Raman Spectra of Hydrocarbons Under Extreme Conditions of Pressure And Temperature: A First-Principles Study

Hydrocarbons are of great importance in carbon-bearing fluids in deep Earth and in ice giant planets at extreme pressure (P)-temperature (T) conditions. Raman spectroscopy is a powerful tool to study the chemical speciation of hydrocarbons. However, it is challenging interpreting Raman data at extreme conditions. Here, we performed ab initio molecular dynamics simulations coupled with the modern theory of polarization to calculate Raman spectra of methane, ethane and propane up to 48 GPa and 2000 K. Our method includes anharmonic temperature effects. We studied the pressure and temperature effects on the Raman bands and identified the characteristic Raman modes for the C-C and C-C-C bonds. To the best of our knowledge, this is the first time that the Raman spectra of hydrocarbons have been calculated at extreme P-T conditions. Our results may help with the interpretation of in situ Raman data of hydrocarbons at extreme P-T conditions, with important implications for studying hydrocarbon reactions in the deep carbon cycle inside Earth and the composition of ice giant planets.