Double Fermi resonance: High pressure Raman investigations on hexachloroethane

Traditional Fermi resonance is a vibrational mixing between a fundamental and a combinational or overtone mode. What will happen when the resonance occurs between a generated fundamental and an overtone mode? In this report, we performed a high pressure Raman spectroscopic investigation on hexachloroethane up to 20 GPa. The origin of the triple bands at 841, 851 and 860 cm−1 were thoroughly discussed, and their intensity evolution in the compression process are further analyzed. The triple bands can be attributed to the quantum mixing between generated v7 fundamental mode and the v2 overtone mode, This intriguing spectral phenomenon is named as double Fermi resonance, and a new mechanism of the interaction of double Fermi resonance is proposed. Pressure provides us a new strategy not only for the mode assignments but also to understand the mechanism underlying the vibrational mixing.