurring after the laser irradiation was observed with SEM. The small crystals grew on the surface of the recorded portions, whose crystallographic axes were arranged with respect to the original film in the same directional relations as those observed by the FT-IR analyses. Consequently, it was proved that the high optical contrast of the recorded portion under the crossed-Nicols polarization is caused by superimposed arrangement of two types of crystalline films, the original film and the recrystallized film, with the specific relations of the crystallographic axes. The optical anisotropy of each crystal film is canceled out by this arrangement.
