High-pressure Raman study of resonance and non-resonance phenomena between the vibron and the libron in crystalline methylene halides

Abstract

High-pressure Raman measurements of crystalline methylene halides have been performed up to 100 kbar at 300 K in a gasketed diamond anvil cell. The pressure-tuning resonances between the vibron and the libron were observed around the crossover pressures of 43 kbar and 25 kbar for crystalline CH₂Br₂ and CH₂BrI, and their vibron-libron coupling constants were determined to be 10.3 cm⁻¹ and 4 cm⁻¹, respectively. The same resonance was not observed for crystalline CH₂ClI; as the pressure increases the libron mode showed only an increase in frequency and just passed through the vibron mode without mutual interaction. From these results, including more CH₂Cl₂ and CH₂I₂, we investigate the dominant factors to determine the possibility of observing pressure-tuning of the vibron-libron resonance and present a systematic explanation of this new phenomenon in crystalline methylene halides.