Phase transitions in benzene under dynamic and static compression

Abstract

AbstractPhase transitions in liquid benzene under laser shocked conditions are studied using high‐resolution time‐resolved Raman spectroscopic 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. Phase transition from liquid benzene to solid benzene (Phase I) is observed below 1.3 GPa, and above this pressure, another phase transition from Phases I and 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 with isothermal compression using DAC. In static pressure measurements, the first phase transition from liquid to solid benzene (orthorhombic) occurs at 0.3 GPa, and the second phase transition from benzene I (orthorhombic) to benzene Phase II (monoclinic) occurs at 2.3 GPa, which is higher than that observed in case of shock compression experiments. The observed high‐resolution spectrum enables us to determine the effect of pressure gradient upon the Raman spectrum of benzene in shocked condition. One‐dimensional 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 reflected shock wave from benzene‐glass interface manifested in terms of red shifting and intensity enhancement of Raman modes at higher pressure is explained in this study.