Raman and infrared spectra in organic layer crystals of squaric acid

The Raman spectra of squaric acid and its deuterated derivative are measured for temperatures from 77 to 450 K covering both ordered and disordered phases. Infrared absorption spectra are investigated for thin films of the single crystals and compared with the Raman spectra. It is found that the vibrational Davydov splitting due to the interlayer interaction is large for the external modes, while it is small for the internal modes of the squaric molecule. This behaviour is explained by the model that the interlayer interaction acts between the hydrogen bonds of adjacent layers and is localized in the region near the hydrogen bonds. The temperature dependence of the frequencies and the linewidths is discussed in relation to the phase transition mechanism. (author)