Phase Transitions in Benzene under Dynamic and Static compression

Phase transitions in liquid Benzene under laser shocked conditions are studied using high resolution time resolved Raman spectroscopy technique. The C-C Ring breathing mode (υ1) of Benzene at 993 cm-1 is analyzed to monitor the phase transitions occurring in the Raman spectra during shock wave propagation in the pressure range 1.3 - 5 GPa. A phase transition from Liquid Benzene to solid Benzene Phase-I is observed below 1.3 GPa and above this pressure another phase transition from phase-I to Phase-II occurs. We also performed, Raman spectroscopic measurements under static compression employing diamond anvil cell (DAC) to compare the effect of shock wave on Benzene compared to isothermal compression using DAC. In static measurement, it is observed that the first transition from liquid to solid benzene phase (orthorhombic) occurs at 0.3 GPa and second phase transition from benzene –I (orthorhombic) to Benzene phase -II (monoclinic) occurs at 2.3 GPa, which is higher than the phase transition pressure measured in case of shock compression. The observed high resolution spectrum enables us to determine the effect of pressure gradient upon the Raman spectrum of benzene in shocked condition. 1D radiation-hydrodynamic simulations were performed to corroborate the Raman spectroscopic results under dynamic compression. Simulated spatial profiles of the shock wave propagation in benzene at different delay times are used to explain the observation of multiple Raman modes in the spectral region 990-1020 cm-1. It is understood that different regions of the sample experience distinct pressure due to shock pressure gradient across the sample. The new Raman modes appearing in this spectral region are attributed to the Raman signals due to phase transition and from differently shocked regions of benzene under dynamic compression. The effect of the higher pressure reflected shock wave from benzene-glass interface is also reported.