Laser-driven shock compression of pyrimidine: Insights from Raman scattering

Pyrimidine (C4H4N2), a nitrogen-rich heterocyclic molecule, is central to advanced green energetic materials, where ring nitrogen enhances both energy yield and stability. Understanding its phase evolution, structural response, and susceptibility to shock initiation under dynamic loading is, therefore, critical for assessing its behavior in extreme environments. Here, we investigate the dynamic response of pyrimidine under laser-driven shocks up to 4.2 GPa and determine its Hugoniot and phase diagram using in situ time-resolved Raman spectroscopy as the sole diagnostic, without any velocimetry methods. From the temporal evolution of Raman modes at 300 and 500 mJ pump energies, we directly extract shock velocities of 2.85 ± 0.05 and 3.09 ± 0.11 km/s, in excellent agreement with one-dimensional radiation-hydrodynamics simulations (2.86 and 3.10 km/s). These measurements, combined with impedance-mismatch analysis using aluminum, enable construction of the Hugoniot equation of state. The shock—particle velocity relationship is found to be linear, us = 1.736 + 1.561 up. Line-shape changes and detailed analysis of time-resolved Raman spectra further reveal a liquid → solid-I (orthorhombic) transition near ∼1 GPa and a subsequent solid-I → solid-II (low symmetry) transition at around 2.1 GPa. This work offers new insights into the dynamic stability and structural transformations of nitrogen-rich molecular systems relevant to advanced energetic formulations and determines the Hugoniot equation of state using time-resolved Raman spectroscopy.