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 to interpret Raman data at extreme conditions. 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, it is the first time to calculate the Raman spectra of hydrocarbons at extreme P-T conditions. Our result may help to interpret in-situ Raman data of hydrocarbons at extreme P-T conditions,  with important implications for studying the hydrocarbon reactions in the deep carbon cycle inside Earth and the compositions of ice giant planets.